Eua zebrina 

Species Report 

May 2020

U.S. Fish and Wildlife Service

Pacific Islands Fish and Wildlife Office

Honolulu, HI
ii

Cover Photo Credits 

Eua zebrina photographs courtesy of D. Clarke and R.J. Rundell. 

Suggested Citation 

USFWS. 2020. Species Report for Eua zebrina. Version 1.1. May 2020 (Version 1.1). U.S. Fish 

and Wildlife Service, Pacific Islands Fish and Wildlife Office, Honolulu, HI. 33 pp. 

Primary Authors 

Version 1.1 of this document was prepared by Adam E. Vorsino, Fred Amidon and James 

Kwon of the Pacific Islands Fish and Wildlife Office, Honolulu, Hawaii. Preparation and review 

was conducted by Gregory Koob, Megan Laut, and Stephen E. Miller of the Pacific Islands Fish 

and Wildlife Office. 
iii

Executive Summary

This Species Report summarizes the status of Eua zebrina, a tropical tree snail in the family 

Partulidae, based on the most recent information. For E. zebrina, this is information from Cowie 

and Cook, 1999.

To assess the viability of E. zebrina, we used three conservation biology principles: resiliency, 

redundancy, and representation, or the “3Rs”. The viability of E. zebrina is evaluated with 

reference to the resiliency of its populations, its redundant occurrence across its native range, and 

its representation in its known habitat types. 

Eua zebrina is known only from the islands of Tutuila and Ofu in American Samoa. Many of the 

120+ partulid species are restricted to single islands or isolated groups of islands. The Samoan 

partulid tree snails in the genera Eua and Samoana are representative of this endemism and 

isolation. Historically, Eua zebrina was abundant on the island of Tutuila, and later reduced to its 

current rare level by the introduction of non-native snail predators.

Systematic surveys conducted in and prior to 1998 suggest an overall decline in distribution and 

abundance of Eua zebrina, with central areas of the National Park of American Samoa 

representing a large proportion of the known extant populations of the species. Though we are 

unaware of any systematic surveys conducted for E. zebrina since 1998, E. zebrina is still 

periodically observed by field biologists.

Threats to Eua zebrina include predation by non-native rats, non-native predatory snails and 

flatworms, as well as habitat destruction due to agriculture, urban expansion, or incursion by 

non-native invasive plants. Stochastic events such as cyclones (hurricanes), are an important 

contributing factor in the expansion of non-native invasive plants. The combined effects of these 

threats is expected to influence the viability of E. zebrina more than any one of the threats alone. 

Conservation efforts that may benefit E. zebrina are focused solely on the control and removal of 

invasive plants from native ecosystems. Efforts to control rats on outlying islets do not affect

known populations of E. zebrina. Due to these threats and limited conservation actions, current 

viability of the species is expected to be low, due to low resilience of its populations, low 

representation in known habitat types, and low redundancy across its geographic range. 
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Table of Contents

Cover Photo Credits........................................................................................................................ ii

Suggested Citation .......................................................................................................................... ii

Primary Authors.............................................................................................................................. ii

Executive Summary.......................................................................................................................iii

List of Tables .................................................................................................................................. v

List of Figures................................................................................................................................. v

Introduction..................................................................................................................................... 1

Regulatory History.......................................................................................................................... 1

Methodology................................................................................................................................... 1

Part 1. Life History and Historical Species Status.......................................................................... 4

Geography ................................................................................................................................... 4

Physical Environment.............................................................................................................. 5

Land-Use ................................................................................................................................. 5

Biological Environment......................................................................................................... 10

Species Description................................................................................................................... 11

Breeding Biology and Lifespan............................................................................................. 13

Habitat ................................................................................................................................... 13

Historical Range........................................................................................................................ 14

Part 2. Current Conditions and Species Status.............................................................................. 14

Current Range and Distribution ................................................................................................ 14

Factors Affecting Viability........................................................................................................ 16

Habitat Loss and Degradation ............................................................................................... 16

Other Factors Affecting Viability .................................................................................... 21

Conservation Efforts that Affect Viability ............................................................................ 23

Resiliency, Redundancy and Representation of Current Populations and the Species............. 24

Tutuila.................................................................................................................................... 24

Ofu......................................................................................................................................... 24

References..................................................................................................................................... 26
v

List of Tables

TABLE 1. CENSUS ESTIMATES OF THE AMERICAN SAMOA POPULATION ON THE ISLANDS OF TUTUILA, TA’Ū, OFU,

OLOSEGA, AND SWAINS. .......................................................................................................................................6

TABLE 2. CENSUS ESTIMATES OF THE SAMOA POPULATION BY ISLAND AND FOR THE ENTIRE COUNTRY...................6

TABLE 3. ESTIMATED ACREAGES OF MAJOR LAND COVER TYPES FOR THE SAMOAN ARCHIPELAGO. ACREAGES ARE 

IN ACRES WITH HECTARES IN PARENTHESIS.........................................................................................................9

TABLE 4. ESTIMATED ACREAGES OF VEGETATION TYPES FOR THE MAIN ISLANDS OF AMERICAN SAMOA. ACREAGES 

ARE IN ACRES WITH HECTARES IN PARENTHESIS...............................................................................................12

TABLE 5. CURRENT SPECIES STATUS OF EUA ZEBRINA IN RELATION TO THE 3 R’S. ..................................................25

List of Figures

FIGURE 1. THE THREE CONSERVATION BIOLOGY PRINCIPLES OF RESILIENCY, REDUNDANCY, AND REPRESENTATION, OR 

THE “3RS”. ............................................................................................................................................................2

FIGURE 2. THE ISLANDS OF THE SAMOA ARCHIPELAGO..................................................................................................5

FIGURE 3. MONTHLY MEAN PRECIPITATION FOR SELECTED AREAS OF THE SAMOAN ARCHIPELAGO. PACIFIC CLIMATE 

CHANGE SCIENCE PROGRAM (PCCSP; 2019)........................................................................................................7

FIGURE 4. MONTHLY MEAN TEMPERATURES FOR SELECTED AREAS OF THE SAMOAN ARCHIPELAGO. PACIFIC CLIMATE 

CHANGE SCIENCE PROGRAM (PCCSP; 2019)........................................................................................................8

FIGURE 5. CYCLONE (HURRICANE) TRACKS FOR THE CENTRAL SOUTH PACIFIC FROM 1898 TO 2019. KNAPP ET AL.

(2010) AND NOAA IBTRACS (2019). ..................................................................................................................9

FIGURE 6. LOCATIONS OF ROADS IN THE SAMOAN ARCHIPELAGO. .................................................................................9

FIGURE 7. MAP OF VEGETATION ON AMERICAN SAMOA. DATA FROM MEYER ET AL. (2017).......................................11

FIGURE 8. EUA ZEBRINA COLOR AND SHELL MORPHOLOGY. PHOTOS COURTESY OF D. CLARKE AND R. J. RUNDELL...13

FIGURE 9. MAP OF EUA ZEBRINA OBSERVATIONS AND VEGETATION ON AMERICAN SAMOA. DATA BY MEYER ET AL.

(2017)..................................................................................................................................................................15
1

Introduction

Eua zebrina is a tropical tree snail in the family Partulidae, and is endemic to the islands of 

Tutuila and Ofu in American Samoa. Snails in this family (which includes three genera: Eua, 

Partula, and Samoana) are widely distributed throughout the high islands of Polynesia, 

Melanesia, and Micronesia in the south- and west-Pacific basin (Johnson et al. 1986, pp. 161–

177; Goodacre and Wade 2001, p. 6; Lee et al. 2014, pp. 2, 6–8). Many of the roughly 120 or 

more partulid species are restricted to single islands or isolated groups of islands (Cowie 1992, p. 

169). The Samoan partulid tree snails in the genera Eua and Samoana are representative of this 

endemism.

Regulatory History

The U.S. Fish and Wildlife Service issued a final listing rule in 2016, in which the Service 

determine endangered status under the Endangered Species Act of 1973, as amended, for two 

endemic American Samoan land snails (Eua zebrina and Ostodes strigatus), the American 

Samoa distinct population segment of the friendly ground-dove, the Pacific sheath-tailed bat, 

(South Pacific subspecies) (Emballonura semicaudata semicaudata), and the mao (Gymnomyza 

samoensis). The effect of this regulation added these species to the List of Endangered and

Threatened Wildlife (U.S. Fish and Wildlife Service. 2016a). There are no additional regulatory 

documents for Eua zebrina, apart from a long list of Candidate Notice of Reviews (CNORs) 

annually from 2005 to 2014 and a proposed rule for listing in 2016. (Go to 

https://www.fws.gov/endangered/ and do a species search for Eua zebrina to get links to the 

CNORs, the proposed rule, and the final rule.)

Methodology

This Species Report is based on the best information available at this time, including peerreviewed literature, gray literature (government, academic, business, and industry reports), and 

expert elicitation. Data gaps were addressed using data available for congeners or otherwise 

similar species, as well as using basic conservation biology principles and plant and animal 

biology to identify the needs of individuals, populations, and species.

This Species Report assesses the ability of Eua zebrina to maintain viability over time. Viability 

is the ability or likelihood of the species to maintain populations over time, i.e., likelihood of 

avoiding extinction. The viability of E. zebrina, we used the three conservation biology 

principles of resiliency, redundancy, and representation, or the “3Rs” (Fig 1; USFWS 2016b, p 

12-16). The viability of E. zebrina was evaluated by describing what the snail needs to be 

resilient, redundant, and represented, followed by comparing this assessment of what is needed 

for viability to the status of the snail in its current condition or its condition based on the most 

recent information. For E. zebrina, the most recent is from Cowie and Cook, 1999.
2

Figure 1. The three conservation biology principles of resiliency, redundancy, and 

representation, or the “3Rs”.

The definitions of the “3Rs” are defined below, and are used to infer the status of the species.

• Resiliency is the capacity of a population or a species to withstand the more extreme 

limits of normal year-to-year variation in environmental conditions such as temperature 

and rainfall extremes, and unpredictable but seasonally frequent perturbations such as 

fire, flooding, and storms (i.e., environmental stochasticity). Quantitative information on 

the resiliency of a population or species is often unavailable. However, in the most 
3

general sense, a population or species that can be found within a known area over an 

extended period of time (e.g., seasons or years) is likely to be resilient to current 

environmental stochasticity. If quantitative information is available, a resilient population 

or species will show enough reproduction and recruitment to maintain or increase the 

numbers of individuals in the population or species, and possibly expand the range of 

occupancy. Thus, resiliency is positively related to population size and growth rate, and 

may also influence the connectivity among populations. 

• Redundancy is having more than one resilient population distributed across the 

landscape, thereby minimizing the risk of extinction of the species. To be effective at 

achieving redundancy, the distribution of redundant populations across the geographic 

range should exceed the area of impact of a catastrophic event that would otherwise 

overwhelm the resilient capacity of the populations of a species. In the report, 

catastrophic events are distinguished from environmental stochasticity in that they are 

relatively unpredictable and infrequent events that exceed the more extreme limits of 

normal year-to-year variation in environmental conditions (i.e., environmental 

stochasticity), and thus expose populations or species to an elevated extinction risk within 

the area of impact of the catastrophic event. Redundancy is conferred upon a species 

when the geographic range of the species exceeds the area of impact of any anticipated 

catastrophic event. In general, a wider range of habitat types, a greater geographic 

distribution, and connectivity across the geographic range will increase the redundancy of 

a species and its ability to survive a catastrophic event.

• Representation is having more than one population of a species occupying the full range 

of habitat types used by the species. Alternatively, representation can be viewed as 

maintaining the breadth of genetic diversity within and among populations, in order to 

allow the species to adapt to changing environmental conditions over time. The diversity 

of habitat types, or the breadth of the genetic diversity of a species, is strongly influenced 

by the current and historic biogeographical range of the species. Conserving this range 

should take into account historic latitudinal and longitudinal ranges, elevation gradients, 

climatic gradients, soil types, habitat types, seasonal condition, etc. Connectivity among 

populations and habitats is also an important consideration in evaluating representation.

The viability of a species is derived from the combined effects of the 3Rs. A species is 

considered viable when there are a sufficient number of self-sustaining populations (resiliency) 

distributed over a large enough area across the range of the species (redundancy) and occupying 

a range of habitats to maintain environmental and genetic diversity (redundancy) to allow the 

species to persist indefinitely when faced with annual environmental stochasticity and infrequent 

catastrophic events. Common ecological features are part of each of the 3Rs. This is especially 

true of connectivity among habitats across the range of the species. Connectivity sustains 

dispersal of individuals, which in turn greatly affects genetic diversity within and among 

populations. Connectivity also sustains access to the full range of habitats normally used by the 

species, and is essential for re-establishing occupancy of habitats following severe environmental 
4

stochasticity or catastrophic events (see Fig 1. for more examples of overlap among the 3Rs). 

Another way the three principles are inter-related is through the foundation of population 

resiliency. Resiliency is assessed at the population level, while redundancy and representation 

are assessed at the species level. Resiliency populations are the necessary foundation needed to 

attain sustained or increasing Representation and Redundancy within the species. 

The assessment of viability is not binary, in which a species is either viable or not, but rather on 

a continual scale of degrees of viability, from low to high. The health, number and distribution of 

populations were analyzed to determine the 3Rs and viability. In broad terms, the more resilient, 

represented, and redundant a species is, the more viable the species is. The current understanding 

of factors, including threats and conservation actions, will influence how the 3Rs and viability 

are interpreted for Eua zebrina.

Part 1. Life History and Historical Species Status

Geography

The Samoan archipelago consists of a remote chain of 13 islands and 2 atolls in the Pacific 

Ocean south of the equator (Figure 2). These islands extend more than 298 miles (mi) (480 

kilometers (km)), lying between 13 and 15 degrees south latitude, and 168 to 172 degrees west 

longitude (Goldin 2002, p. 4). The islands date to the early Pleistocene and were formed as hotspot shield volcanoes, with the older islands located on the western end of the chain (ThornberyEhrlich 2008), pp. 16, 28). The archipelago is divided into two political entities, American 

Samoa, an unincorporated territory of the United States, and the independent nation of Samoa 

(Craig 2009, p. 5). The Samoan archipelago lies approximately 2,600 miles (4,184 km

southwest of Hawaii (Goldin 2002, p. 4).

American Samoa consists of five high islands and two atolls:

• Tutuila (the largest island; 54 square (sq) mi (140 sq km));

• Aunuu (1 sq mi (2 sq km)) off the southeast end of Tutuila; 

• Ofu and Olosega (3.5 sq mi (9 sq km)) separated by a narrow channel now spanned by a 

bridge; 

• Ta‘ū (15 sq mi (39 sq km)); 

• Rose Atoll (1.5 sq mi (4 sq km), a National Wildlife Refuge with two uninhabited islands, 

Rose and Sand); and

• Swains Island (0.6 sq mi (1.5 sq km)), which is politically part of American Samoa, but 

geologically and biologically part of the Tokelau archipelago. 

These islands and atolls range in elevation from the high peak of Mt. Lata on Ta‘ū at 3,170 feet 

(ft) (966 meters (m)) to 4 to 6 ft (1 to 2 m) above sea level (asl) at Rose Atoll (Goldin 2002, pp. 

5-6). 
5

 

 

Figure 2. The islands of the Samoa archipelago.

Physical Environment

The Samoan archipelago lies within the tropics, where it is hot, humid, and rainy year-round. 

The wet season is from October to May, with a slightly cooler and drier season from June 

through September (Figures 3 and 4). Temperatures average about 81.5 degrees Fahrenheit (F) 

(27 degrees Celsius (C)) at lower elevations. Rainfall averages 125 inches (in) (318 centimeters 

(cm)) annually at lower elevations, but can vary greatly depending upon topography, reaching 

300 in (750 cm) or greater annually in the mountain areas. Tropical cyclones (called hurricanes

or typhoons north of the equator) are a common natural disturbance in the Samoan Archipelago 

(Figure 5), and occur at intervals of between 1 to 13 years (Goldin 2002, p. 7).

Land-Use

In 2010/2011, the population of American Samoa and Samoa was 55,519 and 187,820, 

respectively (U. S. Census Bureau 2010; Samoa Bureau of Statistics (SBS) 2011, p. 14). Ninetyeight percent of the American Samoa population occurs on the island of Tutuila (Table 1) while 

76% of the Samoa population occurs on ‘Upolu (which includes Manono and Apolima islands, 

Table 2). Census data collected since 1920 for American Samoa shows initial increases in 

populations on the Manu‘a Islands (Ta‘ū, Ofu and Olosega) and then declines by 2010 (Table 1). 

Tutuila shows a steadily increasing population from 1920 to 1980 and then some fluctuation in 

subsequent years. Samoa shows a steadily increasing population from 1951 to 2011with the 

population on Savai‘i remaining stable from 1981 to 2016 while ‘Upolu shows a steady increase 

during that period (Table 2).
6

Table 1. Census estimates of the American Samoa population on the 

islands of Tutuila, Ta’ū, Ofu, Olosega, and Swains.

Year American 

Samoa

Island

Tutuila Ta‘ū Ofu Olosega Swains Island

19201 8,056 6,185 1,155 361 355 -

19301,2 10,055 7,809 1,243 466 438 99

19401,2 12,908 10,164 1,588 500 509 147

19501,2 18,937 15,954 1,698 576 545 164

19602 20,051 17,250 1,661 605 429 106

19703 27,159 24,973 1,320 412 380 74

19803 47,283 45,524 1,138 345 249 27

19904 46,773 45,043 1,136 353 225 16

20005 57,291 55,876 873 289 216 37

20105 55,519 54,359 790 176 177 17 1

Unkown (1956) , 2 Levin and Wright (1974), 3 U. S. Census Bureau (1982), 4 U. S. 

Census Bureau (1992), 5 U. S. Census Bureau (2010)

Table 2. Census estimates of the Samoa population by 

island and for the entire country. 

Year1 Samoa Island

‘Upolu Savai‘i

1951 84,909 - -

1961 114,427 - -

1971 146,647 - -

1981 156,349 113,199 43,150

1991 161,298 116,248 45,050

2001 176,710 133,886 42,824

2011 187,820 143,418 44,402 1

All data from Samoa Bureau of Statistics (SBS) (2011 p. 14, 17).

Due to the steep topography of the islands, human habitation is primarily located along the 

coastlines (Figure 6). On American Samoa, small-scale agriculture occurs inland from villages in 

former lowland rainforest areas on slopes that sometimes exceed 45 degrees (Atkinson and 

Medeiros 2010, p. 4). Approximately 11 percent (5,456 acres (ac), 2,208 hectares (ha)) of 

American Samoa is classified as developed (i.e., urban areas, roads, infrastructure, and 6 percent

(2,824 ac, 1,143 ha) of American Samoa is classified as agriculture (Table 3). The majority of 

the development and agriculture in American Samoa occurs on Tutuila where the largest human 

population resides (Table 1 and Table 3). 

In Samoa approximately 3 percent (21,869 ac, 8,850 ha) is classified as developed and 25 

percent (174,449 acres, 70,597 hectares) is agriculture. The majority of the development and 

agriculture occurs on ‘Upolu, where the majority of the human population resides (Table 2 and 

Table 3).
7

Figure 3. Monthly mean precipitation for selected areas of the Samoan archipelago. Pacific 

Climate Change Science Program (PCCSP; 2019).
8

Figure 4. Monthly mean temperatures for selected areas of the Samoan archipelago. Pacific 

Climate Change Science Program (PCCSP; 2019).
9 

Figure 5. Cyclone (Hurricane) tracks for the central South Pacific from 1898 to 2019. Knapp et 

al. (2010) and NOAA IBTrACS (2019). 

Figure 6. Locations of roads in the Samoan archipelago.

Table 3. Estimated acreages of major land cover types for the Samoan archipelago. Acreages are 

in acres with hectares in parenthesis. 

Land cover1 

Samoa American Samoa

Savai‘i ‘Upolu Minor Islands Tutuila Ofu/

Olosega Ta‘ū

Developed 6,934 (2,806) 14,794 (5,987) 141 (57) 5,179 (2,096) 101 (41) 175 (71)

Agriculture 77,905 (31,527) 96,037 (38,865) 507 (205) 2,664 (1,078) 42 (17) 119 (48)

Forest 281,593

(113,957)

124,830

(50,517) 722 (292) 23,262 (9,414) 2,797 (1,132) 8,300 (3,359)

Scrub 29,734 (12,033) 17,957 (7,267) 171 (69) 1,564 (633) 52 (21) 1,873 (758)

Grassland/

Herbaceous 16,131 (6,528) 21,730 (8,794) 37 (15) 440 (178) 35 (14) 625 (253)

Mangrove 67 (27) 801 (324) 15 (6) 79 (32) 0 (0) 0 (0)

Wetland 306 (124) 726 (294) 67 (27) 72 (29) 7 (3) 17 (7)

Open Water 91 (37) 692 (280) 10 (4) 62 (25) 0 (0) 0 (0)

Barren 7,811 (3,161) 247 (100) 49 (20) 432 (175) 94 (38) 128 (52)

1 All data from (Meyer et al. 2017)and the Ministry of Natural Resources & Environment (MNRE) (2014). 
10

Biological Environment

Mueller-Dombois and Fosberg (1998, p. 361) describes seven general categories of vegetation in 

the Samoan archipelago: 

• Littoral vegetation generally refers to vegetation that is directly exposed to salt spray and 

occurs along the coasts. It includes a variety of forest, scrub, and herbaceous 

communities (see Whistler (1992) and Mueller-Dombois and Fosberg (1998) for detailed 

descriptions). 

• Lowland rainforests occur below approximately 1,600-2,000 ft (500-600 m) elevation 

and include a variety of subtypes based on the dominate canopy species. 

• Montane rainforests occur above 1,600-2,000 ft (500-600 m) elevation and are generally 

cooler and rainier then lowland forests. 

• Cloud forest and cloud scrub is limited to Savai‘i, ‘Upolu, Ta‘ū and Olosega and are 

wetter than montane forests due to the persistence of clouds. 

• Wetland vegetation includes mangroves, swamp forest, coastal marsh, and montane 

marsh and bog areas. Montane marsh and bog areas are limited to Savai‘i and ‘Upolu. 

• Vegetation on recent volcanic surfaces includes scrubby woody vegetation, ferns, and 

grasses that may represent pioneer rainforest. 

• Modified vegetation includes all areas that were formerly dominated by native vegetation 

but were altered due to human activity (e.g., urban areas, agriculture areas, secondary 

forest, etc.). 

Approximately 70% of American Samoa is forested with the majority classified as lowland 

tropical rainforest (Figure 7, Tables 3 and 4). Montane rainforest makes up a smaller percent of 

the forest areas and is primarily restricted to Ta‘u with a small amount on Tutuila (Figure 7, 

Table 4). (Approximately 58% of Samoa is also forested (Table 3).)

The native flora of the Samoan archipelago (plant species that were present before humans 

arrived) consisted of approximately 550 taxa, 30 percent of which were endemic (species that 

occur only in the American Samoa and Samoa) (Whistler 2002, p. 8). An additional 250 plant 

species have been intentionally or accidentally introduced and have become naturalized, with 20 

or more of these considered invasive or potentially invasive in American Samoa (Whistler 2002, 

p. 8; Space and Flynn 2000, pp. 23-24). 
11

Figure 7. Map of vegetation on American Samoa. Data from Meyer et al. (2017).

Species Description

The biology of Samoan partulid snails has not been extensively studied, but there is considerable 

information on the partulid snails of the Mariana Islands (Crampton 1925a, pp. 1–113;, Cowie 

1992, pp. 167–191; Hopper and Smith 1992, pp. 77–85) and Society Islands (Crampton 1925b, 

pp. 5–35; Crampton 1932, pp. 1–194; Murray et al. 1982, pp. 316–325; Johnson et al. 1986, pp. 

167– 177 and 319–327). The life history traits of these well-studied partulids are very similar, 

and unless otherwise stated, the biology of these Partulid snails will act as a proxy for that of the 

endangered Eua zebrina.
12

Table 4. Estimated acreages of vegetation types for the main islands of American Samoa. 

Acreages are in acres with hectares in parenthesis. 

Type1 Subtype Tutuila Aunu‘u Ofu/Olesega Ta‘ū

Littoral

Vegetation

Herbaceous 

Strand 26 (11) 22 (9) 1 (<1) 572 (231)

Littoral Forest 608 (246) 35 (14) 295 (119) 338 (137)

Littoral Scrub 117 (47) 4 (2) 9 (4) 17 (7)

Pandanus Scrub 18 (7) 0 (0) 0(0) 0 (0)

Wetland

Vegetation

Coastal Marsh 7 (3) 34 (14) 7 (3) 7 (3)

Mangrove Scrub 

& Forest 79 (32) 14 (6) 0 (0) 0 (0)

Swamp Forest 64 (26) 29 (12) 0 (0) 10 (4)

Lowland 

Rainforest - 20,507 (8,299) 0 (0) 480 (194) 761 (308)

Montane 

Rainforest - 95 (38) 0 (0) 92 (37) 2,809 (1,137)

Cloud Forest & 

Scrub - 0 (0) 0 (0) 0 (0) 546 (221)

Vegetation on 

Recent Volcanic 

Surfaces

- 223 (90) 0 (0) 0 (0) 0 (0)

Modified 

Vegetation

Fernlands 3 (1) 2 (1) 13 (5) 34 (14)

Managed Land 10,296 (4,167) 139 (56) 406 (164) 513 (208)

Secondary 

Forest 0 (0) 53 (22) 1,696 (686) 5,421 (2,194)

Secondary 

Scrub 1,214 (491) 14 (6) 37 (15) 78 (31)

Open Water - 62 (25) 11 (4) 0 (0) 0 (0)

Terrestrial

Non-vegetated - 431 (175) 18 (7) 93 (38) 129 (52)

1 Data from Meyer et al. (2017).

Appearance

Eua zebrina is tropical tree snail in the family Partulidae, and is endemic to the islands of Tutuila 

and Ofu in American Samoa. Eua zebrina varies in color, ranging from almost white to palebrown, dark brown or purplish, with or without a zebra-like pattern of flecks and lines (Cowie 

and Cook 1999 pp. 29–30). Most shells have transverse patterning (distinct coloration 

perpendicular to whorls) with a more flared aperture (i.e., tapered or wide- rimmed shell lip) than 

species of the related genus Samoana (Cowie et al. 2017)(Figure 8).

Measurements and Mass

Adult Eua zebrina shells usually vary between 0.7 and 0.8 in (18 to 21 mm) in height, and 

between 0.4 and 0.5 in (11 to 13 mm) in width (Cowie and Cook 1999 pp. 29–30) (Figure 8).
13

Life History

Snails in the family Partulidae are predominantly nocturnal arboreal herbivores that feed mainly 

on partially decayed and fresh plant material (Cowie 1992 pp. 167, 175; Murray et al. 1982, p. 

324). Though Eua zebrina is considered to be primarily herbivorous, individuals have been 

observed eating other living non-partulid snails (Cowie 1992, p. 175). 

Breeding Biology and Lifespan

Partulids are slow growing and hermaphroditic (Cowie 1992, pp. 167, 174), and sexual maturity 

is reached in about 11 months. Adult partulid snails give birth about every 20 days, producing 

approximately18 offspring per year over at least a 5 year lifespan (Cowie 1992, pp. 174, 179–

180). Eggs develop within the maternal body and hatch internally or immediately after extrusion; 

they may or may not receive nourishment directly from the parent prior to extrusion (Cowie 

1992, p. 174). Some species in the family are known to be self-fertile, but most partulids rely 

predominantly on out-crossing (Cowie 1992, pp. 167, 174).

Habitat

Partulids can have a single preferred host plant or multiple host plants, in addition to having 

preference toward anatomical parts of the plant (i.e., leaves, branch, or trunk). Habitat 

Figure 8. Eua zebrina color and shell morphology. Photos courtesy 

of D. Clarke and R. J. Rundell.
14

partitioning may occur among three partulids on Tutuila (Murray et al. 1982, pp. 317–318). Eua 

zebrina is commonly found on leaves, but is also be found on trunks and branches, as well as on 

the ground in the leaf litter (Cowie 1992, p. 175). A survey conducted by Miller (1993, p. 6) 

found all live snails on understory vegetation beneath an intact forest canopy.

Historical Range

Eua zebrina was historically known only from the island of Tutuila (Cowie and Cook 2001, p. 

49), but in 1998 a single population was found on the island of Ofu (Cowie and Cook 1999, p. 

30) (Figure 9). Until 1975, it was considered widespread and common (Cowie 2001, p. 215). 

The large number of collections (927) of this species from Tutuila between the 1920s and 1960s 

indicate this species was widely distributed and abundant; some collections included hundreds of 

specimens (Cowie and Cook 2001, p. 154). In addition, the large number of shells of this species 

used in hotel chandeliers on Tutuila also suggest that this species was historically an abundant

resource (Cowie 1993, p. 1).

Part 2. Current Conditions and Species Status

Current Range and Distribution

A review of long-term changes in the American Samoa land snail fauna based on surveys from 

1975 to 1998 and pre-1975 collections characterized 3 of 12 species as being stable in numbers, 

with the rest described as declining (Solem 1975, as cited in Cowie 2001, pp. 214–216; Miller 

1993, p. 13; Cowie 2001, p. 215). Eua zebrina was one of those land snail species. In the 1993

survey, 34 live E. zebrina were found at two of nine previously occupied sites on Tutuila; shells 

were found at four of the nine sites (Miller 1993, pp. 11–13). Twenty-three of the 34 Eua zebrina 

were seen on the offshore island of Nu‘usetoga (Miller 1993, p. 24). This small island is 

approximately 70 m high and located 100 m offshore of north-central Tutuila. It is forested with 

moderately open understory. No introduced snails were seen on the island, although rats are 

probably present, as indicated by rat-damaged shells. This population may represent an isolated 

remnant of an ancestral lineage isolated in prehistoric time when Nu‘usetoga was still connected 

to Tutuila (Miller 1993, pp. 13).

In 1998 (Cowie and Cook 1999) on Tutuila, American Samoa, E. zebrina was seen alive on 

vegetation at 80 of 186 sites (timed, untimed, and incidental sites) visited. Eua zebrina was seen 

alive for the first time (Cowie and Cook 1999, pp. 13, 22; Cowie 2001, p. 215) on Ofu on 

vegetation at 1 of 20 sites (timed, untimed, and incidental sites) visited. No E. zebrina have ever 

been recorded from Olosega or Ta‘ū. During this 1998 survey, 1,102 live Eua. zebrina were 

recorded on Tutuila, and 88 live E. zebrina were recorded on Ofu (Cowie and Cook 1999, p. 30; 

Figure 9).
15

Figure 9. Map of Eua zebrina observations and vegetation on American Samoa. Data by Meyer 

et al. (2017).

The uneven distribution of 1,102 snails at 186 locations (sites and incidental sites) on Tutuila, 

and the low number of occupied sites (two of nine previously occupied sites, ; 22.2%; Miller 

1993, pp. 11–13; and 80 of 186 surveyed sites, 43.0%; Cowie and Cook 1999, pp. 13, 22)

suggests an overall decline in distribution and abundance:

• 728 snails (66.1% of all observed snails) were in five locations (2.7% of all sites; 6.2% of 

all occupied sites): one incidental site (Vatia powerline trail; Cowie and Cook 1999, 

Appendix 7 p. 99), and three timed sites plus one incidental site all in one area (Amalau 

Valley at about 8 to 42 meters elevation; Cowie and Cook 1999, Appendix 1, p. 61, 

Appendix 7 p. 99);

• 228 snails (20.7% of all observed snails) were distributed over nine sites (4.8% of all 

sites; 11.3% of all occupied sites ): seven timed sites on five ridges, and two incidental 

sites on two ridges (72 m to 454 m elevation; Cowie and Cook 1999, Appendix 1, p. 61

and Appendix 7, p. 99);

• The remaining 146 (13.2%) observed snails were in 66 sites (35.5% of all sites; 82.5% of 
16

all occupied sites): 13 timed and 9 not-timed sites, plus 44 incidental sites, with fewer 

than 10 snails recorded at each site (Cowie and Cook 1999, Appendix 1, p. 61 and 

Appendix 7 p. 99).

On Tutuila, the sites with the highest numbers of Eua zebrina (except one site, Amalau) are 

concentrated in the central area of the National Park of American Samoa: Toa Ridge, Faiga

Ridge, and eastward along Alava Ridge to the Vatia powerline trail (Cowie and Cook 1999, 

p. 30). 

We are unaware of any systematic surveys conducted for E. zebrina since 1998; however, E. 

zebrina are still periodically observed by American Samoan field biologists (Miles 2016, in 

litt.). 

Factors Affecting Viability

Habitat Loss and Degradation

The following sections provide information on habitat loss and degradation due to agriculture 

and development, introduced feral pigs, introduced predatory snails, and introduced plants.

Habitat Destruction and Modification by Agriculture and Development

Several thousand years of subsistence agriculture and more recent plantation agriculture has 

resulted in the alteration and reduction in forest area on lower elevation arable land throughout 

American Samoa (Whistler 1994, p. 40; Mueller-Dombois and Fosberg 1998, p. 361). Fifty-five 

percent of the island of Tutuila has slopes of less than 45 percent where land-clearing for 

agriculture or development is feasible (ASCC Forestry Program 2010, p. 13; Department of 

Marine and Wildlife Resources 2006). Currently, agriculture and urban development covers 

approximately 23 percent of Tutuila and three to four percent of the Manu‘a Islands of American 

Samoa (Ofu, Olosega, and Ta‘u). 

Farmers are increasingly encroaching on steep forested areas, and agriculture on Tutuila has 

spread from low elevation plots to plots in middle and high elevation areas (ASCC 2010, p. 13).

Agricultural area on Tutuila has expanded by 59% (from 1,675 acres to 2,664 acres) since 1970, 

but decreased by 68% (from 507 acres down to 161 acres) on the Manu‘a Islands (Ofu, Olosega, 

and Ta‘u) (Pereira, 1981 p. 68; Ministry of Natural Resources & Environment (MNRE), 2014, 

Table 1). This loss of forest area likely reduces habitat resilience, and may directly contribute to 

the decline of Eua zebrina through the loss of populations of native snails. An increase in

housing is also projected to occur in some rural forests along the northern coastline of Tutuila, 

and in a few scattered areas near existing population bases with established roads (Stein et al. 

2014, p. 24). These areas are outside of known snail locations within National Park of American 

Samoa, but they do include forested habitat where snails may occur.

The development and maintenance of roads and utility corridors, and to a lesser extent, trails has 

caused habitat destruction and modification in or adjacent to populations of Eua zebrina on 

Tutuila (Cowie and Cook 1999, pp. 3, 30). Development and agriculture on Tutuila is increasing. 
17

The population and agricultural land area on this island has increased by approximately 118% 

(Table 1) and 59% (Pereira, 1981 p. 68), respectively, since 1970. Along the Alava Ridge road 

overlooking the western end of Pago Pago Harbor, and in the areas surrounding the Amalau 

inholding on the north central coast within National Park of American Samoa these factors pose 

a threat to populations of Eua zebrina (Whistler 1994, p. 41; Cowie and Cook 1999, pp. 48–49). 

The Amalau Valley area holds 4 of the 5 largest populations of Eua zebrina representing 57.6 

percent of all known individuals. In addition, construction activities, regular vehicular and foot 

trail access, and road maintenance activities cause erosion and the increased spread of nonnative 

plants, resulting in further destruction or modification of habitat (Cowie and Cook 1999, pp. 3, 

47–48). 

Available information for the Manua Islands (Olosega, Ofu and Ta‘ū) does not indicate that 

agriculture and development are a current threat to the single known population of Eua zebrina. 

The population and agricultural land area on these islands has declined by approximately 45% 

(Table 1) and 68% (Pereira, 1981 p. 68), respectively, since 1970.

Land conversion to agriculture on steeper topography at elevations above the coastal plain will 

accelerate if the human population continues to grow, or if changes in the economy shift toward 

commercial agriculture (Department of Marine and Wildlife Resources 2006, p. 71). This is 

especially true for Tutuila. 

Habitat Destruction and Modification by Nonnative Plant Species

Nonnative plant species can adversely modify native habitat and render it unsuitable for native 

snail species (Hadfield 1986, p. 325). Although some Hawaiian tree snails have been recorded on 

nonnative vegetation, it is more generally the case that native snails throughout the Pacific are 

specialized to survive only on the native plants with which they have evolved (Cowie 2001, p. 

219). Cowie (2001, p. 219) reported few observations of native snails, including Eua zebrina, in 

disturbed habitats on Tutuila. 

Nonnative plants can degrade or destroy native habitat in Pacific island environments by: (1) 

modifying light availability by altering canopy structure; (2) altering soil–water regimes; (3) 

modifying nutrient cycling; (4) converting native-dominated plant communities to nonnative 

plant communities; and (5) increasing the frequency of landslides and erosion (Smith et al. 1985, 

pp. 217–218; Cuddihy and Stone 1990, p. 74; Matson 1990, p. 245; D’Antonio and Vitousek 

1992, p. 73; Vitousek et al. 1997, pp. 6– 9; Atkinson and Medeiros 2006, p. 16). Nonnative plant 

species often exploit disturbance caused by other factors such as tropical cyclones (hurricanes), 

agriculture, development, and feral ungulates. In combination, these disturbances reinforce or 

exacerbate their negative impacts to native habitats. 

Although the island of Tutuila contain many areas that are relatively free of human disturbance 

and nonnative plant invasion and largely represent pre-contact vegetation, the threat of invasion 

and further spread by nonnative plant species is of concern (Space and Flynn 2000, pp. 23–24; 

Craig 2009, pp. 94, 96– 98; Atkinson and Medeiros 2006, p. 17; ASCC Forestry Program 2010, 

pp. 15, 20). Of the approximately 20 or more nonnative pest plant species in American Samoa, 

at least 10 (see below) have altered or may alter the habitat of Eua zebrina). The following 
18

provides a brief description of these nonnative pest plants (Atkinson and Medeiros 2006, p. 18; 

Space and Flynn 2000, pp. 23–24; Craig 2009, pp. 94, 96–98; ASCC Forestry Program 2010, p. 

15):

• Adenanthera pavonina (red bean tree, coral bean tree, lopa), native to India and Malaysia, 

is a medium-sized tree up to 50 ft (15 m) high that invades intact forests as well as 

disturbed sites, and can quickly form large stands (GISD 2006). In American Samoa, it is 

invasive in secondary forests, but also has the ability to become more widely established 

on Tutuila and the Manu‘a Islands (Space and Flynn 2000, p. 4). It is considered to have 

negative impacts on the native forests in American Samoa because the trees produce 

large quantities of seed, grow on a variety of soils, and can overtop many native trees and 

eventually form monotypic stands (Space and Flynn 2002, p. 5).

• Castilla elastica (Mexican rubber tree, pulu mamoe), native to tropical America, is a 

medium-sized tree 15 to 30 ft (5 to 10 m) high that can invade intact forest where it 

reproduces prolifically and can crowd out native species (National Park of American 

Samoa 2012, in litt.). It has displaced significant areas of lowland forest in Samoa, and is 

now considered to be an important threat to native forests in American Samoa (Atkinson 

and Medeiros 2006, p. 18).

• Cinnamomum verum (cinnamon, tinamoni), native to south Asia, is a fast-growing, 

medium-sized tree up to 30 ft (9 m) high with aromatic bark and leaves. It forms dense 

root mats that inhibit establishment of other plants, and can shade out other tree species 

and thus create monotypic stands. On Tutuila, it is actively spreading in the ridge forests 

of Mt. Matafao, Matuu, and Maloata (Space and Flynn 2000, p. 4; National Park of 

American Samoa 2012, in litt.).

• The shrub Clidemia hirta (Koster's curse), native to the New World from Mexico to 

Argentina, grows to be 6.6 ft (2 m) in height, forms a dense understory, shades out native 

plants, and prevents their regeneration (Wagner et al. 1985, p. 41; Smith et al. 1985, p. 

64). On Ta‘ū, it has become a serious problem in the unique summit scrub community 

(Whistler 1992, p. 22).

• Falcataria moluccana (albizia, tamaligi) is native to Moluccas, New Guinea, New 

Britain, and the Solomon Islands and is established in American Samoa. It is a tree that 

can reach 131 ft (40 m) in height and has a wide-spreading canopy. It grows rapidly and 

outcompetes slow-growing native trees by reducing light availability, and its abundant, 

high-nutrient litter alters soil chemistry (GISD 2008). Its shallow root system may lead to 

soil instability and landslides (Atkinson and Medeiros 2006, p. 17). 

• Funtumia elastica (African rubber tree, pulu vao), is a medium-sized tree up to 100 ft 

(30 m) tall native to tropical Africa (U.S. Department of Agriculture–Agricultural 

Research Service (USDA) 2006). This tree is invasive because of its “parachute seeds” 

that can disperse long distances and germinate in sunny or shady conditions (Whistler 

2002, p. 122). Funtumia has become a dominant subcanopy and understory tree in the 

western half of Upolu where it can form monotypic forests (Pearsall and Whistler 1991, 

p. 30). It is also established and becoming dominant on eastern Savaii (Whistler 2002, p. 
19

122). This species has the potential to become a major problem in American Samoa due 

to its proximity and the volume of traffic with Samoa (Space and Flynn 2000, p. 12).

• Leucaena leucocephala (wild tamarind, lusina, fua pepe), a shrub native to the 

neotropics, is a nitrogen-fixer and an aggressive competitor that often forms the dominant 

element of the vegetation (Geesink et al. 1999, pp. 679–680). It crowds out native 

species and re-sprouts vigorously after cutting, and seeds can remain viable for 10 to 20 

years (Craig 2009, p. 98). It is established in American Samoa.

• Merremia peltata (Merremia, fue lautetele), is indigenous to the Samoan Archipelago 

where it is a sprawling, or high-climbing vine that can invade areas following 

disturbances such as land-clearing and tropical cyclones (hurricanes). This fast-growing 

vine can smother plantation and forest trees (Craig 2009, p. 98).

• Mikania micrantha (mile-a-minute vine, fue saina), native to tropical America, is a 

scrambling or climbing herbaceous vine, that retards forest regeneration with its 

smothering growth (Whistler 1994, p. 42). This sun-loving, shade-intolerant vine is a 

major pest of plantations and forests on all major American Samoa islands (Space and 

Flynn 2000, p. 5; Craig 2009, p. 94).

• Psidium cattleianum (strawberry guava, kuava) is a tall shrub or small tree that forms 

dense stands in which few other plants can grow, displacing native vegetation through

competition. The fruit is eaten by feral pigs and birds that disperse the seeds throughout 

the forest (Smith et al. 1985, p. 200; Wagner et al. 1985, p. 24). It is thought to have 

been cultivated in American Samoa for more than 40 years and has become naturalized in 

lowland rainforest on western Tutuila.

• Spathodea campanulata (African tulip, faapasi), native to tropical Africa, is a large tree 

up to 80 ft (24 m) or more in height with showy red-orange tulip-like flowers and pods 

containing hundreds of wind-dispersed seeds (Pacific Ecosystems At Risk (PIER) 2016). 

It is established in American Samoa and is particularly invasive in low-to mid-elevation 

forests, and can spread in open agricultural land, waste areas, and intact native forest, 

forming dense stands that shade out other vegetation (GISD 2010).

Predation by Nonnative Snails

At present, the primary threat to long-term survival of the native snail fauna in American Samoa 

is predation by the nonnative rosy wolf snail (Euglandina rosea), the most commonly 

recommended biological control agent of the giant African snail (Achatina fulica), which also is 

invasive in American Samoa. Numerous studies show that the rosy wolf snail feeds on endemic 

island snails and is a causal factor in their decline and extinction (Hadfield and Mountain 1980, 

p. 357; Howarth 1983, p. 240, 1985, p. 161, 1991, p. 489; Clarke et al. 1984, pp. 101–103; 

Hadfield et al. 1993, p. 327 and pp. 616-620; Murray et al. 1982 pp. 150–153; Cowie 2001, p. 

219). 

In 1980, the rosy wolf snail was released on Tutuila to control the giant African snail (Lai and 

Nakahara 1980 as cited in Miller (1993, p. 9). By 1984, the rosy wolf snail was considered to be 

well established and widely spread dispersed at all elevations on Tutuila (Eldredge 1988, pp. 
20

122, 124–125). In 2001 Euglandina rosea was reported as widespread within the National Park 

of American Samoa on Tutuila (Cowie and Cook 2001, pp. 156–157). Live individuals have 

been observed within meters of partulids on Tutuila, including Eua zebrina and Samoana conica

(Miller 1993, p. 10). Shells of E. zebrina and S. conica were found on the ground at several of 

the locations surveyed on Tutuila, along with numerous shells and an occasional live rosy wolf 

snail (Miller 1993, pp. 13, 23–28). While there are no records of introduction of the rosy wolf 

snail to the Manu‘a Islands (Ofu, Olosega, and Ta‘ū), this species has been reported on Ta‘ū

(Miller 1993, p. 10). 

Predation by several other nonnative carnivorous snails, Gonaxis kibweziensis, Streptostele 

musaecola, and Gulella bicolor, also are a threat to Eua zebrina and other native land snails. 

Several species of Gonaxis also were widely introduced in the Pacific in attempts to control the 

giant African snail. These species have been implicated in contributing to the decline of native 

snail species in the region (Cowie and Cook 1999, p. 46). Gonaxis kibweziensis was introduced 

on Tutuila in American Samoa in 1977 (Eldredge 1988, p. 122). This species may be restricted to

Tutuila (Miller 1993, p. 9, Cowie and Cook 1999, p. 36), and based on dead shells observed on 

the ground, is not as common as the rosy wolf snail (Miller 1993, p. 11). 

Two predatory snails have been recorded in American Samoa: Streptostele musaecola (8.7 mm 

long by 2.7 mm wide) on Tutuila, Ta‘ū, and Ofu, and Gulella bicolor (6mm long by 1.8 mm 

wide) on Ofu (Cowie and Cook 1999, pp. 36–37). The potential impacts of these two species on 

the native fauna are unknown; both are much smaller than Euglandina rosea (76 mm long by 28 

mm wide) and Gonaxis kibweziensis (20 mm long by 12 mm wide), and were rarely observed 

during surveys (Cowie and Cook 1999, pp. 36–37, 46). However, Solem (1975 as cited in Miller 

1993, p. 16) speculated that S. musaecola might have a role in the further decline of native 

species.

Predation by the New Guinea or Snail- Eating Flatworm

Predation by the nonnative New Guinea or snail-eating flatworm, Platydemus manokwari, is a 

threat to Eua zebrina. The extinction of native land snails on several Pacific Islands has been 

attributed to this terrestrial flatworm, native to western New Guinea (Ohbayashi et al. 2007, p. 

483;Sugiura 2010, p. 1,499). In the 1990s, it was released in Samoa in an unsanctioned effort to 

control the giant African snail (Achatina fulica) (Cowie and Cook 1999, p. 47). In 2002, the 

flatworm was not believed to be present in American Samoa (Cowie 2001b, p. 18). However, by 

2004, it had been found on the islands of Tutuila and Ta‘ū (Craig 2009, p. 84).

Although mostly ground-dwelling, the New Guinea flatworm has also been observed climbing 

trees to feed on partulid tree snails (Hopper and Smith 1992, p. 82). It has contributed to the 

decline of native tree snails due to its ability to ascend into trees and bushes (Sugiura and 

Yamaura 2010, p. 741). Areas with flatworms usually lack partulid tree snails or have declining 

numbers of snails (Hopper and Smith 1992, p. 82). Because E. zebrina feeds on the ground as 

well as in shrubs and trees, it faces increased risk of predation by the New Guinea flatworm 

(Cooke 1928, p. 6). 
21

Predation by Rats

Rats are known to prey upon arboreal snails endemic to the Pacific islands, and they can 

devastate native snail populations (Hadfield et al. 1993, p. 621). Rat predation on tree snails has 

been observed on the Hawaiian Islands of Lāna‘i (Hobdy 1993, p. 208; Hadfield and Saufler 

2009, in litt, p. 4), Moloka‘i (Hadfield and Saufler 2009, p. 1,595), O‘ahu (Hadfield et al., 1993, 

p. 616) and Maui (Hadfield 2006, in litt.). Three species of rats are present in American Samoa: 

The Polynesian rat (Rattus exulans), probably introduced by early Polynesian colonizers, and 

Norway (R. norvegicus) and black ( R. rattus) rats, both introduced subsequent to western 

contact (Atkinson 1985, p. 38; Cowie and Cook 1999, p. 47; Department of Marine and Wildlife 

Resources 2006, p. 22). Polynesian and Norway rats are abundant in American Samoa, but 

insufficient data exist on the populations of black rats (Department of Marine and Wildife 

Resources 2006, p. 22).

Frequent evidence of predation by rats on Eua zebrina was observed at several locations on 

Tutuila (Miller 1993, pp. 13, 16). Shells of E. zebrina were damaged in a fashion that is typical 

of rat predation; the shell is missing a large piece of the body whorl or the apex (Miller 1993, p. 

13). Frequent evidence of rat predation was also observed on E. zebrina, and other native land 

snails, during subsequent surveys (Cowie and Cook 1999, p. 47). 

Other Factors Affecting Viability

Tropical Cyclones (Hurricanes)

Tropical cyclones are a common natural disturbance in the tropical Pacific and have impacted

American Samoa with varying frequency and intensity (Figure 5). Hurricanes adversely affect 

the ground snail and tree snail habitat by destroying vegetation, opening the canopy, and 

modifying light intensity and moisture, leading to the formation of disturbed areas that are open 

to invasion by nonnative plant species (Elmqvist et al. 1994, p. 387; Asner and Goldstein 1997, 

p. 148; Harrington et al. 1997, pp. 539–540; Lugo 2008, pp. 373–375, 386). These cyclonemediated changes destroy or modify habitat elements (e.g., stem, branch, and leaf surfaces, 

undisturbed ground, and leaf litter) that are essential in sustaining the snails’ life-history. In 

addition, high winds and intense rains from tropical cyclones can also dislodge individual snails 

from leaves and branches of host plants and deposit them on the forest floor where they may be 

crushed by falling vegetation or exposed to predation by nonnative rats and snails (Hadfield 

2011, pers. comm.).

Low Numbers of Individuals and Populations

Species that have experienced a decline in numbers and range reduction are inherently 

vulnerable to extinction from loss of habitat, predation, and localized catastrophes such as severe 

storms, diseases, climate change, or demographic stochasticity (Gilpin and Soule 1986, pp. 24–

34; Pimm et al. 1988, p. 757; Mangel and Tier 1994, p. 607). Conditions leading to this level of 

vulnerability are easily reached by island species, such as Eua zebrina, that are in small isolated 

populations. Small, isolated populations can exhibit reduced levels of genetic variability, which 
22

can further diminish the species’ capacity to adapt to environmental changes, thereby increasing 

the risk of inbreeding depression and reducing the probability of long-term persistence (Shaffer 

1981,p. 131; Gilpin and Soule 1986, pp. 24– 34; Pimm et al. 1988, p. 757). For Eua zebrina, 82.5 

percent of all occupied sites had fewer than 10 snails (mean occurrence = 2.2 snails per site; 

median = 2.0 snails; mode = 1 snail). The threat of small population size could seriously 

jeopardize the continued existence of Eua zebrina. 

Effects of Climate Change

There are no climate change studies that directly address impacts to the specific habitats of Eua

zebrina. The scientific assessment completed by the Pacific Science Climate Science Program 

(Australian Bureau of Meteorology (BOM) and Commonwealth Scientific and Industrial and 

Research Organization (CSIRO). 2011, Vol. 1 and Vol. 2) provides general projections or trends 

for predicted changes in climate and associated changes in ambient temperature, precipitation, 

hurricanes, and sea level rise for countries in the western tropical Pacific region including Samoa 

(used as a proxy for American Samoa).

Although there is no specific information on the impacts of the effects of climate change to Eua

zebrina, increased ambient temperature and precipitation and increased severity of hurricanes 

will exacerbate threats to this species, as well as provide additional stresses on its habitat. The 

probability of extinction are increased by climate-change impacts (Intergovernmental Panel on 

Climate Change 2007, p. 48), especially given the restricted range of E. zebrina, and the small 

number of populations. 

“The negative impact on E. zebrina caused by hurricanes was strongly suggested by surveys 

that failed to detect any snails in areas bordering agricultural plots or in forest areas that 

were severely damaged by three hurricanes (1987, 1990, and 1991).”(Miller 1993, p. 16).

Synergistic Effects

While each of the above factors negatively affect Eua zebrina, it is likely that synergistic effects 

of two or more factors will greatly increase the loss of populations, and contribute significantly 

to extinction. Climate change is an especially pervasive threat to the species, since projected 

increases in ambient temperature and storm severity will exacerbate other direct threats to E. 

zebrina in American Samoa. The combined effects of environmental, demographic, and 

catastrophic-event stressors, especially on small populations, can lead to a decline that is 

unrecoverable and results in extinction (Brook et al. 2008, pp. 457–458). The impacts of any one 

of the stressors described above might be sustained by a species with larger, more resilient 

populations, but in combination, habitat loss, predation, small-population risks, and climate 

change have the potential to rapidly affect the size, growth rate, and genetic integrity of a species 

like E. zebrina that persists as small and isolated populations. 
23

Conservation Efforts that Affect Viability

Invasive Plant Species Control

The National Park of American Samoa (NPSA) was established in 1988 to preserve and protect 

the tropical forest and archaeological and cultural resources, to maintain the habitat of flying 

foxes, to preserve the ecological balance of the Samoan tropical forest, and, consistent with the 

preservation of these resources, to provide for the enjoyment of the unique resources of the 

Samoan tropical forest by visitors from around the world ((Public Law 2006, 1988). Under a 50-

year lease agreement, beginning in 1993, between local villages, the American Samoa 

Government, and the Federal Government, approximately 8,000 ac (3,240 ha) of forested habitat 

on the islands of Tutuila, Ta‘ū, and Ofu are protected and managed (American Samoa 

Government and National Park of American Samoa (NPSA) 1993).

Several programs and partnerships to address the threat of nonnative plant species have been 

established and are ongoing in American Samoa. Since 2000, the National Park of American 

Samoa has implemented an invasive plant management program that has focused on monitoring 

and removal of nonnative plant threats. The nonnative plant species prioritized for removal 

include lopa (Adenanthera pavonina), pulu mamoe (Castilla elastica), tamaligi (Falcataria 

moluccana), lusina (Leucaena leucocephala), and strawberry guava (Psidium cattleianum)

(Togia 2015, in litt.). In particular, efforts have been focused on the removal of the tamiligi from 

within the boundaries of the National Park of American Samoa as well as in adjacent areas

(Hughes et al. 2012).

Thrips (Liothrips urichi) were introduced to American Samoa in the 1970s as a biocontrol for the 

weed Clidemia hirta (Tauili’ili and Vargo 1993, p. 59). This insect has successfully controlled 

Clidemia on Tutuila. Though Clidemia is still common and widespread throughout Tutuila, the 

thrips inhibit its growth and vigor, preventing it from achieving ecological dominance (Cook 

2001, p. 143).

In 2004, the American Samoa Invasive Species Team (ASIST) was established as an interagency 

team of nine local government and Federal agencies. The mission of ASIST is to reduce the rate 

of invasion and impact of invasive species in American Samoa with the goals of promoting 

education and awareness on invasive species and preventing, controlling, and eradicating 

invasive species. In 2010, the U.S. Forest Service conducted an invasive plant management 

workshop for Territorial and Federal agencies, and local partners (Nagle 2010 in litt.). More 

recently, the National Park of American Samoa produced a field guide of 15 invasive plants that 

the park and its partners target for early detection and response (National Park of American 

Samoa 2012, in litt.).

Predator Control

A proof of concept rat eradication project was conducted to restore habitat for the Mao 

(Gymnomyza samoensis) and other priority species by removing the predation threat of the 

Polynesian rat (R. exulans). The project was conducted on two uninhabited islands off of the 

eastern end of ‘Upolu, Samoa, both of which are not known to have E. zebrina: Nu‘utele (267 ac 

(108 ha)) and Nu‘ulua (62 ac (25 ha)) (Tye 2012, in litt). The demonstration project aimed to 
24

eradicate the Polynesian rat from both islands through aerial delivery of poison baits. Postproject monitoring detected rats on Nu‘utele, suggesting that rats survived the initial eradication 

effort or were able to recolonize the island (Tye 2012, in litt.). Though this project did not 

directly affect E. zebrina populations, it did demonstrate an approach to actively manage rats and 

help foster future predator control conservation efforts. There are no known efforts directed at 

controlling predatory snails or flatworms. 

Resiliency, Redundancy and Representation of Current Populations and the 

Species.

Tutuila

The island of Tutuila is the primary refuge for Eua zebrina in American Samoa, as it 

encompasses almost all of the remaining known individuals. The 1998 snail survey by Cowie 

and Cook (1999) found that approximately 93% of the all living E. zebrina individuals were seen 

alive on Tutuila, with the rest occurring on Ofu (Cowie and Cook 1999, pp. 13, 22; Cowie 2001, 

p. 215). The Tutuila snail populations occur in the forested habitat (approximately 2,533 ac 

(1,025 ha)) of the National Park of American Samoa, which is mostly protected from clearing for 

agriculture and development. Given this limited level of protection, the species has moderate to 

low resilience to stochastic perturbations. The geographically defined populations on Tutuila 

represent a limited subset of the known range and habitat for E. zebrina (Figure 9), which was 

once spread widely throughout Tutuila. Thus redundancy and representation are also currently 

considered to be moderate to low (Table 5).

Ofu

Very few collections have been documented from the island of Ofu in American Samoa, it is 

currently known from only 1 of 58 sites surveyed in the Manu‘a Islands (Ofu, Olosega, and 

Ta‘ū) from which 88 specimens were recorded (Cowie and Cook 1999, p. 30). The predatory 

Rosy Wolf Snail and New Guinea Flatworm are not known to be present on Ofu, and the single

population of Eua zebrina is a significant refuge for the species (Cowie 2001, p. 217). Due to 

the lack of predators and the decline in population (Table 1) and agriculture (Pereira 1981, p. 68) 

on the island of Ofu, resiliency of the population is moderate. However, very few individuals 

have been found on the island, and they have only been found from one location. Thus 

redundancy and representation are both low (Table 5). 

Though it is also anticipated that synergy among threats to Eua zebrina may result in greater 

impacts to the species than any one stressor, because the Rosy Wolf Snail and New Guinea 

Flatworm are not found on Ofu there are fewer factors on Ofu that threaten the single known 

population. If the threats in the future match that of those on Tutuila, the single extant 

population on Ofu would be projected to have little to no resiliency, redundancy or 

representation.

Given the status of these populations, based on 1998 information from Cowie and Cook (1999) 

and sporadic recent field sightings, the overall viability of Eua zebrina is low (Table 5). New 

surveys could greatly improve the information used to assess the viability of this snail.
25

Table 5. Current species status of Eua zebrina in relation to the 3 R’s.

Current Assessment of Species Viability

3 R’s1 Tutuila Ofu All Locations

Resiliency Moderate to Low Moderate Moderate

Redundancy Moderate to Low Low to None Low

Representation Moderate to Low Low to None Low

1 3 R’s (USFWS 2016b, p 12-16)
26

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