ON the NATIVE FAUNA of ALDABRA ATOLL, SEYCHELLES Ferai Cats

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THE IMPACT OF THE FERAL CAT (FELIS CATUS) ON THE NATIVE FAUNA OF ALDABRA ATOLL, SEYCHELLES

Wendy SEABROOK*

Ferai cats (Fe lis catus) have colonized and caused considerable damage to island ecosystems throughout the world (Merton, 1978 ; Croxall et al., 1984 ; Veitch, 1985). In this paper the results of an examination (1985-86) of the distribution and ecology of the ferai cat on Aldabra Atoll in the Indian Ocean (9.5" S., 46.5" E.) are summarized, and the impact of cats on native fauna is assessed. Prior to this study very little was known about the cat population. Only sightings and signs of them had been reported on the four main islands of the Atoll (Stoddart, 1971 ; Racey Nicoll, 1984). Considering Aldabra's status as a World Heritage Site and its rich endemie fauna (eg. Giant tortoise (Geochelone& gigantea) and White-throated rail ( Dryo limnas cuvieri aldabranus)), a preliminary ecological study on the cats was necessary to design an eradication or control programme. Any proposai to eliminate an alien predator from an island should be justified on biological grounds, and the likely ecological consequences must be considered (Merton, 1978), especially if, as on Aldabra, other alien predators are also present. Because of the considerable financial investment and persan-power required, eradication and control programmes must also be evaluated with reference to other mana gement objectives. Aldabra is one of the finest examples in the world of a raised coral atoll ecosystem (Stoddart Savy, 1983). The climate is semi-arid (Farrow, 1971), with a mean annual rainfall of 1 089 mm (Stoddart, 1983). Dense Pemphis acidula scrub covers most of &the atoll, except for the mangrove forests fringing the central lagoon, and mixed scrub and open vegetation (grassland, parkland, etc.) near the coast and at the far east end of Grande Terre (Fig. 1). The denseness of the vegetation, combined with the extreme ruggedness of the terrain restricts access, and probably accounted for the relatively late settlement of Aldabra in the late nineteenth century.

HISTORY

Ferai cats were reportedly introduced onto Aldabra at the end of the last century, in an effort to control the already prevalent Black rat ( Rattus rattus)

• University of Aberdeen, Dept. ofZoology, Aberdeen, Scotland. Current address : University of Sydney, School of Biological Sciences, Sydney 2006, NSW, Australia.

Rev. Eco/. {Terre Vie) , vol. 45, 1990

- 135 - 12

MANGROVE THICK SCRUB OPEN VEGETATION Ill � [] 0 km

Figure 1. - Aldabra Atoll. population (Anon., 1920 ; Stoddart, 1971). Abbott (1893) and Fryer (1911) considered cats to be confinedto Grande Terre (Fig. 1), but it is likely that a ferai population also became established on Ile Picard, where the atoll's only perma nent settlement was established. No further information is available until 1967. Between this date and 1984, incidental sightings of cats and their tracks were recorded. Stoddart (1971) summarizes these for the period 1967-69, and the more recent information was assembled by the author (Seabrook, 1987). From 1967-84 evidence of cats was found mainly on Grande Terre, although cats or their tracks, were reported on ali the four main atoll islands (Fig. 2). On Grande Terre Stoddart (1971) considered that the restricted distribution of sightings and signs near field buts and camp sites reflected the cats' actual distribution, but this may be more due to the concentration of observers activities in these areas. On Ile Polymnie only two records have been reported (one of a cat calling and the other of tracks (Alexander, H.G.L. in Racey Nicoll, 1984)), and on Ile Malabar three sightings have been made (the last in 1977, when two cats were also beard calling to each other) as weil as five& track records. No domesticated individuals have been kept on Ile Picard since 1974, but the continued presence of fe rai individuals has been confirmed by sightings of tracks.

CURRENT DISTRIBUTION

Current information was obtained from surveys of cat tracks, direct obser vations and trapping programmes (Seabrook, unpubl.). The track surveys were restricted mainly to soft substrata on the coastal areas where, with exception to the muddy margins of sorne of the inland pools, spoor impressions can best be found (Fig. 3b). On Grande Terre, cat tracks were surveyed from March 1985 to February 1986 : between Anse Eccosse (GR.061 070) - Anse Galet on an average of 3.4

- 136 - e Huis 1 Field camps 0 Sightings of cats ô Cat !racks

Figure 2. - Distribution of cat and cat track sightings 1967-85 (1967-69 from Stoddart 1971). consecutive mornings a month, and Dune Jean-Louis - Anse du Bois, 3.8 consecutive mornings per month (Fig. 3). Except for along the north shore, the remainder of the coast was surveyed in July and November 1985. The areas on the north coast of Grande Terre and coastal areas on the other atoll and lagoon islands were examined at irregular intervals, but at least fivetimes during the two years. Cat traps were set in the Cinq Cases, Takamaka (335 055), Anse Eccosse - Anse Tamarind (GR. 078 015) and Dune Jean-Louis - Anse du Bois areas. Trapping effort totalled 1 438 trap nights, and yielded 16 cats. Cat observations (230 hours plus incidental) were not systematically arranged by area as in most inland areas access is only possible along eut trails, and were mainly concentrated between Anse Ecosse - Anse Tamarind and Dune Jean-Louis - Anse du Bois. Cats ranged throughout Grande Terre and Ile Picard, but did not occur on any of the other atoll or lagoon islands. On Grande Terre, although the distribution of captures and sightings was restricted Fig. 3a), tracks were fo und extensively along the coast (Fig. 3b). Cats were also caught and/or seen during this study, or reported previously, in ali the major vegetation types (as classified by Gibson Phillipson, 1983) on the island. Cat tracks were also fo und throughout the year on the beaches between Anse Ecosse · - Anse Tamarind and Dune Jean-Louis& - Anse du Bois (Seabrook, 1989). On Ile Picard, cats appeared less abundant. None were seen, and their tracks were found infrequently even on the main beach, which extends for approximately a third of the length of the island. The single track record on one of the north coast beaches does confirm, however, that they are not restricted to the southern part of the main island, where their other tracks were found.

DIET

Limited information on the di et of the Aldabran cats was obtained from the examinati on of 52 scats and gut contents of the 16 captured individuals (Table 1).

137 - e Huts 0 Observations l:l. Cats trappcd

Figure 3. - Distribution of cat sightings & captures (a), and cat track sightings (b) during 1985/86. Thick line, areas where possible to detect tracks ; thin !ines, where tracks seen July and/orNovember surveys on the west and south coast of Grande Terre, and at other times elsewhere.

With the exception of the crabs and turtle hatchlings, the prey items consumed are commonly reported components ofthe diet offeral cats (Fitzgerald Karl, 1979 ; Garnett unpubl. ; Todd unpubl. ; Konecny, 1987). The results may be biased & towards foods exploited in the coastal environment, because ali the scats and two cats were obtained in this area (Table II). There were significant differences between the scat and gut contents of the 14 cats caught in the inland areas

(combined stomach intestine - scats G = 50.24, d.f.8, p < 0.001 ; G Test Sokal Rohlf, 1981). These may reftect differences in prey item detectability & between the gut and scat contents, but generally they appear real, with insects, & especially cockroaches, geckos, and skinks important in the inland areas, and turtle hatchlings and fish in the coastal. It is likely that many more species form part of the diet, but were not detected because of the small number of guts and scats examined.

138 TABLE 1

Percentage fr equency of occurence of prey items in the scat gut contents of fe rai cats on Aldabra. &

(S = stomach, 1 = intestine, R = rectum).

Prey items Gut contents (n = 6) Scat contents

s s + 1 R (n = 52)

Crustacea 3.8 Crabs Geograpsus spp . 12.5 12.5 1.9 Grapsus tenicrustatus• 12.5 18.8 18.8 18.8 7.7 lnsects 3.8 Orthoptera 1.9 Pternoscriptus a/dabrae 6.3 6.3 1.9 Mantodea Polyspi/ota peruginosa 18.8 B1attaria Blattaria sp . 43.8 25.0 43.8 50.0 3.8 Reptiles Gekkonidae 7.7 Phelsuma abbotti 6.3 12.5 12.5 abbotti Hemidactylus 6.3 6.3 mercatorius Scincidae Cryptob/epharus 6.3 6.3 18.8 boutonii Che1oniidae Chelonia mydas 12.5 12.5 12.5 6.3 90.4 (hatch1ings) Fish 13.5 Birds 12.5 12.5 12.5 12.5 7.7 Mammals Rattus rattus 25.0 12.5 25.0 43.8 25.0 Vegetation Tha/assia spp . 9.6

• A1so found in gut of cat killed 8/71 by J. Stevenson.

ASSESSMENT OF IMPACT

No species are known to have become extinct on Aldabra as a result of cat predation. This is surprising as, of all the species which are known to have become extinct on islands elsewhere due to introduced predators, 26 % have done so as a result of cat predation (King, 1985). More importantly, virtually all of the Aldabran fauna are potentially vulnerable to cat predation, because the same or related species are known to be eaten by cats on Aldabra and/or elsewhere (Benson Penny, 1971 ; Macfarland et al., 1974 ; Apps, 1983 ; Prys-Jones in Racey Nicoll, 1984 ; Rauzon, 1985 ; Konecny, 1987 ; Seabrook, unpubl.). Three reasons are& proposed : 1.& Sorne vulnerable species have survived in cat-free areas by having ranges

139 - TABLE Il

Comparison between the scat and gut contents of thefe ra/ cats in coasta/ and in/and areas on Aldabra.

Prey items % frequency of occurrence of prey items

Inland (n = 14) Coast (n = 52)

Stomach Rectum Scats et Intestine

Crustaceans 14.29 14.29 13.46 Insects 14.29 21.43 00.00 Cockroaches 50.00 57.14 3.85 Geckos & Skinks 28.57 21.43 7.69 Turtle hatch1ings 14.29 7.14 90.38 Birds 00.00 7.14 7.69 Rats 28.57 50.00 25.00 Fish 00.00 00.00 13.45 Seagrass 00.00 00.00 9.60

not contiguous with that of cats during the vulnerable stages of their life history ; 2. The coexistence over an evolutionary timescale of the island's fauna with a predatory land crab fa una (i.e. Coconut crab Birgus latro, and Cardisoma spp.) may have enabled the evolution of anti-predator responses that were effective also against both rats (see Atkinson, 1985) and cats ; 3. Black rats were reportedly introduced before cats (sometime prior to 1893 Atkinson, 1985), and may have caused the extinction of sorne species vulnerable to predation before any scientific record was made of their presence (eg. there appears to be no obvious reason wh y the White and Brown boobies (Sula dactylatra S. /eucogaster) and the Sooty tern ( Sterna fu s ca ta) do not breed on A1dabra, other than the lack of predator-free nesting space or the proximity of Assumption& and perhaps Cosmoledo Islands, that offer better nesting sites for these species (Diamond, 1979)). lt is worth examining the first reason in more detail. Survival of the flightless White-throated rail and possibly also the Brush warbler has seemingly been due to the incomplete colonization of the whole atoll by cats. Although there have been a few reports of cats on all the four islands, populations seem only to have become established on Grande Terre and Ile Picard. The Brush warbler, whose current status is unclear (Roberts, 1987) was formerly found on Ile Malabar, and while it is not known if it had also inhabited the cat-infested islands, the rail did. The contraction of the rails' range has been ascribed to the combined influence of cat and rat predation (Penny Diamond, 1971). However, rather than the cat « probably helping » to cause their demise (Brockie et al., 1988, p. 23), cats are probably the more influential& predator as the rails continue to survive on Ile Malabar and Polymnie, where rats are far more abundant than on Grande Terre (Whitlaw, unpubl.). The absence of all ground-nesting species from the main islands of the atoll, when much of it is suitable, is one of the most « striking features of the Aldabran

140 - sea-bird distribution » (Diamond, 1979 : 232). In addition, because guano can radically change vegetation at and around the nests and roosts of sea-bird colonies (Diamond, 1979), this has led sorne to suggest that the « elimination of these sea-birds from the larger islands, with the loss of nu trient input from guano, has had major consequences for the atoll as a whole » (Brockie et al., 1988 : 23). Diamond (1979) attributed the distribution of the ground nesting species, and sea-birds to the mangroves fringing the lagoon, to the presence of rats. However, cat predation is likely to be at least as influential. Ali the sea-bird species nesting on Aldabra are reported to be taken by cats elsewhere (Stonehouse, 1962 ; Apps, 1983 ; Korte, 1984 ; Feare, 1984 ; Rauzon, 1985 ; Konecny, 1987 ; Anderson, unpubl.), and in general larger predators are able to subdue larger prey species (Moors Atkinson, 1984). The nests oflarger birds are usually reported to escape predation by R. rattus (Atkinson, 1985). On Ascension Island, the virtual eradicati&on of the frigatebirds, boobies and tropicbirds coincided with the introduction of cats, not rats, which had been introduced over one hundred years previously (Stonehouse, 1962). Sorne of the land bird species, which are fo und as adults throughout the atoll, have ranges which do not overlap with the cat population during the more vulnerable stages of their life history (ie. nestling and pre-fledgling). Cattle egrets ( Bubulcus ibis), Dimorphic egrets ( Egretta garzetta dimorpha) and Malagasy Squacco Herons ( Ardeo/a idae) only nest on a restricted number oflagoon islets (Benson Penny, 1971), which are cat, and probably also rat-free, and the Sacred ibis (Threskiornis aethiopica abbotti) nests on low scrub in seasonally flooded pools. Only& one ground nesting bird, the Malagasy nightjar ( Caprimu/gus madagascariensis), continues to breed on the major atoll islands (Prys-Jones Diamond, 1984). The Aldabran& ferai cat population is likely to have other more subtle hnpacts. One is that cats reduce the population size of prey species, making them more vulnerable to stochastic extinctions (Gilpin Soulé, 1986). Cats have reduced prey species density elsewhere (Fitzgerald Karl, 1979), and on a New Zealand island, after the cat population was eradicated,& the densities of sorne of the land bird species subsequently increased (Veitch,& 1983). Unfortunately on Aldabra, little information is available on differences in population density of cat prey species between cat-free and cat-infested islands. The different densities of giant tortoises result at least partially from the influence of man (Gaymer, 1973), but cat predation could be the cause of the relatively lower densities of rats reported on Grande Terre in comparison to Iles Malabar, Polymnie, and Picard (Whitelaw, unpubl.). Other cat prey species may be able to compensate for }osses through increased reproductive output, or by having life history strategies which are able to withstand heavy predation }osses. Many of the Aldabran land birds compen sate for predation }osses of eggs by repeatedly re-laying (Prys-Jones Diamond, 1984), and limited information is available which suggests that cats may reduce Giant tortoise recruitment. Of 100 newly emerged tortoise hatchlings& marked on Grande Terre and Ile Malabar, one year later 19 were fo und on the latter, but only 6 on Grande Terre (Swingland Coe, 1979). Coe Swingland (1984) consider that the hatchlings and young appear more affected by high levels of predation, in particular by the Rail, Ibis, Rat,& Cardisoma spp. and& coconut crabs, than fo od availability. If predation is important, why is survival lower on Grande Terre, where the Rail is not found and rats at lower densities 1 suggest that cats are the more significant predator. ?

- 141 - CONCLUSION

The impact of fe rai cats on the Aldabran ecosystem, and threat to the survival of the last remaining flightless bird in the Indian Ocean, are of sufficient magnitude to recommend that the cat population should be eradicated or controlled. However, both of these would be very difficult to achieve. Cats have been successfully eradicated from several islands around the world (see Todd, unpubl. ; Rauzon, 1985 ; Veitch, 1985), but Little Barrier Island (New Zealand) at 3 076 ha is the largest yet successfully attempted. It took 38,880 per son days and four years to rid the island of just 150 cats (V eitch, 1985). Aldabra is much bigger and much of it inaccessible. In addition, many of the techniques employed successfully elsewhere would be unsuitable on the atoll (eg. poisonous hait leg traps). Control programmes could be used to reduce cat densities in restricted particularly sensitive areas, but on Aldabra there are really no such areas.& The population as a whole could be controlled. This has been achieved on Marion Island, using Fe line panleucopaenia (FPL) (Feline enteritis) (van Reins burg et al., 1987). FPL is a host specifie, highly contagious disease, which is resistant to environmental factors and induces high mortality in susceptible populations (van Aarde, 1984). But there are difficulties with this method (see Howell, 1984 ; Veitch, 1985 ; van Reinsburg et al., 1987), and its success in controlling the population over the long term has yet to be seen.

SUMMARY

Ferai cats ( Felis catus) have had devastating effects on island ecosystems throughout the world. The history, distribution and ecology of the ferai cat on Aldabra Atoll (a World Heritage Site) is summarized, and used with information fr om studies ferai cats elsewhere, to assess their impact on native fauna. Despite virtually ali of the Aldabran fauna being potentially vulnerable to cat of predation, no native species have (apparently) become extinct. Cats are however having an impact, the most dramatic being that ali the sea-birds and many land birds restrict their nesting to the few cat-free sites, and that the flightless White-throated rail ( Dryolimnas cuvieri aldabranus) survives on two islands where cat populations have yet to become established. These impacts are sufficient to recommend that the cat population should be eradicated, but in practice this would be impossible using the currently available techniques suitable for use on Aldabra.

RÉSUMÉ

Les chats harets (Felis catus) ont exercé une influence catastrophique sur beaucoup d'écosystèmes insulaires dans le monde. L'auteur passe en revue ce que l'on sait de l'histoire, de la distribution, et de l'écologie des chats introduits sur l'atoll d'Aldabra (un site du «Patrimoine Mondial ))), et tente d'évaluer leur impact sur la faune autochtone, compte tenu de leur action en d'autres localités. Bien que tous les animaux d'Aldabra constituent des proies potentielles pour le chat, aucune espèce indigène n'a apparemment disparu de son fait. Le chat a

142 - cependant eu un impact certain sur la faune, le plus important étant que tous les oiseaux de mer et beaucoup d'oiseaux terrestres ne nichent plus que là où il n'y a pas de chats. Par ailleurs, le Râle aptère d'Aldabra ( Dryolimnas cuvieri a/dabra nus) ne survit actuellement que sur les deux îlots où le chat n'a pas encore pénétré. On ne peut donc que recommander l'éradication des chats d'Aldabra, mais les techniques actuellement disponibles ne permettent guère de le faire, à cause des conditions spéciales de l'atoll.

ACKNOWLEDGEMENTS

The field work could not have been undertaken without the assistance of Messrs. R. Lorenque, H. Charles, A. Cedres, L. Denousse, G. de Mousac & P. Roberts and logistical support from Messrs. L. Chong Seng & L. Mole. My thanks to Drs H. Kruuk & Prys-Jones for their advice, and to Professors G.M. Dunnet & P.A. Racey for use of the facilities at the University of Aberdeen, Scotland. This research was undertaken white the author was employed by the World Wildlife Fund for Nature & the Seychelles Islands Foundation.

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145