39

Zonation Pattern, Succession Process and Invasion by Aliensin Species-poor Insular Vegetation of the Galápagos Islands
Syuzo ITOW
Professor Emeritus, Nagasaki University
Menoto 2-34-12, Nagasaki, 852-8144 Japane-mail : syuito@net.nagasaki-u.ac.jp

Abstract
The Galápagos Islands, located 1,000 km west of the South American coast on the equator in theeasternmost Pacific, are of volcanic origin. The vascular plant flora is poor and disharmonic,
comprising ca. 200 endemic taxa. The vegetation is altitudinally arranged, roughly related to the increase in precipitation, from the maritime and dry zones in the lowlands, through the transition andmoist zones, to the treeless highland zone. The zonation is deflected upward in elevation on theleeward side of each island and on leeward islands, due to the rainshadow of the southeastery tradewinds which bring moisture to the islands. The treelessness of the highlands is attributed to thefluctuation of weather conditions between extreme dryness and extreme moisture saturation, to which
no tree species adapted to such fluctuation has migrated and in which no endemic trees have evolved.
Of the flora, an endemic genus Scalesia (Compositae) has attracted special attention. The genus consistsof 15 heliophilous species, of which 12 are shrubs distributed in the dry lowlands and three are trees
found in the moist zone. All the species are allopatric in distribution. Scalesia pedunculata, thelargest tree of the genus, 12 m high and 15 cm in DBH, predominates in the moist zone of Santa Cruz
Island. The canopy population of the dense forest is a cohort of the same age, and nearly all the trees
die synchronously at maximum maturity, triggered by a large amount of El Niño rainfall or by extreme
drought in La Niña years. Such a stand-level dieback resets the cohort generation, and then self-cyclic
succession starts with no successors or shade-tolerant trees. The same was observed in an S. cordata
forest on Sierra Negra Volcano of Isabela Island. Alien plants, introduced intentionally or accidentallyto the islands, are spreading into semi-natural and natural areas as well as in disturbed habitats. Aliens
have increased since the first half of the 19th century and now exceed 600 in number of species.
Naturalized aliens include tree species that establish self-recruiting populations filling gaps in habitats
and tree niches, since the flora is poor in tree species. El Niño rainfall creates favorable conditions forinvasive aliens and accelerates their expansion to semi-natural and natural vegetation. Some of theendemic taxa are declining to endangered status due to invasion by alien plants and grazing by introduced animals like goats and donkeys. The vegetation is changing not only in agricultural and town
areas but also in part of the Galápagos National Park despite its strict regulations for protection andconservation.
Key words: alien plants, ecological disharmony, endemism, floristic disharmony, habitat, invasiveness,niche, species richness, stand-level dieback, vegetation zonation.

1. Introduction
The Galápagos Islands lie in the easternmost equatorial Pacific, almost on the opposite side of the globe
from Japan. The archipelago has been well known
since Charles Darwin’s books: The Voyage of the
Beagle (Darwin, 1845) and The Origin of Species(Darwin, 1859). Since then the archipelago has
attracted a number of biologists, who gave various
nicknames to the archipelago, of which “Noah’s Arcof the Pacific” (Eibl-Eibesfeldt, 1960) well indicatesthe biological nature of the archipelago. I named it
the “Cradle of Evolution Theory” (Itow, 1966a, 1983),
after Darwin’s description, “Hence, both in space andtime, we seem to be brought somewhat near to greatfact--that mystery of mysteries--the first appearance of
new beings on this earth” (Darwin, 1845).
Animals well known as peculiar to the Galápagos
Islands include a penguin, flightless cormorant,
finches, gulls, land and marine iguanas, giant tortoises
and many others. The tameness of those animals is
also attractive to visitors from all over the world.

40

S. ITOW

These animals have been thought of as major
members of the showcase of organic evolution in thearchipelago. On the other hand, the plant life is not
so familiar nor attractive to visitors in general,
although seven genera and ca. 200 taxa of vascularplants are endemic to the archipelago, and thevegetation is also unique in its species constitution,
distribution pattern and succession process.
My major interests have been on the ecologicalpatterns and processes in the vegetation in whichplants and animals have evolved and are evolving, as
well as on the insular plant life of endemic genus
Scalesia (Compositae), since my first visit in 1964 asa member of the Galápagos International Scientific
Project (GISP) organized jointly by the California
Academy of Sciences and the University of California
at Berkeley. Up to that time, infrequent visitors
sketched and reported vegetation of several islands
from the standpoint of their own speciality, but thevegetation itself was still open to ecological analysis.
To fill this information gap, I visited the islands andcarried out most of my field investigations on naturalvegetation in 1970 and 1978. Since then I have also
occasionally visited them to gather ecological recordsof vegetation and plant life in general, in the course ofwhich I had to widen my concerns to the menace ofalien plant introductions to the islands and to conservation issues.
The present paper aims to illustrate the patterns
and processes in Galápagos vegetation and to state a
few words about alien plants, from my viewpoint ofplant and vegetation ecology, integrating not only mystudies in the Galápagos but also studies carried out
by my colleagues of Japan and other countries.
Botanical nomenclatue in the present paper follows
Wiggins and Porter (1971) and Lawesson et al. (1987)
as a whole, Eliasson (1974) and Hamann and WiumAndersen (1986) for the Genus Scalesia, and Soria etal. (2002) for introduced species.

Fig. 1

2. Environmental Setting and Floristic
Background
2.1 Volcanology and geology
The Galápagos Islands lie on the equator, 9501,150 km west of the South American coast in theeasternmost Pacific (Fig. 1). All the islands arevolcanic in origin, created by the Galápagos Hotspot
located under the Nazca Plate. The plate moves
east-southeast, 5-6 cm per year, and therefore, theeasterly islands of San Cristóbal and Española are old,
and westerly ones like Fernanndina and Isabela areclose to the hotspot and volcanologically young (Cox,1983; Geist, 1996). Geist (1996) assumes an island
age of 4.0-6.0 million years for the easternmost San
Cristóbal, and 0.3-0.7 million years for westernmost
island of Fernandina and Wolf Volcano of Isabela,based on calculations of plate movement, distancefrom the hotspot and the subsidence rate of theislands.
Basaltic lava is the major volcanic ejecta, whichbuilds shield volcanoes in the archipelago. Scoria
and ashes are ejecta also to be noted, building scattered parasitic cones on the flanks of shield volcanoes.
The volcanological origin of all the islands means that
all terrestrial life was originally absolutely absent
from the islands. The present-day substrates which
support plant life are principally lava, partly scoria
and ash, and the accompanying soils, particularly on
the easterly old islands, that are weathered from theoriginal ejecta and mixed with organic material.
2.2 Meteorology and climatology
The Galápagos Islands lie in the southeast tradewind zone in the eastern Pacific, and the climate isrelatively cool and dry for the equatorial latitude.
This is due to the cold South Equatorial Current thatflows westward around the Galápagos Islands, bringing the cold waters of the Peru, or Humboldt, Current
which originates in the Antarctic Ocean and flows

Map of the Galápagos Islands, showing elevations of islands and volcanoes, andagricultural and town areas (hatched) excluded from the National Park area.

Vegetation of the Galápagos Islands

northward along the South American coast.
The Galápagos year can be divided into two seasons: a warm and rainy season from January to May
and a cool “garúa” season from June to December(Fig. 2). In the rainy season of a normal year, thesoutheast trade winds are weak, the South Equatorial
Current flows south of the archipelago, warmer water
comes from the north, and precipitation occurs in theform of showers. The precipitation in this season
accounts for more than two thirds of the annual rainfall, especially at low and middle elevations. In the“garúa”, or cool, season, the trade winds are strong,
the cold South Equatorial Current surrounds thearchipelago, and precipitation occurs in the form ofdrizzle, which is called “garúa” in the Galápagos. In
this season, the high elevations of the windward
islands are usually enveloped heavily by clouds andreceive high amounts of precipitation. In El Niño
years, which come every 3-6 years, with a major event
every 15-20 years, the precipitation and temperature
are higher throughout the year. Figure 2 shows theprecipitation and temperatures during the 1982-1983
El Niño. In La Niña years, which usually follow El
Niño years, the precipitation and temperatures are low.
Figure 2 also gives data of the 1985 La Niña.
The descriptions of climate and weather given
above are for Santa Cruz, which is a centrally locatedhigh-elevation island in the archipelago. The same isnearly true for southerly located, windward, highelevation islands and volcanoes, like San Cristóbal,Floreana, and the volcanoes of Cerro Azul and Sierra
Negra on Isabela Island, which the trade winds hit
directly. The leeward islands and slopes, on theother hand, receive less precipitation. The vegetation
zonation pattern shows the climatological differences
between windward and leeward islands and slopes.

Fig. 2 Air temperature and precipitation patterns at Charles
Darwin Research Station (CDRS), Academy Bay, on
the south coast of the centrally located island of Santa
Cruz. Figures show averages of normal years andanomalies in El Niño and La Niña years.

41

2.3 Floristic disharmony
Porter (1983) states that most of the Galápagos
native flora is derived from Central and Tropical
America and the rest from Andean, Caribbean orpantropical regions. He also estimates the number oforiginal introductions, based on the morphology ofseeds and fruits adaptive to natural agents of introduction such as birds, winds and oceanic drift. For example, plants bearing berries or drupes are thought tohave been introduced by birds internally, wetland
plants bearing minute seeds by birds externally, plants
bearing minute seeds with parachutes or wings by air,and plants bearing salt-tolerant and buoyant seeds or
fruits by oceanic currents. Based on this, the indigenous flowering plant flora (including endemics andnon-endemics, excluding human-introduced species)
has been derived from 306 introductions by means of
natural agents, and the fern flora has been derived
from 107 introductions, all by air except a waterfern
by birds. Of the 306 natural introductions of flowering plants, 243 (or 79% of the total) were by birds
internally in the digestive tract, or externally attached
to feathers or webbed feet. Twenty-eight (9%) by
wind, and 35 (11%) by oceanic drift. Nearly 80% of
natural introductions were by birds, probably because
birds carrying seeds from the continent would
deliberately aim for other land, including islands,
while seeds buoyant on air or sea currents have very
little probability of arriving on small remote islands inthe vast ocean.
In all cases, birds and winds carry only small seeds,
and not large and heavy ones. Natural agents cannot
carry all the flora of the source area, but only selected
ones. This is the cause of floristic disharmony, or unbalanced constitution of the flora, of remote oceanicislands. The consequences can be seen in the fact that,among the Galápagos’ indigenous flora, Agavaceae,
Araceae, Cannaceae and Palmae are absent,
Bromeliaceae, Hypoxidaceae, Iridaceae, Najadaceae,
Ericaceae, Viscaceae, and Vitaceae are families represented by only one species, whereas families with unusually high percentages are small seeded Cyperaceae
and Gramineae, externally bird-carried or air-born
Compositae, internally bird-carried Boraginaceae,
Cactaceae and Solanaceae. The exception is Leguminosae, which bears somewhat large seeds inside ofcapsules which float and are subject to oceanic drift.
Floristic disharmony is one of the important characteristics of oceanic remote island biota. The endemism rate is 7% among the ferns and allied taxa, and
51% among the flowering plants.
2.4 Species poverty
Another aspect of disharmony is the poverty oftree and shrub species in the Galápagos Islands, ascompared with continental areas.
Tree species
poverty can be shown by species diversity indices.
The indices I used in my previous study (Itow, 1988)
were Fisher’s alpha index (Williams, 1947) and42

S. ITOW

Hurlbert’s S(n) index (Hurlbert, 1971). The alphaindex can be calculated by the formulaS = α ln (1 + N/α),
where S is the total number of species and N is thetotal number of individuals found in the sampled community, and the alpha value indicates the theoreticalnumber of species represented by single individuals inthat biotic community. Larger values indicate thatthe community consists of a larger number of species.
Hurlburt’s index can be calculated by the formula
proposed recently by Morisita (1996),
S(100) = Σ [1 − (1 − 100/N) xi],
where xi is the number of individuals in the i-th
species, and N is the total number of individuals in thesampled community.
I used both the alpha and S(100) indices to compare
the species-richness of trees and shrubs exceeding
3 cm diameter at breast height (DBH) between
oceanic islands and continental areas (Itow, 1988).
Since I have not collected any such data from continental rainforests, I cited data from Thailand andupper Amazonia. I also studied natural forests in
Micronesia (oceanic islands), the Ryukyus (continental islands), the Bonins (Ogasawara) (oceanicislands) and Japan (continental islands). Table 1
compares the index values between continental habitats and oceanic island habitats. Figure 3 shows theparallel relation of continental species-richness withclimatic favorableness represented by Kira’s warmthindex (Kira, 1977). As seen in this graph, theGalápagos Archipelago is located in a subtropicalclimate, and the trees and shrubs in the oceanic Galápagos, Micronesia and Bonins are definitely poor interms of saturation with tree species, nearly half therichness expected based on the climate, as comparedwith continental forests.
In interpreting ecologically the poverty of tree
species on oceanic islands, key concepts are ‘habitat’and ‘niche.’ Habitat means the particular type, or site,of environmental conditions integrated by all factors
affecting plant life. The vegetation zones are habitat
types in the present study. Niche means the particular status or role of a species in a biotic community.
In the present paper, tree, shrub, liana (climbing plant),
and herb or grass are plant life-forms corresponding tobroad plant niches. The poverty of tree species on
oceanic islands implicitly suggests the vacancy of thetree niche in some habitats. This is an ecologicalaspect of floristic disharmony that should be noted as‘ecological disharmony’ to understand the vegetationand ecosystem characteristics of oceanic islands. It
will be discussed in detail in the sections on gradientanalysis, succession process and invasion by aliens.

Table 1 Comparison of diversity values of natural forest communities as averages and ranges between mainlands
and oceanic islands in the subtropical and tropical climates of East Asia, the Pacific and South America.
(After Itow, 1988)
Mainlands and Continental islands

Oceanic islands

Ryukyus

Bonins

Subtropical
WI
Alpha
S(100)
Tropical

11.6
26.0

190 - 217(8.0-16.4)(21.0-32.2)

Thailand
WI
Alpha
S(100)
Equatorial South America
WI
Alpha
S(100)

Fig. 3

28.0
43.0

4.15
13.2

182 - 205
(2.4-6.5)(9.0-17.3)

Micronesia
244(26.2-30.0)(41.5-44.4)

Amazonian upstreamc. 24026.5(22.6-42.6)
40.5(37.2-42.6)

3.4
11.7

210 - 260
(1.8-4.9)(8.5-15.0)

Galápagos
2.1
7.8

200 - 208
(0.8-4.4)(4.0-13.9)

A regression of species diversity values with the climatic favorableness as represented by Kira's
warmth index in continental areas, showing the gaps in values of oceanic islands from thecontinent. (after Itow, 1988) Arrows indicate values of Galápagos forests. For indices, see text.

Vegetation of the Galápagos Islands

3. Patterns in Vegetation Zonation
3.1 Rainshadows
One purpose of my vegetation studies in the
Galápagos Islands was to find patterns in space andtime, as well as to describe unknown plant communities. I adopted the gradient analysis (Whittaker,1967) as a method of detecting spatial patterns, andused elevation as the first gradient axis. Fieldworkon uninhabited islands was hard because of bushes
and lava fields that were difficult to traverse. In
some cases, I observed vegetation and landscapesfrom the boat, and studied carefully the records madeby previous visitors. The vegetation zones I recognized ranged from the maritime zone along thecoastline, through the dry zone, transition zone, andmoist zone at middle elevations, to the highland zone.
Figure 4 is a schematic illustration of vegetation zonation in an east-west cross-section of the archipelago
from the easterly located windward island of San
Cristóbal, through Santa Fé, and the centrally locatedSanta Cruz and Pinzón, to the leeward volcano of
Alcedo and westernmost Fernandina, along the line ofvolcanologically old to new volcanoes.

Fig. 4

Schema of vegetation zonation from the volcanologically old, meteolorogically windward island of San
Cristóbal, through three centrally located islands, tothe new and leeward volcano of Alcedo, Isabela
Island, and Fernandina.

The vegetation zones, excepting the maritime zone,
are deflected upward in elevation on the leeward
islands, and the same is true on the leeward north sideof individual islands. As described before in the
Meteorological section, the southeast trade winds
bring moisture to the islands, and clouds develop
better and precipitation occurs more heavily on thewindward side. The upward deflection of the zones
is due to rainshadows of the trade wind (Itows, 1971,
1975, 1992).
Zone boundaries can be identified by changes inzone characteristics, which are given below briefly,
but are obscure in some cases where interrupted byrecent lava flows.
3.2 Characteristics of the vegetation zones
Maritime zone (Fig. 5a):Coasts along the shoreline are characterized by43

salt-spray resistant vegetation. There are three main
habitat types in the maritime zone. They are sandbeach, lava coast, and mangrove. (Hereafter asterisks show species endemic to the Galápagos Islands.)
Major plants on the beaches are Amaranthus sclerantoides*, Sporobolus virginicus, Tiquilia darwinii*, T.
galapagoa*, and Ipomoea pes-caprae. Shrubs ondune crests are Cryptocarpus pyriformis, Vallesia
glabra, Nolana galapagensis* and Scaevola plumieri.
Plants common on lava coasts are Sesuvium edmonstonei* or S. portulacastrum in the first belt and C.
pyriformis in the second. Mangroves of four species,
Rhizophora mangle, Avicenia germinans, Laguncularia racemosa and Conocarpus erecta, are foundon either lava or mud substrate in calm coves andinlets.
Dry zone (Figs. 5b & 5c):
The vegetation is decidedly xerophytic, consistingof white-barked trees and shrubs such as Bursera
graveolens (most common), Croton scouleri*(narrow-leaved varieties), Piscidia carthagenensis,
Castela galapageia*, Cordia lutea, Tournefortia
pubescens* and Lantana peduncularis*. Some ofthem are drought-deciduous. Gigantic plants ofOpuntia species* and Jasminocereus thouarsii* areprominent in some areas. The other plants restricted
to the dry zone are thorny Leguminosae trees andshrubs such as Acacia species, Prosopis juliflora andParkinsonia aculeata.
Of the endemic genus
Scalesia (Compositae), shrubby species are restricted
to the dry zone. The absence of terrestrial and epiphytic ferns is one of the characteristics of the dry
zone vegetation, except in cracks in lava. The coloration of the vegetated dry-zone landscape is white togray due to the white-barked trees and shrubs. The
surface of the substrate is nearly completely coveredby unweathered lava blocks with little acumulation oforganic material in which plants can root.
Transition zone (Fig. 5d):This zone is literally transitional from xerophytic
to mesophytic. The surface of the substrate consistsof lava blocks and weathered soil. Bursera graveolens trees reach a good size, up to 10 m high.
The undergrowth of the forest is denser than in the dryzone. Ferns begin to appear in the ground layer ofthe forest and on tree trunks as epiphytes. One of theprominent characteristics is abundant growth of filamentous lichens such as Ramalina and Usnea species.
On Santa Cruz, upper parts of the transition zone
support sporadic growth of Scalesia pedunculata*
trees, which predominate in the lower half of the
moist zone of this island.
Moist zone (Figs. 5e, 5f & 5g):
The mesophytic moist-zone vegetation is characterized by abundant growth of shrubs, herbs andterrestrial ferns in the forest and by green and brown
bryophytes that festoon tree branches. On the islandsof San Cristóbal and Santa Cruz, the moist zone canbe subdivided into three sub-zones, Scalesia pedun-

44

S. ITOW

culata* forest (Fig. 5e), Zanthoxylum fagara forest,
and Miconia robinsoniana* scrub (Fig. 5g). On islands other than San Cristóbal and Santa Cruz, theMiconia scrub is absent. On the western islands,
Scalesia cordata* forest occupies the lower part of the
moist zone on two southern volcanoes of Cerro Azul
and Sierra Negra in Isabela, and Scalesia microcephala* (Fig. 5f) on three volcanoes of Alcedo,
Darwin and Wolf in the northern half of Isabela andon Fernandina. On those volcanoes and islands, themoist-zone vegetation is low in term of tree height,
and sparse in terrestrial herbs and ferns and in epiphytic bryophytes. The sparse vegetation on thosenorthwestern volcanoes is apparently due to volcanologically young substrates as a result of the closeness to the Galápagos hotspots and reduced precipitation in the rainshadows of the southeast trade winds.
The moist zone vegetation has been heavily

disturbed by man on the southerly located inhabitedislands of San Cristóbal, Floreana and Santa Cruz andthe windward volcano Cierra Negra of Isabela. The
central parts of these islands and the volcano havebeen disturbed and converted to farmland and pastures.
Nowadays Scalesia pedunculata forests are restricted
to only the north side of Santa Cruz and are foundvery sparsely on San Cristóbal, Floreana and Santiago.
Scalesia cordata forests are also nearly gone from theCierra Negra Volcano of Isabela.
Highland zone (Fig. 5h):
The present zone is characterized by treeless vegetation, and has been called by different names such as
the grassy zone (Stewart, 1911), mountain zone
(Svenson, 1935), upland zone (Bowman, 1961), fernsedge zone (Wiggins & Porter, 1971), evergreen fern
meadow or pampa (Werff, 1979), and broad-leaved
herb vegetation, meadow or pampa (Hamann, 1981).

Fig. 5 Vegetation of different zones. a: Maritime vegetation on sand beach, Rábida. Top-left is sparse forest of
Bursera graveolens. b: Dry zone landscape as seen from Isla Eden on the northwest side of Santa Cruz. c:Dry zone vegetation on lava field at Villamil, Isabela. d: Bursera graveolens forest in the transition zone on
Santa Cruz. e: Dense forest of Scalesia pedunculata at Los Gemelos, Santa Cruz. Top-left is the transitionzone to the dry zone on the north side of the island. f: Sparse forest of Scalesia microcephala at a middle
elevation on the east side of Alcedo, Isabela. g: Miconia robinsoniana scrub at Cerro Media Luna, Santa
Cruz. Photo taken in 1991 when Cinchona pubescens (green trees) had already invaded sporadically. h:Treeless highland vegetation at Cerro Crocker, the highest point of Santa Cruz. The brake at center is anendemic tree-fern, Cyathea weatherbyana.

Vegetation of the Galápagos Islands

As these names suggest, major plant species of thepresent zone are ferns, grasses, sedges and herbs.
Trees are absent and shrubs are restricted to leeward
slopes and depressions, while the endemic tree fern,
Cyathea weatherbyana*, is prominent in some areas.
There are several types of plant communities in the
Santa Cruz highlands, which will be described later indetail. The highland treeless vegetation of other islands is also varied in types and species composition..
3.3 Reason for highland treelessness
As to the reason for the treelessness, there havebeen two hypotheses. One attributes the treelessness
to the greater wind velocity and decreased precipitation (Stewart, 1911: p.286; Bowman, 1961: p.18;
Wiggins & Porter, 1971: p.27). The other takes an
opposite view, which is that the high air humidity
blocks evapotranspiration and mineral uptake byplants (Werff, 1978: p.31). Both views are individually partly true to weather conditions of the highlands, since the dry phase appears infrequently during
the warm rainy season, especially so in La Niña years,
and the wet phase appears frequently in the garúa
season.
The rainfall patterns in the Galápagos vary greatly
within and between seasons and months (Grant, 1986).
The highlands become infrequently completely dry forweeks and months owing to strong dry winds, particularly during the La Niña warm season, while they
become very wet during the garúa season due to heavydrizzle and fogs which last for weeks and months
without a break.
My hypothesis on the treelessness is as below.
There are tree species that adapt to dry conditions, likedesert plants, and there are tree species that adapt towet conditions, like wetland plants. The dry extremes, even once in many years, will kill trees adapted

North side
(leeward)

Fig. 645

to wet or moist habitats, and the wet extremes killthose adapted to dry conditions. In the Galápagos
Islands, however, no tree species adapted, or resistant,to both extremely dry and extremely wet conditions
has migrated to or evolved in this oceanic archipelago.
I proposed this hypothesis at the Galápagos Botany
Workshop in 1987, which is cited in p. 595 of Vegetation of the Tropical Pacific Islands (Mueller-Dombois& Fosberg, 1998). Unfortunately, my hypothesis has
been verified by the vigorous invasion of the Santa
Cruz treeless highlands by the tree Cinchona
pubescens in the 1990s. (For details, see the Alien
chapter of the present paper.)3.4 Detailed gradient analysis of Santa Cruz
vegetation
Fieldwork on Santa Cruz has revealed the rainshadow effects on vegetation zonation in detail (Itow,
1971, 1975). In 1964 and 1970, the Zanthoxylum
fagara natural forest still kept its physiognomic
feature, a brownish appearance due to brown liverworts, Frullania festooning the tree branches, but it
had already lost its natural conditions due to animalgrazing and agricultural exploitation.
On the leeward north side of the same island, on
the other hand, the zones are largely deflected upwardin terms of the zonation, and, on the north side,
Zanthoxylum forest and Miconia scrub are absent
while the Scalesia forest is directly contiguous to thetreeless highlands (Fig. 6). The gradient analysis ofvegetation gives us not only the zonation pattern but
also the distribution ranges of individual plant species
in relation to elevation and substrate conditions, onboth the windward and leeward sides of the island(Fig. 6).
In 1970 and 1978 when I conducted most of myfieldwork, the natural vegetation on Santa Cruz wasSouth side
(windward)(Above) Zonation pattern of vegetation on Isla Santa Cruz. The rainshadow effect can be seen in the upward deflection of
zonation on the leeward northside of the island.
(Below) Gradient analysis of vegetation in relation to elevation and substrates as represented by the coverage of lava on the
ground surface, showing distributions of selected plant species and individual plant communities. (After Itow, 1992)

46

S. ITOWDry zone

Diversity value

undamaged or little disturbed on both the south andnorth sides of the island, excepting the Scalesia forest
and Zanthoxylum forest in the moist zone. The same
was true of the highland zone, which supported six
types of plant communities. Each of them was dominated individually by Paspalum conjugatum, Pteridium aquilinum-Blechnum polypodioides, Lycopodium
clavatum, Hypericum uliginosum, Polypodium
tridens*, and Pernettya howellii* (Itow, 1990). The
order of the above descriptions is roughly parallelfrom moist to dry habitats in the highlands (Fig. 7).
In addition, there were three types of Sphagnumdominated vegetation. They were fens dominated by
S. cuspidatum var. serrulatum, raised bogs dominatedby S. erythrocalix and vertical bogs (Fig. 8) dominatedby the same bog moss (Itow & Weber, 1974). All thehighland plant communities listed above were treeless,but many of them have been changed gradually sincethe 1990s due to the invation by an alien tree,
Cinchona pubescens.
The gradient analysis also detects a pattern of tree
species richness along the elevation gradient. Bothof the diversity indices I adopted, Fisher’s alpha andHurlburt’s S(n), tell us how many species can be foundin the community. Figure 9 shows the change inindex values for woody plants with trunks larger 3 cm

Diversity value

Fig. 7 Transect of the Santa Cruz highland, showing the distributions of eleven types of plant communities, including fens and bogs.
1. Miconia robinsoniana scrub, 2. Pteridium aquilinum / Blechnum polypodioides communnity, 3. Cyathea weatherbyanastand, 4. vertical bog, 5. Paspalum conjugatum community, 6. fen, 7. Polypodium tridens community, 8. Lycopodium
clavatum community, 9. raised bog, 10. Pernettya howellii community, 11. Scalesia pedunculata forest. (After Itow, 1990)

Fig. 9Trends in woody species richness along the elevation
gradient on the north and south sides of Santa Cruz.
Note higher values in the transition zone, showing a
mixture of dry zone and moist zone species, and low
values in the Scalesia forest.

DBH along the elevation. The arid and transitionzone plant communities consist of seven to ten woody
species, while the moist zone Scalesia forest consistsof only three to five species. The forest canopy is
predominantly composed of Scalesia pedunculata, atree to like Compositae, 12 m high and 15 cm in DBH
on average. The forest is definitely poor in tree andshrub species.

4. Ecology of Endemic Genus Scalesia

Fig. 8 Vertical bog formation on wind-exposedwall of parasite volcano.

My special interest has gradually become focusedon the endemic genus Scalesia (Compositae) since myearly field studies in 1964 and subsequent ones inVegetation of the Galápagos Islands

1970, because I frequently encountered different
Scalesia speices in the dry and moist zones on nearly
all the islands I visited. Different species of shrubby
Scalesia are found in the dry zones on different islands, and different tree species in the moist zones on
different islands. I thought that no vegetation study
in the Galápagos would be complete without ecological studies of this genus.
4.1 Patterns in speciation and distribution
There are 15 species, four subspecies and six varieties, of which three species and one variety are trees
and 12 species four subspecies and five varieties areshrubs (Eliasson, 1970; Hamann & Wium-Andersen,1986). Ono (1967, 1971) first studied the chromosomes of the genus and revealed 68 chromosomes insomatic cells for all of the six species he studied.
Eliasson (1970, 1974) confirmed the same results innine taxa he studied. Ono (1967, 1971) and Eliasson(1974) suggested Helianthus, Thitonia or Viguiera as
the closest relatives of Scalesia, although Schilling etal. (1994) showed the close affinity of Scalesia to thegenus Pappobolus, an Andean endemic, based on a
study of chlorophyll DNA. Apparently, Scalesia
species have evolved from an ancestral herbaceous
plant to 12 shrubby species and three tree species.
All the species are allopatric in the present-day
distribution.
Hereafter I treat the five main
volcanoes of Isabela as different individual islands,
because they originated as separate islands whichwere then connected to neighboring volcanoes by low
and vast lava fields.
To understand speciation pattern in this genus, I
accepted two assumptions. The first is biogeographic,that larger islands are able to harbour more species, as
MacArthur & Wilson (1967) predicted and verified.
The second is related to the time available for
speciation, that is, older islands have more years for
speciation and therefore support more species than
younger islands. Reflecting a combination of these
two assumptions, older and larger islands actually do

Fig. 1047

harbour more species than newer and smaller islands(Fig. 10). High-elevation islands support a tree species in their moist zones, while low-elevation, small
islands, volcanologically either new or old, harbour
only one shrub species in their dry lowlands. Largebut young islands harbour one shrub species and one
tree species in their dry and moist zones, respectively,
whereas large and old islands harbour a few to several
shrub species in their dry zones and one tree species intheir moist zones.
All the shrub species, excepting Scalesia baurii
subsp. baurii, which is restricted to Pinzón, are distributed in the dry lowlands, while three of the tree
species are found in the moist zones on middle- andhigh-elevation volcanoes and islands. I have collected specimens of all shrub species but one, S.
atractyloides, from their habitats, all of which are outcrops of lava or unweathered substrates scatteredamong sparse forest of Bursera graveolens or similar
sparse vegetation in the dry zone. The habitat isapparently strongly heliophilous.
Of the three tree species, Scalesia microcephalaforests are sparse in density and 3-5 m high in the
middle elevations of Fernandina and the volcanoes
Wolf, Darwin and Alcedo of northern Isabela (cf. Fig.
5f). Scalesia cordata forests are dense and 8-10 m
high in the middle elevations of Sierra Negra andCerro Azul, southern Isabela.
The Scalesia
pedunculata forest is dense and 8-12 m high in the
moist zones of Santiago, Santa Cruz, Floreana, andSan Cristóbal.
I also collected ripe seeds of Scalesia in 1970 for
Ono’s (1971) chromosomal study of five species,
some of which I sowed in a greenhouse in Japan.
The seedlings grew fast to one meter and flowered six
months after germination. In S. pedunculata, thelargest tree species in the genus, the trunk reaches
12 m in height and 15 cm in DBH, with annual rings
up to 0.5-1.0 cm thick. The wood of this species issoft with a large pith at the center (Fig. 11). Fast
growth, flowering in the germination year and large

Relationship of the number of Scalesia species with the relative age and size of volcanoes and islands.
Numerals affixed to volcano and island names are their maximum geological ages cited from Geist (1996).

48

Fig. 11

S. ITOW

Scalesia pedunculata trunk. Note the large pith atcenter and width of annual rings.

pith are apparently herbaceous traits of the plant.
Hamann (1979) suggested its life span may be 15-20
years.
In the remote past in a geological sense, the moist
habitat and tree niche were nearly vacant in the
Galápagos. Tree Scalesia species have evolved froma herbaceous ancestor, filling the vacancies in both
moist habitat and the tree niche. The present Scalesia forest is low in species richness in Whittaker’s(1972) sense, ranging S(100) = 3-5 (Itow, 1988, 1992).
Of the three tree Scalesias, S. pedunculata is
distributed in the most fertile habitat of the southerly
to centrally located high-elevation islands of San
Cristóbal, Floreana, Santa Cruz, and Santiago. The
natural forests of this species have been nearly extirpated from San Cristóbal and Floreana, due to early
settlement and exploitation since the 19th century, andfrom Santiago due to heavy grazing and browsing byferal goats for a century. Fortunately the naturalforest of this species has remained on the north side ofSanta Cruz until now.

4.2 Succession process of Scalesia pedunculata
forestThere was a dense forest of S. pedunculata in the
Los Gemelos area at a middle elevation ca. 550 m
above sea level on Santa Cruz, when I visited there in1970 and 1978 (Fig. 12a).
Since then I haverecorded the diameter of trunks of selected S.
pedunculata trees (not density in quadrat) in 1978,
1981, 1987, 1991, 1995, 1998 and 2002 (Fig. 13).
The 1982-1983 El Niño event brought a large amountof rainfall, 10 times that of a normal year, and nearly
all the Scalesia trees died synchronously. This was agood example of stand-level dieback of forest(Mueller-Dombois, 1988, 1992). New cohorts germinated in 1985 and produced 100% ground coverage(Fig. 12b). As seen in those pictures, other trees
associated with S. pedunculata were quite sparse, andonly of Zanthoxylum fagara and Psidium
galapageium. The stand is nearly purely dominatedby S. pedunculata, with no shade-tolerant tree species.
As the species is heliophilous, the seeds germinate
only when mature trees fall down and sunny habitat iscreated. Thus the succession is collapse-and-recruiting,or self-cyclic (Itow & Mueller-Dombois, 1988; Itow,

Fig. 13

Fig. 12(a) Mature forest of Scalesia pedunculata at Los
Gemelos, Santa Cruz in 1981, two years prior to the
1982-1983 stand-level dieback. (b) Young
generation at the same area in 1986, one year after
the new cohort germinated in 1985.

Change in size distribution of Scalesia pedunculata
at the Los Gemelos area of Santa Cruz. The 1978apopulation (top) was a four-year old generation
established on abandoned soils. The 1978b and1981 populations were approaching the mature stage.
In 1982-1983, a heavy El Niño occurred, and the oldgeneration died back synchronously. The 1987
population consisted of a few old trees and many of
the new generation. The figures of 1978a-1991 are
reproduced from Itow and Mueller-Dombois (1988)
and Itow (1995). New measurements made in 1995,
1998 and 2002 are added.

Vegetation of the Galápagos Islands

1995; Fig. 14). The same was observed in a dense
forest of S. cordata on Sierra Negra Volcano of Isabela
Island (Lawesson, 1988). In the sparse forest of
Scalesias, stand-level dieback does not occur but the
population recruits with continuous generation underopen canopy (Lawesson, 1988; Shimizu, 1997).
In 1997, the second heaviest recent El Niño
occurred, but complete stand-level dieback did notoccur in the Los Gemelos forest, because the standwas young and premature with an age of ca. 12 years
after the 1985 cohort germination, while it occurred ina dense Scalesia forest at La Caseta around 200 m
above sea level (Hamann, 2001), which is the
low-elevation limit of Scalesia forests.
In the S. pedunculata forest at Los Gemelos immediately after the 1997 El Niño, the above-ground biomass was ca. 60 kg ha-1, which is one-fourth to onesixth that of the Meterosideros polymorpha forest in
Hawaii. Mineral nutrients such as soluble potassium,and exchangeable calcium and magnesium are rich in
the forest soils, almost 5.5 to 10 times those in the
Hawaiian forest soils (Kitayama & Itow, 1999). The
foliage of the canopy was nearly 100% in coverage
and was thought to be approaching maturity, butstand-level dieback has not yet occurred as of 2002,
when the forest was 17 years old. This is because
the trees seem to be dying gradually, leaving gaps inthe canopy and creating sunny spots on the forest floor
where new seedlings appear. The forest may berecruited by continuous generation, as Shimizu (1997)
predicted.
4.3 Hypothesis on stand-level dieback
The question on stand-level dieback is why thedieback occurs in dense monospecific cohort forestsof Scalesia pedunculata and S. cordata but not in

Biomass

Pioneer

Climax species
Successor

49

sparse forests of the same species. A clue to thisquestion seems to be in the growth habits of thespecies on the one hand, and the self-thinning processof the population on the other. Based on samplings
and measurements made in 1998 at Santa Rosa, 350 m
in elevation on the northwest side of Santa Cruz,
Kitayama and Itow (1999) have revealed allometric
relationships between the DBH and dry weight ofwood and foliage in S. pedunculata as below:
Ww = 0.01540 (D 2) 1.609059 (r 2 = 0.99), and
Wf = 0.01769 (D 2) 0.77946 (r 2 = 0.76),
where Ww and Wf are the dry biomass (kg) of woodand foliage, respectively, and D is the trunk diameter(cm) at breast height over bark. It should be notedthat the growth rate in wood biomass is as large as
twice the net biomass increment in foliage. This factsuggests that in every individual tree, the photosynthetic assimilation gain by the foliage may not provide enough energy to support the respiration loss inthe wood and roots at a mature point, which seems tobe the life span of the tree.
In 1970 at La Caseta, located at 200 m in elevationon the southwest side of Santa Cruz, there were 10
stands of different sizes (ages) of S. pedunculata.
Within each of the stands, individuals were nearly thesame size, and could be of a single-aged cohort. The
average DBH was 1.4 cm in the youngest stand and9.4 cm in the oldest stand. The population density
per 100 m2 was 325 in the youngest and 36 in theoldest stand. Figure 15 shows the self-thinning process of density in a series from young stands to older
ones, as the forest develops.
Here the biomass of individual trees and that of thestand can be calculated, using the allometric equations
given above. Figure 16a shows the increase in standbased wood biomass in relation to forest development,
in which the population density decreases and DBH
increases. The stand-based foliage biomass (Fig.
16b), however, stays the same, or decreases a little, as
the forest develops, although some irregularities areseen.
This fact predicts hypothetically that the standbased assimilation gain by foliage may not provide
enough energy to support the respiration loss in woodand roots at a mature stage of the forest’s development,
and that a synchronous dieback may occur in such an

PioneerTime
Fig. 14

A schematic illustration of (above) self-cyclic
succession of heliophilous Scalesia pedunculata as
represented by synchronous stand-level dieback andsubsequent cohort regeneration, with a comparison to(below) normal succession with heliophilous pioneers,
followed by successors, and finally shade-tolerant
climax species, which is found in species-rich
subtropical regions in the nortthern hemisphere.

Fig. 15 Self-thinning of Scalesia pedunculata population
density per 100 m2 during forest development. Data
from La Caseta collected in 1970.

50

S. ITOW

even-sized cohort mature stand. Actually a synchronous stand-level dieback did occur in 1982-1983 (Fig.
13). The dieback plays an important role in settingback the population to a new cohort generation, creating a sunny habitat for seed germination of thisheliophilous species. Each individual of the cohort
grows again and will approach its senescent stage,
thinning its population, unless the cohort encounterssome disturbance like disease or insect attacks. Thenthe stand must die back again synchronously when an
environmental anomaly like heavy rainfall or extreme
drought acts as a trigger. Then again the dieback sets
the population to a new, next cohort and a new
succession starts. The whole series of ecologicalevents is a self-cyclic succession or a buildup-collapse
succession, which includes the phases of initial buildup of the new cohort, its growth, senescence, anddieback (Fig. 14).
Synchronous diebacks in Scalesia species will
occur in dense and even-sized cohort mature populations, when they encounter environmental anomalies
like heavy rainfall (Itow & Mueller-Dombois, 1988;
Lawesson, 1988) or extreme drought (Kastdalen,1982). In canopy-broken forests of the same species,
as Shimizu (1997) described as continuous regeneration, the population recruits continuously with newseedlings that grow at sunny spots on the forest floor,
and individual old trees die when they reach theirmaximum maturity, or when they are damaged bydisease or insect attack.

ab

Fig. 16

Changes in (a) wood and (b) foliage biomass of the
Scalesia pedunculata stand (100 m2) during forest
development, adapted by the allometric equations
given in the text.

5. Invasion of Island’s Ecosystem by Alien
Species
5.1 Background
Since the islands of the Galápagos Archipelago areof volcanic origin, and were originally free from any
terrestrial life, all the plant species are aliens, in asense, originally introduced from outside by natural
agents such as winds, birds, and oceanic currents.
Plants that were so introduced and established on theislands and plants evolved from those plants are
named as indigenous, or native. The aliens or exotics
treated in the present paper are plants that have been
introduced intentionally by man or accidentally in
relation to human activities.
Since introductions of alien plants, either intentional or accidental, are associated with human activities, a brief human history of the archipelago isneeded here in order to understand the background ofalien plants. Since the discovery of the islands in
1535, they were occasionally visited by buccaneers in
the 17-18th and frequently by whalers in the 18-19th
centuries. Man first settled there in the first half ofthe 19th century on the island of Floreana, and then at
the turn of the century on the island of San Cristóbal
and on the Sierra Negra Volcano of Isabela Island.
Settlement on Santa Cruz began in the 1920s.
Human habitations were restricted to areas on the
coasts and at middle elevations where one or both ofarable land and drinking water were available.
The archipelago was designated a National Park in
1959, and the Charles Darwin Research Station(CDRS) and the Galápagos National Park Service(GNPS) were inaugurated in 1964 and 1968, respectively. Up to that date, goats and donkeys had run
wild on many of the uninhabited islands as well as on
inhabited islands, and cows and horses also ran wildon the four inhabited islands. Since ecotourism
started under strict regulation in the mid-1970s, plant
and animal life and the unspoiled scenery of the
volcanic archipelago attracted visitors from all over
the world. The consequent inflow of man and cargo
increased year by year. The human population has
multiplied tenfold in half a century, from ca. 1,400 in
1950, via ca. 4,000 in 1974 and ca. 9,700 in 1990, to
ca. 16,000 at the turn of the 21st century. Human
habitation is still restricted to mid-elevation agricultural zones and the coastal towns of the four islands of
San Cristóbal, Floreana, Santa Cruz, and Sierra Negra
Volcano of Isabela. Galápagos National Park occupies 97% of the land surface, which is zoned into four
categories with different protection regulations, which
are the Maximum protection zone, Primitive zone,
Special use zone and Visitors’ zone (Galápagos
National Park Service, 2000).
5.2 Increase in alien plants
In the study of introduced plants, the first step isdistinguishing alien species from indigenous ones.

Vegetation of the Galápagos Islands

This step is easy for intentionally introduced plants
such as ornamentals, fruits and crops cultivated ingardens and farmlands, and also for those plants
useful for man even if they are spreading in naturaland semi-natural habitats. It is not so easy, however,to distinguish accidentally introduced herbs andgrasses from indigenous species of natural introduction by winds, birds, and oceanic drifts. For this step,
I largely followed categories in Porter (1983) and
Lawesson et al. (1987), where plants are classified as
endemic, non-endemic native, introduced, and cultivated or garden escapes. They treated the last two
categories as alien species, but their evaluation was
not always the same for some particular herbs andgraminoids. For example, one author put species X(e.g. Desmodium glabrum) in the introduced category
but the other in the native category. In the present
study, I classified such species into the alien category,
and therefore, the number of alien species became
larger than the enumeration in either Porter (1983) or
Lawesson et al. (1987).
The second step is to trace back to the earlier lists
to grasp of the increase in aliens, enumerating thealien species as defined above in each of the pastcollection lists. For this step, I treated the alien species that were listed in an earlier list, as present in thelater lists even if they were not listed under any
categories in the latter. This is because the lists earlier than Porter (1983) were different in focus; onelisted cultivated plants, the others did not. Thus thenumber of alien species increases cumulatively fromthe early days to the present, adding new records ofaliens.
The first collections of aliens were made by
Charles Darwin during the 35 days of his stay in 1835
(Hooker, 1847). He collected 209 species, of which
14 are supposed as aliens in the present definition. A
few botanical explorations followed, and immediately
after the turn of the century, the California Academyof Sciences carried out a year-long scientificexpedition in 1905-1906, in which 100 alien species
were listed (Stewart, 1911). A few more investiga-

Fig. 17

Increase of alien plant species during the century
and a half since Darwin's collections. Counts after
1990 are only the total numbers cited. For definitionof aliens and methodology of count, see text.

51

tions followed, until the CDRS was launched in 1964.
Since then the CDRS’s botanists have conducted
extensive and intensive investigations by themselves,and have supported field work by visiting scientists.
The 1987 checklist of the flora (Lawesson et al.,1987) enumerated 255 introduced and cultivatedspecies (310 in my count being based on the present
definition as above). In the course of the present
study, I counted ferns, monocotyledons and dicotyledons separately. Until 1990, no ferns had been
listed as introduced; monocots and dicots occupied
about 20%-80%, respectively, of the total alien species
in each of the lists studied. Figure 17 shows theincrease in those aliens during a little more than acentury and half since Darwin’s biological explorations. Recent enumerations of total numbers after
1990 are cited from the CDRS’s botanists, Jaramillo(1999) and Tye (2001b).
The increase in alien plants is apparently dependent on two factors: real increase due to genuine new
introductions, and amounts and focus of field research.
At present, the CDRS’s botanists enumerate ca. 600
species (Tye, 2001b), which includes three introduced
fern species (Tye, Personal communication, 2002).
The alien species listed are not spread evenly over theislands but are largely confined to agricultural andurban areas. Relatively few alien species have invaded the National Park area but some of these are
now widespread even on uninhabited islands (Tye,Personal communication, 2002).
5.3 Habitats and niches for aliens
Hereafter the present chapter focuses on alienplants that spread and recruit by their own dispersal
and reproductive abilities in natural, semi-natural andman-altered habitats, irrespective of introduction purposes, means and processes, but excludes plants thatwere intentionally introduced and are so far still only
found in cultivation in agricultural and town areas.
Examples of the latter are ornamentals (e.g. Chinarose, Hibiscus rosa-sinensis L.) and crop plants (e.g.
sweet potato, Ipomoea batatas Lam.).
The conceptual frame to understand invasion of
insular ecosystems by aliens should consist of thehabitat and niche for individual species. In thepresent study, the habitat can be conceived along twoaxes. One is the altitudinal zonation from maritimeand dry zones, through the transition and moist zones,
to the highland zone. This gradient runs parallelwith the altitudinal increase in annual precipitation,
though its monthly distribution in the different zones
differs somewhat. The other is the degradation axis,
which ranges from natural, through semi-natural, todisturbed or man-altered. ‘Natural’ means truely
natural conditions that support vegetation composedof endemic and indigenous non-endemic plant species.
Individual vegetation zones support natural plantcommunities, as described in the previous chapters.
‘Disturbed’ or ‘man-altered’ indicates habitats that

52

S. ITOW

have been completely destroyed by man and then used
for human life. Examples are agricultural areas likecrop lands and pastures, heavily trampled trails, parklands in residential areas, gardens, and so forth.
‘Semi-natural’ indicates, for example, forests fromwhich timber trees have been extracted in the past,
habitats that were cleared once in forested areas andnow support plant communities established after suchlight disturbance.
Forest-edge communities areanother good example of the ‘semi-natural’ category.
The concept ‘niche’ in the present study means thestatus or place of a species within a plant community.
Trees, shrubs, lianas (climbing plants), and herbs or
grasses are plant life-forms corresponding to niches.
In each of the life-forms, pioneer, successor and climax species are niches in a series of plant succession.
An introduced plant species can be conceived as
occupying a habitat range and a niche. For example,
Persea americana, one of the invasive tree species,
occupies a moist habitat and tree niche in the
Galápagos; Passiflora edulis occupies a moist habitat
and a liana niche.
The susceptibility of habitat to aliens depends inpart on the species poverty of the area (Cronk & Fuller,
1995), which implies vacancy of niches. As discussed in the previous chapters of the present paper,
the flora of oceanic islands is poor and disharmonic inan ecological sense, and, therefore, insular vegetation
is susceptible to aliens in its vacant habitats andvacant niches. Good examples in the Galápagos are
the moist fertile habitat with deep soils and the treeniche in it, where tree species of Scalesia have
evolved from an herbaceous ancestor to fill the habitatand niche, and where invasive trees like Cinchona
pubescens and Cedrela odorata are invading, filling
gaps in a moist habitat and a tree niche. The same istrue of species-poor vegetation on other oceanicislands (Brockie et al., 1988). Myrica faya is invading species-poor vegetation of the Hawaiian volcanoes(Brockie et al., 1988; Vitousek, 1989); Pinus
luchuensis, Casuarina equisetifolia and Bischofia
javanica are invading species-poor natural forests ofTable 2the Bonin (Ogasawara) Islands, Japan (Okutomi et al.,
1985; Shimizu & Tabata, 1985; Shimizu, 1988). The
same is true again of liana in the moist habitats of the
Galápagos, where Passiflora colinvauxii has evolved
and is climing over trunks and branches of Scalesia
pedunculata trees. This liana is the sole species in thisniche, where an invasive alien liana of Passiflora
edulis is spreading and replacing endemic P. colinvauxii, filling gaps that are still vacant in this fertile
habitat.
5.4 Invasiveness and invasive species
In order to understand invasiveness of aliens, it isimportant to understand their ecology. Invasivenessis related to traits such as seed morphology, seeddispersibility, seed dormancy, shade tolerance, droughttolerance, and others. There have been few studiesof these aspects of Galápagos aliens up to now, but astudy on Rubus niveus, one of the worst invaders,revealed traits relating to invasiveness such as
phenological patterns, size of seed bank, viability ofseeds in the seed bank, and germination patterns thatlead to high invasveness of the species (Landazuri,
Personal communication, 2002). Articles referringto introduced plants are also informative regarding
invasiveness of individual aliens (cf. Brockie et al.,
1988; Hamann, 1984, 1991; Jaramillo, 1999; Loope etal., 1988; MacDonald et al., 1988; Shimizu, 1997;
Soria et al., 2002). Table 2 indicates the invasiveness of several selected aliens, which are somewhat
subjectively evaluated, from my own observations anddescriptions in the above cited literature, into threeranks of high (+++), medium (++), and low (+) for
each of the traits and characteristics.
The present section hereafter describes some of theworst invaders in the Galápagos.
Cinchona pubescens Vahl. (Rubiaceae), or
quinine tree, is one of the worst invasive tree species
in the Galápagos. A farmer first planted this invasive
tree on one property in 1946, and naturalized plants
were found in the Santa Cruz highlands in 1971
(Hamann, 1984).
The present species bears a

Invasiveness of selected six aliens evaluated with their traits and characteristics, with a comparison to three endemics.
1
Cinchona
pubescens

Aliens234PerseaCedrela Psidium
americana odorata guajava

5
Rubus
niveus*6Lantanacamara7
Scalesia
pedunclata

Endemics
8
Psidium
galapageium

9Lantanapedunclaris

Dormancy

+++
+++
air
+

+
+
gravity
+

+++
+++
air
?

+++
++
animal
+++

+++
+++
bird
+++

++
+++
bird
+++

++
+
gravity and air
-

++
++
bird
+++

+
++
bird
+++

Germinationin shadein light

++
+++

+++
++

++
+++

++
+++

++
+++

++
+++

+++

++
++

+++

Seedling
Shade tolerance
Drought resistance

++
++

+++
++

+++
++

++
++

+++
++

+
+++

+

++
-

+++

+++

+

++

+++

+++

+++

-

+

+

+++

+

++

+++

+++

+++

+++

+

++

Total score
22
15
20
23
25
23
10
* Evaluation after Landazuri (pers. com. 2002); others are mine with comments by Tye (pers. com. 2002).

15

15

Seed
Productivity
Dispersibility

Sproutingafter cutting
Age to maturity

Vegetation of the Galápagos Islands

number of wind-dispersed seeds and has spreadwidely in the moist and highland zones of Santa Cruz.
Since it favors moist habitats and resists dry conditions as well, the plants have invaded the highlands
where the habitat fluctuates between extremely wet
and extremely dry conditions and where the tree niche
has been vacant from any indigenous tree species.
The invasion is still restricted to Santa Cruz. Figure
18 illustrates its expansion in the originally treelesshighlands.
Psidium guajava L. (Myrtaceae) or guava, is oneof the worst invasive trees, originally planted inorchards and gardens for fruit. The fruit, ca. 5 cm indiameter, harbours many small seeds inside and feral
introduced animals, like donkeys, cows, horses andgoats, carry the seeds in their digestive tract anddisperse them into semi-natural and natural vegetation.
The plant favors moist habitats and has expanded in
the moist zone of the four inhabited islands: San
Cristóbal, Floreana, Santa Cruz and the Sierra Negra
Volcano of Isabela. On the uninhabited island ofSantiago, this species was also planted but has not yet
spread. The plants on Sierra Negra Volcano were
found abundantly in and around the agricultural areaof Santo Tomás in 1970 when I visited first there, andhave expanded since then into the originally treelesshighlands, where cows and horses run wild, up to highelevations close to the caldera rim in 1998. Its
sprouting ability after cutting and burning is strong(Macdonald et al., 1988; Lawesson & Ortiz, 1990;

Fig. 18

Invasion of the treeless Santa Cruz highlands by
aggressive Cinchona pubescens. Photos (fromtop down) taken in 1970, 1991 and 2001.

53

Shimizu, 1997). On Santa Cruz, P. guajava was infrequently found in the moist zone in 1964, but abundantly in pastures in the upper moist zone (which
originally supported Zanthoxylum forest) to the highlands in 2000.
Syzygium jambos Alston, rose apple or jambos, isanother invasive species of the same family. The
species has invaded disturbed areas in the San
Cristóbal moist zone, and forms nearly monospecificthickets there, although it is not yet as serious on otherislands.
Cedrela odorata L. (Meliaceae), West Indian cedar,
was originally planted as shade and for timber alongtrails and paths in the moist zone between Bellavista
and Santa Rosa on the south side of Santa Cruz. The
tree is 30 m high and 70 cm DBH, and the wood is
hard and used for furniture. It produces wind-dispersed seeds and invades semi-natural and natural
habitats as well as disturbed areas (Lawesson & Ortiz
1990; Soria et al., 2002), in some cases, reaching
distant regions north of the central mountain range ofthe island.
Persea americana Mill. (Lauraceae), avocado, is afruit tree, originally planted around farmers’ houses in
the moist zone. It bears large fruits, 15 cm long and
10 cm across, having a large seed, 5 cm in diameter,
inside. The seed is dispersed only by gravity butrolled by wild-running animals into nearby areas.
Seedlings and saplings are highly shade-tolerant andestablish well in colonies competing with other plants.
Once established, the trees grow up to 30 m high and
70 cm in DBH, and persist many years, dispersing
seeds around the mother trees and expanding theirdistribution range gradually. Groves of this species
can be seen not only in populated agricultural areas
but also in the abandoned agricultural area of
Alemania, on the Sierra Negra Volcano of Isabela(Shimizu, 1997).
Rubus niveus Thunb. (Rosaceae) or blackberry, isa recently introduced and badly invasive shrub species.
The plant is armed with prickles on arching stems thatcan grow to 4 m long. In my fieldwork on San
Cristóbal, no plants were found in 1970 and 1978, anda few in forest-edge habitats in the transition zone in1986. The first sighting of this species was made in1983 on the same island (Lawesson & Ortiz, 1990).
Thereafter it has expanded its range up to the highest
part of the island, covering forest-edges andherbaceous vegetation. Originally the Galápagos
flora was poor in shrub species adapted to open
habitats like forest edges in the moist zone; apparently
Rubus niveus fills this vacant shrub niche in thespecies-poor, ecologically disharmonic vegetation. It
now is one of the commonest plants found along roads
and paths and along forest-edges from the transitionzone up to the highland zone of San Cristóbal. I do
not know precisely the year of its introduction to
Santa Cruz; no plants were seen in 1964, 1970, 1978
and 1986 but a few were found at Santa Rosa in 1991.

54

S. ITOW

Since then the species has spread along roads andpaths in the moist zone, and further into natural forestsof Scalesia pedunculata at Los Gemelos along theroad that crosses the island (Fig. 19). As described
previously in Table 2, its invasiveness, as evaluated bysuch traits as seed production, dispersal, seed bank,dormancy, sprouting after cutting, etc., is very high(Landazuri,
Personal
communication,2002).
Recently Soria et al. (2002) reported naturalization of
three other Rubus species, R. glauca, R. adenotricos,
and R. megalococcus on Santa Cruz. Their invasiveness is unknown but probably high.
Passiflora edulis Sims (Passifloraceae), or passion
fruit, is a perennial liana or creeping plant. A native
species of this genus, Passiflora foetida var. galapagensis Killip, is a member of the dry zone vegetation.
This is also a perennial liana, climbing trees andshrubs or creeping along open ground. P. edulis was
originally introduced into farmers’ settlements for its
delicious fruits. It is adapted to a humid climate, andthe fruit harbors many seeds that can be dispersed

Fig. 19through digestive tracts of animals. Its seedlings andsaplings are shade-tolerant. Vines elongate 10 m or
more, climbing trees and creeping along the ground.
In the Scalesia forest zone, there is an endemic liana,
Passiflora colinvauxii Wiggins, which has evolved
from an ancestral Passiflora species, adapting to themoist fertile habitat and occupying a liana niche in the
Santa Cruz moist zone. Invasive P. edulis is competing with and replacing this endemic in the Scalesia
forest. Two other introduced species of the same
genus, P. ligularis A. Juss. and P. quadrangularis L.
are similar to P. edulis in their behavior (Lawesson &Ortiz, 1990).
Table 3 gives a list of the 15 worst invasive plants
in the Galápagos, selected rather subjectively based on
my own observations and experiences since my first
visit in 1964 and thereafter. The list also describes
species traits and characteristics that are thought to berelated to invasiveness, as well as present situations of
distribution and invasion.

Ground vegetation of Scalesia pedunculata forests at Los Gemelos, invaded by aggressive Rubus
niveus (above) and Caesalpinia bonduc (below). Photos taken in 2000 at Los Gemelos, Santa
Cruz. The forest was free from those aliens in 1995.

Vegetation of the Galápagos Islands

Table 355

A list of fifteen worst invasive aliens in the Galápagos, with descriptions of present status.

TREES:
Cedrela odorata L. (Meliaceae) (See text)
Cinchona pubescens Vahl. (Rubiaceae)(See text)
Persea americana L. (Lauraceae) (See text)
Psidium guajava L. (Myrtaceae)(See text)
Syzygium jambos (L.) Alston (Myrtaceae)(See text)
Syzygium malaccense (L.) Merr. & Perry (Myrtaceae)(Frequently established tree around houses and farmlands in the moist zone of San Cristobal,
prominent due to its large tree size and red flowers)
SHRUBS:
Caesalpinia bonduc (L.) Roxb. (Leguminosae)(Troublesome creeping shrub having strong prickles on stems; large seeds not so dispersive;
covering densely cleared fields, sometimes invading into Scalesia pedunculata forest at
Los Gemelos and El Chato, Santa Cruz. The present species covers the forest floor
densely and prevents germination of Scalesia seeds)
Lantana camara L. (Verbenaceae)(Originally introduced as ornamental, produces berries dispersed by birds, spread along paths
and trails, sometimes into semi-natural and further natural vegetation, competing with endemic
L. pedunlularis in the dry and transition zones.
Ricinus communis L. (Euphorbiaceae)(Originally introduced for pharmaceutical or medicinal purposes, spreading along paths and trails
and around houses in the transition to the moist zones, and invading natural vegetation in the arid zone.)
Rubus niveus Thunb. (Rosacee)(See text.)
HERBS AND GRASSES:
Bryophyllum pinnatum (Lam.) Oken (Crassulaceae)(Originally introduced as ornamental; dispersed by minute seeds and vegetative reproduction fromfallen leaves, spreading as ruderal along paths and trails in the transition and moist zones, andinvading into natural forests, where it blankets the ground and replaces the natural herb layer.)Pennisetum purpureum Schumach (Gramineae)(Originally introduced as fodder plant; propagates by vigorous rhizome elongation as well as
wind-born seeds; spreads along roads and trails, and invades semi-natural habitat along forest
edges, but not deeply into natural forests.)
LIANAS:
Cucumis dipsaceus C.G. Ehrenb. ex Spach. (Cucurbitaceae)(Recently introduced vine; prominent due to its yellow fruits, many seeds inside, dispersedby birds, but distributed mainly along roads and paths in the dry to moist zones,
ocassionally into natural vegetation. A prominent weedy vine.)Mormordica charantia L. (Cucurbitaceae)(Widely spread in the dry and transition zones, prominent due to yellow flowers and red colorof split fruit, seeds dispersed by birds. Invasive and prominent in the arid zone.)
Passiflora edulis Sims. (Passifloraceae) and two other species. (See text.)

5.5 Distribution chart
The present-day distribution of aliens in the
Galápagos depends on some other factors as well as
the susceptibility of the habitat and invasiveness of thespecies. These factors include feral animals andrainfall in El Niño years. Herbivorous mammals likegoats and donkeys destroy natural vegetation andcreate habitats susceptible to invasion by alien plants.
A small El Niño appears every three to six years in
the Galápagos, bringing large amounts of precipitation.
Since most of the alien plants in the Galápagos
originate from moist or humid regions of the tropics
and subtropics, and favor humid weather and climates,
they can invade the mid-elevation moist zone thatoriginally supported Scalesia forests. The moist
zone is poor in tree species and therefore susceptible
to moisture-favoring invaders. Heavy rains in El
Niño years create moist habitats even in the transitionand dry zones, where invasive aliens find favorable
conditions that year and leave their seeds in the soil.
Thus aliens originating in moist regions of the conti-

nent invade and spread into species-poor insular vegetation and habitats.
To understand the present-day distributions ofaliens, time is another factor of importance. The time
elapsed since the first introduction of a species allows
its spread into agricultural and town areas, and further
to natural habitats, depending on its invasiveness.
Figure 20 is a distribution chart that consists of
two axes: vegetation zones on the Y ordinate andhabitat degradation on the X abscissa. The chartshows the present-day distribution of individual aliens
on Santa Cruz Island, integrating susceptibility ofhabitat, invasiveness of aliens and time elapsed.

6. Concluding and Prospective Remarks
Up to this chapter, the present paper does not referto the threatened status of the Galápagos endemic
flora. The matter is difficult for visitors to treat
because a thorough search of the flora throughout thearchipelago is needed. Lawesson et al. (1987) tried

56

S. ITOW

first to evaluate each of the endemic and non-endemic
native plant species in terms of the IUCN Red Data
categories, based on their intensive botanical explorations of the archipelago. Adsersen (1989) summarized 144 rare plants of individual islands, andsuggested the threats to the rare plants to be erosion,
fire, land use, introduced animals and introduced
plants. Lawesson, former botanist of the Charles
Darwin Research Station, summarized the status of 12
endemics and eight non-endemic natives as
Endangered and 12 endemics and four non-endemic
natives as Vulnerable (Lawesson, 1990). Tye, thepresent-day botanist at the CDRS, studied the matterby mapping individual endemic species distribution as
well as conducting field investigations, and concluded
that, of 195 of the endemic taxa, Three were extinct,
18 critically endangered, 30 endangered, 78
vulnerable, 16 near threatened, and 50 of least concern(Tye, 2001a). Major threats to endemic plants areferal grazing mammals like goats, donkeys and cows
on several islands like San Cristóbal, Floreana, Santa
Cruz, Santiago and Isabela, and aggressive alienplants, which disperse seeds, leave them in soils,
compete with native species with their vigrous growth,and spread themselves in natural and semi-natural
habitats.
The protection and conservation ofendemic plant species are one of the top priorities,
which should be ranked in close relation to eradicationand control of invasive alien plants, as well as of
introduced animals.
The vegetation is an organized assemblage of component plant species. In Galápagos, it is still theobject of ecological research and investigation fromthe habitat-related viewpoints of species constitution,

Fig. 20

distribution patterns, succession series, and degradation processes due to invasion by aliens, and so forth.
Up to now, the vegetation has been studied well on the
inhabited islands of San Cristóbal, Floreana, Santa
Cruz and Sierra Negra Volcano of Isabela, as reviewed
in the previous chapters, while that of the uninhabitedislands is less well-known regarding its patterns andprocesses, although Hamann (1981) described various
plant communities of remote islands and volcanoes.
Distributions of individual species, zonation patternsof vegetation and species composition of individual
communities on individual islands and volcanoes are
the major objects of vegetation ecology. The vegetation thus investigated and studied can be, and shouldbe, mapped in documents of vegetation at particularages (Itow, 1966b; Atkinson, 1990). If any of theislands, regions or areas had been thus mapped, themaps would, and will, give us an insight into vegetation changes of natural succession or man-induced
deterioration.
Another important aspect in future vegetation
studies is transect vegetation studies from the newborn western islands close to the Galápagos hotspot,
through centrally located islands, to the volcanologically older eastern islands. Such transect studies
will reveal the developmental process of ecosystems
on volcanic islands over millions of years, as
Kitayama (1996) and Kitayama et al. (1997) did in the
Hawaiian volcanoes.
The wide-ranged surveys and investigations ofvegetation as proposed above will reveal endangered
vegetation types and communities. The Zanthoxylum
fagara forest, originally present in the Santa Cruz
upper moist zone prior to 1964 (Bowman, 1961), isDistribution charts of seven aliens on Santa Cruz, in relation to vegetation zones and habitat degradation. Five endemic
counterparts, Psidium galapageium, Lantana peduncularis, Passiflora colinvauxii and P. foetida var. galapagensis, andPortulaca howellii (absent in Santa Cruz but present on nearby Isla Plaza), are given for comparison.

Vegetation of the Galápagos Islands

already gone (Werff, 1979), and fens and bogs (Itow& Weber 1974; Hamann, 1981), and other wetlandcommunities and Cyathea weatherbyana thickets are
endangered (Lawesson, 1990). The protection andconservation of particular endangered communities,
and their restoration as well, should be a high priority,
since endemic and non-endemic native species can be,
and should be, preserved in their original ecosystems,
which are really the stage of the organic evolution
theater of the Galápagos where organisms have evolved and are evolving.

Acknowledgements
During the course of my vegetation studies on remote islands and uninhabited areas since my second
visit in 1970 when my studies started, I owe in earnest
thanks for much logistic support to the Charles
Darwin Research Station. The Galápagos National
Park Service (GNPS) permitted me to perform
collecting work and export the collected material.
On field trips, many people were my companions,
without whom I would not have conducted my fieldwork well. They include Tjitte de Vries, Daniel
Weber, the late Robert Silverglied, and Jacinto
Gordillo in 1970; J. Gordillo and the late Arnaldo
Tupiza in 1978; Jonas E. Lawesson, J. Bosco Nowak
and Kunito Nehira in 1986; Dieter Mueller-Dombois
in 1987; Ondina Landazuri in 1995; Kanehiro
Kitayama in 1998; and Alan Tye, Mark Gardener,
Patricia Jaramillo, Heinke Jaeger, Yasuhiko Miya,
Kazumi Matsuoka, Iván Aldaz, O. Landazuri and J.
Gordillo since 1998 up to 2002. The late Dr. Ira L.
Wiggins, Stanford University, kindly identified specimens I collected on my field trips in 1964 and 1970,
and permitted me access to sheets that had alreadybeen approved for publication in Wiggins & Porter’s(1971) Flora of the Galápagos Islands. A. Tye andK. Kitayama gave me valuable suggestions andcomments on my studies and on a part of the draft ofthe present paper. To those people and organizations,
I send my sincere thanks.

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(Received on 7 March 2003, Accepted on 4 July 2003)