59

Ecological Diversification of Insular Terrestrial Reptiles:
A Review of the Studies on the Lizard and Snakesof the Izu Islands
Masami HASEGAWA
Department of Biology, Faculty of Science, Toho University,
Miyama, Funabashi-shi, Chiba, 274-8150 Japane-mail: mhase@bio.sci.toho-u.ac.jp

Abstract
Terrestrial vertebrates of the Izu Islands are the subset of species indigenous or endemic to theJapanese main islands, but their species composition is extremely disharmonic both taxonomically andecologically. Mammals, amphibians and freshwater fishes are impoverished due to isolation, the smallarea, reduced habitat diversity and lack of permanent freshwater habitats, whereas birds and reptiles arerather rich in endemic species or subspecies with unique ecological characteristics, and those groups ofvertebrates maintain higher population densities particularly on the islands without mammaliancarnivores. The snake Elaphe quadrivirgata, a top predator on most of the islands, shows remarkablediversification in food habits and body size in accordance with inter-island difference in intensity ofinterspecific competition with other snake species and in availability of prey. The lizard Eumeces
okadae, a small insectivorous predator in intermediate trophic level, shows variation in life history traits
in relation to different predation pressures. The lizard populations exhibiting delayed maturity andlower reproductive rates due to less than annual reproduction exist only on the islands without snakes ormammalian carnivores. Introduction of the weasel for controlling rat pests in agricultural products almost extirpated lizard populations with little adaptation to novel predators, and subsequently caused
significant changes in entire island food web structures.
Key words: disharmonic biota, food web, island population, life history, predation by exotic carnivores

1. Introduction
Extinction is much more frequent on islands thancontinents. Most extinctions in bird, mammal andreptile populations occurring in the past 350 years
were recorded from islands (King, 1985; Groombridge,1992). Extinction is now accelerating, and continental populations are becoming threatened as theirnatural habitats have been destroyed and fragmented
to small areas. The populations in those fragmented
habitats are highly vulnerable to extinction, and theextirpation of each small population ultimately lead to
species extinction.
Why are insular populations so susceptible toextinction? The proposed reasons are that the species
exist only on a single or few islands or that populations consist of few individuals. Environmental variability and demographic stochasticity make small
populations much more susceptible to extinction thanlarger ones. Anthropogenic detrimental impacts suchas habitat destruction, fragmentation, deterioration,
over-exploitation and hunting, introduction of exotic

species and infectious diseases, together make thosesmaller island populations much more vulnerable toextinction (Primack, 1995).
Nevertheless, not all species on certain islands
would become extinct even if they were exposed tosimilar environmental changes. Some species would
be extirpated, but others would survive the disaster.
The same species living on the several similar islands
would undergo different population trends even if they
suffered identical environmental challenges. Due tostochastic variability, we cannot predict which populations will become extinct or survive, though certain
proportions are predicted to become extinct. Differences among species in susceptibility to extinction, insome cases, might be due to some deterministic
factors. Differences among species in physiological,
behavioral, ecological and life historical attributes are
the major deterministic factors responsible for interspecific differences in extinction rates.
Such deterministic factors would explain higher
extinction rates of endemic species on remote, oceanicislands, which suffer frequent introduction or inciden-

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M. HASEGAWA

tal invasion of exotic species. The followings is a
scenario explaining why certain species on islands
with disharmonic biota are vulnerable to extinction
under the threat of carnivore introduction.
1) Biotas of oceanic islands are usually disharmonic in terms of taxonomic composition, because
only the taxa that can do over-seas dispersal canbecome potential colonists.
2) Those disharmonic biotas are characterized by
the lack of large predators that require extensive homeranges to sustain their high energetic demands.
Consequently, taxa belonging to the next lower trophic
level such as insectivorous birds or lizards are able tomaintain high population densities, which in turn put
significant predation pressure on the organisms inlower trophic levels.
3) The insectivorous birds or lizards on suchoceanic islands have to live with permanent deficiency
of food and space. As an adaptation to thoseenvironmental regimes, certain behavioral and life
historical attributes such as relaxation of predator
awareness, prolongation of parental care and reduced
reproductive output will be developed.
4) Intended or incidental introduction of exotic
predators like carnivorous mammals rapidly extirpate
those native birds and lizards having little anti-predator adaptation, and food web structures change
extensively in comparison with the conditions before
introduction.
In this review, I try to show that the above-mentioned scenario is consistent with long-term changes
in vertebrate and some invertebrate' fauna of the Izu
islands, Japan. I will also discuss the evolutionary
aspects of community organization in island
environments.

all. The native population of the weasel Mustela
itatsi is restricted to Ohshima, and the other insular
populations currently established in this region are ofartificial origin. The insectivorous mammals (moles
and shrews) and rodents are also poorly represented.
The native species are the wood mouse Apodemus
speciosus, the white-toothed shrew Crocidula
dsinezumi and two species of bat (Hasegawa &Moriguchi, 1989; Takada et al., 1999; Motokawa et al.,2000). Motokawa et al. (2000) suggested a highpopulation density of C. dsinezumi on the Izu Islands,
though this species is not so common in other areas ofJapan.
Twenty-one to 38 species of birds are known tobreed in this island group. The scarcity or absence ofcommon terrestrial bird species of the mainland(raptors, pheasants and woodpeckers) characterizes
the Izu Islands avifauna (Higuchi, 1973). The greyheaded buzzard eagle (Butaster indicus) is the only
putative top predator in the insular food web, because
other common raptorial species (Aquila chrysaetos,
Spizaetus nipalensis, Pernis apivorus, Accipitergentilis, A.nisus and A.gularis) are absent (Higuchi,
1973; Hasegawa et al., 1996). There are suspected
breeding populations of the common buzzard Buteo
buteo and peregrine falcon Falco peregrinus (Higuchi,1973), but their statuses were not confirmed by arecent survey (Hasegawa et al., 1996).
The reptilian fauna is not so rich in comparison to
the mainland. The lizard Eumeces okadae is distri-

2. Disharmonic Vertebrate Fauna
The Izu archipelago is a group of volcanic islands,
which are situated linearly from north (Ohshima:34°
43’ N) to south (Aogashima:32°29’ N) over a distanceof 230 km (Fig. 1). Areas of inhabited major islands
range from ca. 3 to 90 km2. A uniformly mild climate, under the influence of the warm temperate waterof the Kuroshio Current, and abundant annual precipitation (2,000-3,000 mm) have made these islands wellvegetated with broad-leaved evergreen forests.
Terrestrial vertebrates of the Izu Islands are extremely
disharmonic both taxonomically and ecologically.
Mammals, amphibians and freshwater fishes are impoverished primarily due to isolation, the small area,
reduced habitat diversity and lack of permanent freshwater habitats.
Mammalian carnivores have very restricted distribution on the Izu Islands. Common carnivorous
mammals found on the main islands of Japan (i.e., thefox Vulpes vulpes, the raccoon dog Nyctereutes
procyonoides, the badger Meles meles, and the marten
Martes melampus) do not occur on the Izu Islands at

Fig. 1The Izu Islands, central Japan. Names of major islands
are shown in the figure.

A Review of the Studies on the Lizard and Snakes of the Izu Islands

buted on all the major islands and several small islets.
The number of the snake species varies from zero to
five by islands. Among the snakes, Elaphe quadrivirgata is a major native predator and has the widest
distribution (Hasegawa & Moriguchi, 1989).
All amphibian species currently distributed on the
Izu Islands are of recently introduced artificial origins.
No systematic survey has been conducted for the
freshwater fishes of the Izu Islands. Species now
present in several water bodies are of introduced
origins.

3. Four Types of Insular Food Webs
The simple insular vertebrate fauna facilitates theidentification of food webs (Fig. 2). From the standpoint of lizards, terrestrial food webs of the Izu islands
are classified into four types based on faunal differences in potential native predators: groups of islands
with just avian; avian and ophidian; and avian, ophidian and mammalian predators; and the fourth type,that on the small islands with rich sea bird colonies.
This sort of classification reflects potential differencesin food web structures present in the Izu islands(Hasegawa, 2001).
The first group consists of three islands(Miyakejima, Aogashima and Hachijo-kojima) thatwere originally free of snake and weasel predators.

Fig. 2

61

The absence of these predators on these islands
apparently made possible the high abundance ofE.okadae (Hasegawa, 1994) and insectivorous bird
species, especially the thrush Turdus celaenops(Higuchi, 1973). In time-constrained censuses conducted during the mating season in the late 1970s andearly 1980s, 100 or more lizards were regularly seen
per hour of survey (Hasegawa, 1994).
MaleE.okadae on those islands were heavily paracitised
during the mating season by the tick Ixodes asanumai
specialized to E.okadae (Hayashi & Hasegawa, 1984).
Lizards on Miyakejima had experienced foodshortages under high population density (Hasegawa,1997) and were presumably forced to eat otherwise
unpalatable prey such as worker ants (Hasegawa,
1994; Hasegawa & Taniguchi, 1993). The fauna ofground and soil dwelling invertebrates of this islandgroup were characterized by a greater abundance ofamphipods and lesser abundances of isopods andearthworms compared to those on other islands(Hasegawa, 1994). Abundances of ants, centipedes,
and spiders seemed to be similar at the time of thesamplings.
The second group consists of populations on
Toshima,Mikurajima,
Kozushima,
Niijima,
Shikinejima and Hachijojima, which lacked only
mammalian predators.
These islands supported
moderate densities of the lizard (Hasegawa, 1994a),

Abundances of predatory snakes, thrushes and three major prey invertebrates (earthworms, woodlice andlandhoppers) of Eumeces okadae on three representative islands. Abundances are shown in terms ofappropriate indices such as number of individuals encountered in time- or distance- constrained survey, or
amount of prey organisms captured per trap-day. Arrows in the figure indicate predation and their thickness
indicate relative frequency of predation revealed by stomach content analysis or direct observations.

62

M. HASEGAWA

and the parasitic load by Ixodes asanumai on male
E.okadae was low (<1%) on these islands (Hayashi &Hasegawa, unpublished data). The lizards were notso food-limited because their stomach contents
consisted of relatively large and palatable prey such as
lepidopteran larvae and earthworms (Hasegawa, 1994;
Hasegawa & Taniguchi, 1996). The snake El.quadrivirgata was moderately abundant and the frequencyof sighting ranged from 0.5/hr on Shikinejima to
2.8/hr on Kozushima during spring censuses in
1981-1988 (Hasegawa & Moriguchi, 1989). Densities of insectivorous birds were low on these islands(Higuchi, 1973), probably due to the high densities ofsnakes, which also preyed on birds (Hasegawa &Moriguchi, 1989; Hasegawa, unpublished data). The
raptorial bird B.indicus, a potential predator on both
E.okadae and El. quadrivirgata have breeding populations consisting of less than 10 pairs on those islands(Hasegawa et al., 1996). Long-term monitoring ofboth E.okadae and El.quadrivirgata suggested that the
prey-predator system has been stable on the second
group of the islands for over nearly two decades.
The third group consists solely of the population
on Ohshima. This island accommodated the highest
diversity of predators. Potential native predators included the weasel Mustela itatsi and five species ofsnakes (El. quadrivirgata, El. climacophora, El. conspicillata, Dinodon orientale and Agkistrodon
blomhoffii). The density of E.okadae on Ohshima
was lowest among the Izu islands, being less than six
adult males seen per hour in the spring. Lizards
experienced no parasitic load and enjoyed high preyavailability (Hasegawa, 1994). The prey-predators
system of Ohshima seemed stable, as was suggestedby the long-term stability of the E.okadae population.
Recent establishment of introduced frogs (Hasegawa,1999), however, might influence dietary compositionsof native predators.
The fourth island group consists of the relatively

Fig. 3small unpopulated islands (1< km2) that accommodate
colonies of several sea bird species (Phalacrocorax
filamentosus, Synthliboramphus wumizusume, Calonectris leucomelas, Oceanodroma tristrami, andLarus crassirostris) (Carter et al., 2002; Moyer, 1959;
Hasegawa, 1984). The islands differ in the presenceor absence of snake predators. On the islands ofJinaijima, Onbasejima, Onoharajima, and Kojine, thelizard E.okadae is only terrestrial vertebrate except
breeding land and sea birds, whereas the islands of
Udonejima, Hanshima, and Tadanaejima are inhabitedby the snake El.quadrivirgata. Because of thesmaller island size, forest vegetation is rarely
developed there, and extensive areas are covered byeither short or tall grasslands. In addition, on
Tadanaejima, dense populations of the grasshopper
Parapodisma sp. and a beetle were observed almost
every summer, and they were capable of defoliating
several bushy trees. Also observed were dense web
spider populations that consumed those insects.
Terrestrial organisms of these small islands would
certainly be under the strong influence by nutritional
input from marine food webs via the breeding
activities of the sea birds (e.g., Polis & Hurd, 1996).

4. Snake Ecology in Relation to Prey andCompetitor Fauna
Snakes are predators high in trophic position, andpredator communities in general may be regulated byprey abundance and interspecific competition(Hairstone, 1989). The snakes of the respective Izu
Islands are thus expected to vary ecologically in
relation to prey and competitor fauna (Fig. 3).
Distributions of the snake species of the Izu Islands are related to their food habits. The principal
prey of mainland El.quadrivirgata is frogs (Uchida &Imaizumi, 1939; Fukada, 1959), but is the lizards andbirds on the islands (Hasegawa & Moriguchi, 1989),

A Gigantic female Elaphe quadrivirgata from Tadanaejima reef (left) in comparison
with an average sized female from Kozushima island (right).

A Review of the Studies on the Lizard and Snakes of the Izu Islands

whereas prey items of El.climacophora are primarily
endothermic vertebrates both on the mainland (Uchida& Imaizumi, 1939; Fukada, 1959) and on the islands(Hasegawa & Moriguchi, 1989). Its flexible feeding
habits may enable El.quadrivirgata to inhabit various
islands with variable prey fauna. Conservative food
habits would limit the distribution of El.climacophora.
The other two species with the most specialized diets(Dinodon orientalis takes small reptiles, and Elaphe
conspicillata, small mammals) are confined only to asingle island (Hasegawa & Moriguchi, 1989).
The patterns of inter-island variation in body sizes
and feeding ecologies differ among the snake species.
In El. quadrivirgata, body size has diverged towardboth ‘giants’ (e.g., Tadanaejima) and ‘dwarfs’ (e.g.,
Ohshima), whereas the body size of El.climacophora
has changed little (Ohshima and Niijima) or dwarfed
slightly (Kozushima) in comparison with the mainland
populations (Fukada, 1972, 1978). The pattern ofbody size variation in El.quadrivirgata is very similar
to that reported for the Australian tiger snake Notechis
ater (Schwaner, 1985). In both species, dwarfism
occurs on islands where the snakes mainly exploit
lizards (body mass < 20 g), and gigantism occurs on
the islands with the colonies of sea birds. Dwarfism
is strongly linked to saurophagy (lizard eating), but
the underlying factors leading to saurophagy may
differ between the two island groups. Dwarfed
N. ater of Roxby I. takes skinks as its major prey,
because skinks are the only potential prey there butabundant, whereas E.quadrivirgata of Ohshima takes
E. okadae as the major prey, although these skinks are
not so abundant and seven other prey types areavailable.
The hypothesis of resource partitioning may
explain such a pattern of prey use. El.quadrivirgata
on Ohshima is a diurnally active specialist to the skink
under the presence of other snake species. By
contrast, the body size of El.quadrivirgata has became
larger on the two-species islands where it is sympatric
only with El.climacophora (i.e., Kozushima) in
accordance with a wider diet breadth. The broader
feeding niche of El.quadrivirgata on two-species
islands than a five-species island is related to acorresponding increase in their abundance.
Gigantism both in the Japanese and Australianinsular snake populations with sea bird colonies isquite interesting, but the exact mechanism may differ
between the two. El.quadrivirgata is the only snake
species on Mikurajima and Tadanaejima, and it exploits a wide array of available prey exclusively. Its
gigantic body size on the two islands may have
evolved (or been attained) directly in response to exploiting plentiful sea birds’ eggs and chicks.
Therefore, available data suggest that differencesin prey fauna are important on offshore islands ofsouthern Australia, whereas interspecific competition
plays an important role in the evolution of body sizefor the insular snake populations of the Izu Islands.

63

5. Life History and Diet of Lizards in
Relation to Predation Regime
The lizard Eumeces okadae (Figs. 4 & 5) a small
insectivorous predator at an intermediate trophic level,shows considerable intraspecific geographic variationin life history traits in relation to different predationpressures. Some E.okadae populations have a suiteof traits including small body size, early maturity,
production of small and many eggs, low frequency offailure in follicle development, annual reproduction,high relative clutch mass and low post-reproductive
mass of females. Whereas others have a suite of the
opposite character states. The populations with therichest predator fauna (Ohshima) exhibit suites of life
history traits characterized by early maturity and highfecundity, and the populations on the islands with only
avian predators (Miyakejima and Aogashima) employ
late maturity and low fecundity. The populations on
the islands where snakes are a major vertebrate predator (Shikinejima, Kozushima and Mikurajima) are
intermediate between these two extremes. The predation regime (age-specificity and intensity) has adirect role in molding the observed divergence of life
history traits. Predation impacts directly upon demographic parameters of lizard populations and indirectly upon prey availability via intraspecific competition under different population densities (Reznick& Endler, 1982). The adaptive significance of life
history traits under different predation regimes is quite
interesting, but its discussion is beyond the scope of
this review.
Differences in the intensity and age specificity of
predation affect population density (Schoener &Schoener, 1978, 1980, 1982), and explain inter-islanddifferences in the abundances of E. okadae. Highmortality of hatchlings relative to adult lizards on
Miyakejima (Hasegawa, 1990b) suggests that
accumulation of long-lived adults results in high density, not high survivorship, of juveniles. The population on Miyakejima attains a density of ca. 4,000/ha
exclusive of hatchlings (Hasegawa, 1990b). The
reverse pattern, low density with predators capable ofconsuming adult lizards, may be true on the otherislands.

6. Food Web Dynamics after Weasel
Introduction
Biological control of rats was the primary purpose
for the introduction of the weasel onto most small
islands in Japan (Shiraishi, 1982). This was the true
for most of the Izu islands. The weasel (M.itatsi)
was introduced onto the four islands (Toshima,
Miyakejima, Hachijojima, and Aogashima; Nishikata,1986), where effective native predators of rats wereabsent or present but in low density. Miyakejima andAogashima are free from snakes and native carnivorous mammals, whereas Toshima has a snake

64

M. HASEGAWA

Fig. 4

Dense basking aggregation of Eumeces okadae observed on April
1983. As many as 10 lizards can be recognized within a square meter.
Every lizard was individually marked by painting different colour
spots on its back. (Photo by H. Higuchi.)Fig. 5

Newly hatched Eumeces okadae from Kozushima with vivid blue tailand creamy white longitudinal stripes on the body.

(El.quadrivirgata) but in low density. Hachijojima
may appear exceptional, because the pit-viper
A.blomhoffii was formerly abundant (Goris, 1967).
However, it is questionable as to whether the pit-viper
was capable of controlling rats to the level at which
their agricultural damage was negligible.
Weasel predation on certain island organisms was
so dramatic that this phenomenon has been repeatedly
highlighted in a number of recent publications (e.g.,
Hasegawa, 1986, 1994, 1999; Moyer et al., 1985;
Hasegawa & Nishikata, 1991; Takagi & Higuchi,1992). In fact, introduction of the weasel caused acatastrophic decline in the endemic lizard Eumeces
okadae (Moyer et al., 1985; Hasegawa, 1986, 1994;
Hasegawa & Nishikata, 1991), as well as in endemic
avifauna such as the seven island thrush Turdus
celaenops and Ijima`s willow warbler Phylloscopus
ijimae (Higuchi, 1981; Takagi & Higuchi, 1992).
Recent frequent outbreaks of herbivorous lepidopteranlarvae on Hachijojima were attributed to the oversimplification of food web structure on this islandcaused by colonization of the weasel (Kunimi, 1982).
The density of E.okadae on Miyakejima was reduced to the level of a thousandth or ten thousandth of

65

A Review of the Studies on the Lizard and Snakes of the Izu Islands

that prior to the introduction of the weasel (Fig. 6).
In the summer of 1993, a decade after the illegal
introduction of the weasel, the average number of
E.okadae seen per hour was only 0.1, being 0.0006 of
the value obtained before weasel introduction. The
decline in the lizard population was first documented
in 1984, followed by a more rapid reduction to an
extremely low level (ca. <1.0 lizards/hr). The periodof such a catastrophic decline of the lizard coincided
well with that of a remarkable increase of the weasel.
Although an insectivorous bird, T.celaenops, actually
decreased from densities of 23.4-33.4 birds/km in
1973-1980 to 6.7-11.1 birds/km in 1990-1991 (Takagi& Higuchi, 1992), this degree of decrease is muchsmaller than that of the lizard. It is reasonable toassume that the birds in general are less vulnerable tocapture by the weasel due to their ability to escape by
flight.
In order to evaluate the weasel’s impact on the
lizard’s potential prey on Miyakejima, pitfall trapsamplings were repeated in 1993 at the same samplingsites as 1983. This time, I saw no E. okadae at all
during the sampling period. Animals trapped in thissurvey were large numbers of carrion beetles (genus
Eusilpha) that had never been seen in such a highabundance in the previous trapping bouts. During
the 1993 sampling, this beetle was also frequently
observed along the roadside. This suggests that considerable changes have occurred in the groundinvertebrate community with the possible local
extinction of lizards in the late 1980s.
Comparisons of food web elements (abundances of
detrivores, omnivores, and carnivores) showed adrastic increase in the carnivorous beetles from 1983
(0.1/trap) to 1993 (61.0/trap). Other changes included a slight increase in earthworms (from 0.2 in1983 to 1.3 in 1993), and a slight decrease in ants(from 34.8 in 1983 to 20.4 in 1993). Before resuming the pitfall trap sampling in 1993, I expected thatamphipods that had been consumed by E.okadae inlarge numbers would show a remarkable increase.
However, there was no substantial difference between
the abundance of amphipods in 1983 and 1993. The
abundance of isopods and spiders did not change
appreciably. In summary, the addition of the weasel
to an island food web as a large carnivore has led a
medium carnivore (lizard) to near extinction, small
carnivores (carnivorous beetles) to drastic increase,and one detrivore (earthworm) to a slight increase.
On the other hand, it apparently did not much affectdensities of other small carnivores (spiders), omnivores (ants) and detrivores (amphipods and isopods).

7. Conservation Implications
It is obvious that the weasel has had a serious
impact on the insular vertebrate and invertebrate fauna.
Whether the introduction of an exotic predator has
significant effects on prey animals might depend not
only on the type of the predator but also on the life
history traits and anti-predation mechanisms of prey
populations. Differences in diet preference and predation intensity among predators certainly result indifferent predation impacts on their prey populations(Winemiller & Polis, 1996).
Evolutionary differences in life history traits
among prey species may be a factor responsible fordifferential vulnerability to exotic predators. The life
history traits of E.okadae on the weasel-free islands ischaracterized by low reproductive rate (biennial reproduction, small clutches of large eggs, and delayed
maturity with a large mature size) and low natural
mortality rate (Hasegawa, 1994). This type of life
history and demographic traits seem to be particularly

Number of lizards/hr

1000

100

10
Oh-shima
1

Kozu-shima
Miyake-jima

0.1
1975

1980

1985

1990

1995

2000

2005

Year
Fig. 6Long-term changes in the abundance of Eumeces okadae on three representativeislands (Ohshima, Kozushima and Miyakejima). Abundance is the mean numberof lizards excluding hatchlings seen per man-hr of census during July, August and
September. Abundance on Miyakejima decreased sharply after an appreciable
increase of the sighting records of Mustela itatsi since 1983.

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M. HASEGAWA

vulnerable to exotic predators, because the balance
between natality and mortality can be upset due to theintroduction of exotic predators. An evolutionary
response towards such high predation intensity is acombination of life history traits leading to highnatality. A suite of traits including annual reproduction, large clutches of small eggs, and earlier maturity
with a small mature size was found in the populationof E.okadae on Ohshima where the weasel is native(Hasegawa, 1994). However, E.okadae can not adjust its life history traits to cope with a weasel population rapidly established within an ecological timescale (<10 yr), despite E.okadae’s capability of reproducing annually when its population density is experimentally reduced (Hasegawa, 1997).
Behavioral traits also vary among populations
under different predation regimes. Anti-predator responses by newly hatched E.okadae was compared
between the islands with and without snake predators(Mori & Hasegawa, 1999). The results were consistent with the hypothesis that lizards from snake
islands have evolved the ability to recognize chemicalcues from snake predators. The lizards from snakefree islands were less efficient at recognition or lacked
recognition of snake chemicals. Such a loss of predator recognition or reduced wariness against predators would explain the greater vulnerability of insular
lizards to exotic predators.
The different islands of the Izu Islands support
biological communities of differing structures andorganizations. This is not only due to different species compositions, but more importantly due todifferent life history traits, resource utilization tactics
and predator avoidance mechanisms exhibited by thecomponent species. The impact of introduced species on the recipient community varies enormously
from case to case (Case & Bolger, 1991), andDiamond and Case (1986) explained much of thisvariation is related to the extent of its prior experience
with functionally similar species. Most species inthe disharmonic insular communities have been
evolved under fewer biological relationships thancontinental counterparts. This evolutionary view ofinsular biological communities is indispensable inconsidering conservation of island biota.

Acknowledgments
I express sincere thanks to Dr. N. Kachi for giving
me an opportunity to write this review. Thanks arealso extending to numerous persons who give help,
encouragement, advise and argument to my long-term
ongoing study of the Izu island biota.

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(Received on 28 April 2003, Accepted on 4 July 2003)