Download THE IMPACT OF THE FERAL CAT (FELIS CATUS) ON THE NATIVE

THE IMPACT OF THE FERAL CAT (FELIS CATUS)
ON THE NATIVE FAUNA OF ALDABRA ATOLL, SEYCHELLES
Wendy S EABROOK *
Ferai cats (Felis catus) have colonized and caused considerable damage to
island ecosystems throughout the world (Merton, 1 978 ; Croxall et al., 1 984 ;
Veitch, 1 985). In this paper the results of an examination ( 1 985-86) 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, 1 9 7 1 ; Racey & Nicoll, 1 984).
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,
1 978), 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, 1 983). The climate is semi-arid (Farrow, 1 97 1 ), with
a mean annual rainfall of 1 089 mm (Stoddart, 1 983). 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. of Zoology, Aberdeen, Scotland. Current address : University of
Sydney, School of Biological Sciences, Sydney 2006, NSW, Australia.
Rev. Eco/. { Terre Vie) , vol. 45, 1 990
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135
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12
10
Ill
10
MANGROVE
�
THICK SCRUB
[]
OPEN VEGETATION
Figure 1 .
-
0
km
Aldabra Atoll.
population (Anon. , 1 920 ; Stoddart, 1 9 7 1 ) . Abbott ( 1 893) and Fryer ( 1 9 1 1 )
considered cats to be confined t o Grande Terre (Fig. 1 ), but i t 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 1 967. Between this date and 1 984,
incidental sightings of cats and their tracks were recorded. Stoddart ( 1 97 1 )
summarizes these for the period 1 967-69, and the more recent information was
assembled by the author (Seabrook, 1 987).
From 1 967-84 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 ( 1 9 7 1 ) 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, 1 984)), and
on Ile Malabar three sightings have been made (the last in 1 977, 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 1 974, but the
continued presence of ferai 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 1 98 5 to February
1 986 : between Anse Eccosse (GR.06 1 070) - Anse Galet on an average of 3.4
-
1 36
-
e Huis 1 Field c a m p s
0 S i gh t i n g s of c a t s
ô Cat !racks
Figure 2. - Distribution of cat and cat track sightings 1967-85 ( 1 967-69 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 1 98 5 . 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 five times during the two
years.
Cat traps were set in the Cinq Cases, Takamaka (3 35 055), Anse Eccosse Anse Tamarind (GR. 078 0 1 5) 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 found
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, 1 983) on the island. Cat tracks were also found
throughout the year on the beaches between Anse Ecosse - Anse Tamarind and
Dune Jean-Louis - Anse du Bois (Seabrook, 1 989).
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 diet of the Aldabran cats was obtained from the
examinati on of 52 scats and gut contents of the 1 6 captured individuals (Table 1).
1 37
-
e Huts
0 Observations
l:l. Cats trappcd
Figure 3 . - Distribution of cat sightings & captures (a), and cat track sightings (b) during 1 985/86.
Thick line, areas where possible to detect tracks ; thin !ines, where tracks seen July and/or November
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 of the diet of feral cats (Fitzgerald & Karl, 1 979 ;
Garnett unpubl. ; Todd unpubl. ; Konecny, 1 987). 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 1 4 cats caught in the inland areas
50.24, d.f.8, p < 0.00 1 ; G Test
(combined stomach & intestine - scats G
Sokal & Rohlf, 1 98 1 ). 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.
=
1 38
TABLE
1
Percentage frequency of occurence of prey items in the scat & gut contents offerai
cats on Aldabra.
(S
=
stomach, 1
Prey items
=
intestine, R
Gut contents (n
s
Crustacea
Crabs
Geograpsus spp.
Grapsus tenicrustatus•
lnsects
Orthoptera
Pternoscriptus a/dabrae
Mantodea
Polyspi/ota peruginosa
B1attaria
Blattaria sp.
Reptiles
Gekkonidae
Phelsuma abbotti
abbotti
Hemidactylus
mercatorius
Scincidae
Cryptob/epharus
boutonii
Che1oniidae
Chelonia mydas
(hatch1ings)
Fish
Birds
Mammals
Rattus rattus
Vegetation
Tha/assia spp.
=
rectum).
=
Scat contents
6)
s + 1
R
(n
=
52)
3.8
12.5
12.5
12.5
18.8
18.8
6.3
18.8
6.3
1 .9
7.7
3.8
1 .9
1 .9
18.8
43.8
25.0
43 .8
50.0
3.8
7.7
6.3
12.5
12.5
6.3
6.3
6.3
6.3
1 8.8
12.5
12.5
12.5
6.3
90.4
1 2. 5
1 2.5
12.5
12.5
13.5
7.7
25.0
1 2.5
25.0
43.8
25.0
9.6
• A1so found in gut of cat killed 8/7 1 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, 1 985). 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, 1 97 1 ; Macfarland et al., 1 974 ; Apps, 1 983 ; Prys-Jones in
Racey & Nicoll, 1 984 ; Rauzon, 1 985 ; Konecny, 1 987 ; Seabrook, unpubl.). Three
reasons are proposed :
1 . Sorne vulnerable species have survived in cat-free areas by having ranges
1 39
-
TABLE
Il
Comparison between the scat and gut contents of the fera/ cats in coasta/ and in/and
areas on Aldabra.
Prey items
% frequency of occurrence of prey items
Inland (n
Crustaceans
Insects
Cockroaches
Geckos & Skinks
Turtle hatch1ings
Birds
Rats
Fish
Seagrass
=
1 4)
Coast (n
=
Stomach
et Intestine
Rectum
Scats
14.29
14.29
50.00
28.57
14.29
00.00
28.57
00.00
00.00
14.29
2 1 .43
57. 14
2 1 .43
7.14
7.14
50.00
00.00
00.00
1 3 .46
00.00
3.85
7.69
90. 3 8
7.69
25.00
1 3 .45
9.60
52)
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 fauna (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, 1 985) and cats ;
3. Black rats were reportedly introduced before cats (sometime prior to 1 893
Atkinson, 1 98 5), 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 why the White and Brown boobies (Sula
dactylatra & S. /eucogaster) and the Sooty tern ( Sterna fus cata) 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, 1 979)).
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, 1 987) 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, 1 97 1 ). However, rather than the cat
« probably helping » to cause their demise (Brockie et al., 1 988, 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
1 40
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sea-bird distribution » (Diamond, 1 979 : 232). In addition, because guano can
radically change vegetation at and around the nests and roosts of sea-bird
colonies (Diamond, 1 979), this has led sorne to suggest that the « elimination of
these sea-birds from the larger islands, with the loss of nutrient input from guano,
has had major consequences for the atoll as a whole » (Brockie et al., 1 988 : 23).
Diamond ( 1 979) 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, 1 962 ; Apps,
1 98 3 ; Korte, 1 984 ; Feare, 1 984 ; Rauzon, 1 985 ; Konecny, 1 987 ; Anderson,
unpubl.), and in general larger predators are able to subdue larger prey species
(Moors & Atkinson, 1 984). The nests of larger birds are usually reported to escape
predation by R. rattus (Atkinson, 1 985). On Ascension Island, the virtual
eradication of the frigatebirds, boobies and tropicbirds coincided with the
introduction of cats, not rats, which had been introduced over one hundred years
previously (Stonehouse, 1 962) . Sorne of the land bird species, which are found 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 of lagoon islets (Benson & Penny, 1 97 1 ), 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, 1 984).
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, 1 986). Cats have
reduced prey species density elsewhere (Fitzgerald & Karl, 1 979), and on a New
Zealand island, after the cat population was eradicated, the densities of sorne of
the land bird species subsequently increased (Veitch, 1 983).
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, 1 973), 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,
1 984), and limited information is available which suggests that cats may reduce
Giant tortoise recruitment. Of 1 00 newly emerged tortoise hatchlings marked on
Grande Terre and Ile Malabar, one year later 1 9 were found on the latter, but only
6 on Grande Terre (Swingland & Coe, 1 979). Coe & Swingland ( 1 984) 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 food
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 ferai 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, 1 985 ; Veitch, 1 985), 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 1 50 cats (Veitch, 1 985). 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 Feline panleucopaenia (FPL) (Feline enteritis) (van Reins­
burg et al., 1 987). FPL is a host specifie, highly contagious disease, which is
resistant to environmental factors and induces high mortality in susceptible
populations (van Aarde, 1 984). But there are difficulties with this method (see
Howell, 1 984 ; Veitch, 1 985 ; van Reinsburg et al., 1 987), 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
from studies of ferai cats elsewhere, to assess their impact on native fauna.
Despite virtually ali of the Aldabran fauna being potentially vulnerable to cat
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.
R É SUM É
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
1 42
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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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