A COMPARISON OF THE DIETS OF FERAL CATS FELIS CATUS AND RED FOXES VULPES VULPES ON PHILLIP ISLAND, VICTORIA ROGER KIRKWOOD, PETER DANN AND MARIA BELVEDERE THE introduction of feral cats (Felis catus) and red foxes (Vulpes vulpes) to Australia in the 1800s had a profound impact on resident ecosystems. Both predators colonised successfully and now are distributed across most of mainland Australia (Saunders et al. 1995; Abbott 2002). They consume mainly ground-dwelling mammals (Coman 1973; Croft and Hone 1978; Jones and Coman 1981; Lapidge and Henshall 2002; Hutchings 2003), but where these are scarce, birds, reptiles, insects and human refuse may become important dietary components (e.g., Bubela et al. 1998; Paltridge 2002). Although they prey on similar species, when compared at the same location differences in diet between the predators are evident (Triggs et al. 1984; Catling 1988; Risbey et al. 1999). Populations of both F. catus and V. vulpes are present on Phillip Island, a 10,000 ha island in southern Victoria (38 15' S, 145 30' E). F. catus probably arrived with European settlers in the early 1800s and a feral population may have established at that time. As the island became increasingly urbanised in the 1900s, feral cat numbers undoubtedly increased. V. vulpes were first introduced in about 1905 (Glidden 1968). In Australia, Phillip Island represents an unusual environment for these predators as there is a nearabsence of small, native, ground-dwelling mammals. On the island, potential prey include introduced mammals - European rabbit (Oryctolagus cuniculus), brown hare (Lepus capensis), black rat (Rattus rattus), house mouse (Mus musculus) and sheep (Ovis aries), as well as some native species - eastern water rat (Hydromys chrysogaster), short-beaked echidna (Tachyglossus aculeatus) and swamp wallaby (Wallabia bicolor) (Andrew et al. 1984). Also, six species of skink, numerous terrestrial and water birds and burrow-nesting seabirds shorttailed shearwaters (Puffinus tenuirostris), > 1 million breeding birds (Harris and Bode 1981) and little penguins (Eudyptula minor), ~26,000 breeding birds (P. Dann, unpubl. data). Within 20 years of their introduction, V. vulpes were recognised as a threat to the native seabirds (Gabriel 1919). Mainly due to V. vulpes predation, the number of E. minor colonies on the island gradually reduced and by ~1980, the penguins were largely confined to one colony at the western end of the island (Dann 1992). Declines in numbers at this colony continued into the 1980s when a feral predator control program was instigated (Dann 1992). The main dietary component of V. vulpes is P. tenuirostris, a migratory seabird that is resident between late September and early May, and O. cuniculus (Norman 1971; Kirkwood et al. 2000). The diet of feral F. catus has not been reported previously. In a study of the impact of F. catus on shearwaters on Phillip Island, however, Lanyon (2000) proposed that over a 50 year period, even low levels of predation could extinguish small shearwater colonies. Here we compare the contemporaneous diets of F. catus and V. vulpes on Phillip Island to assess the relative impacts of these predators. Felis catus and V. vulpes were collected between 1983 and 1994 using a variety of techniques, including cage trapping, snaring and shooting aided by spotlights (night hunts). The principal aim of the predator control program was to protect penguins at the western end of the island. Consequently, all F. catus and V. vulpes were caught within 5 km of the western end which comprised mostly open grazing and grassland, coastal reserve and a small (< 100 house) urban development. Kirkwood R, Dann P and Belvedere M, 2005. A comparison of the diets of feral cats Felis catus and red foxes Vulpes vulpes on Phillip Island, Victoria. Australian Mammalogy 27: 89-93. Key words: Oryctolagus cuniculus, European rabbit, Puffinus tenuirostris, short-tailed shearwater, Eudyptula minor, little penguin, diet. R Kirkwood and P Dann, Phillip Island Nature Park, PO Box 97, Cowes, Vic. 3922, Australia. Email: rkirkwood@penguins.org.au. M Belvedere, 18 Stradbroke Road, Boronia, Vic. 3155, Australia. Manuscript received 10 November 2003; accepted 10 August 2004.
90 AUSTRALIAN MAMMALOGY Within 12 h of being killed, the animal s stomach was opened and the contents were weighed. The contents were rinsed over a 2-mm sieve until the water ran clear, then sorted macroscopically. Recognisable food items were recorded to provide a frequency of occurrence (FOO). Vertebrate prey were identified to species (mammals by their hair following the technique of Brunner and Coman 1974, and birds by their feathers) based on microscopic comparisons with a reference collection. Insects were identified to families and the presence of plant material was noted. To assess differences in diet composition between the predators, we applied χ 2 tests with Yates correction to the FOO data (Fowler et al. 1998). Significance was taken at the P < 0.05 level. As the FOO technique may exaggerate the contribution of small dietary components (Corbett 1989), we also assessed the by-mass contributions. Components in each stomach were weighed to the nearest gram. Only stomachs that contained > 20 g of contents were included, to avoid the possibility of differential degradation rates (e.g., slow digestion of grass) biasing the analysis. The prey masses were converted to proportions per stomach so that each individual contributed equally to the predicted diet. Diet overlap between the predators was assessed using a modified percentage similarity index (%PSI; Schoener 1970; Goldsworthy et al. 2003). In total, 277 F. catus were collected, 78% (217) on night hunts, 14% in cage-traps, 4% in snares and 2% road-killed. The method of kill was not recorded for the remaining 2%. Dietary components were recognised in 239 stomachs. The most commonly occurring components were: plant fragments, O. cuniculus, M. musculus, R. rattus and P. tenuirostris (Table 1). Greater than 20 g of recognisable food matter was taken from the stomachs of 166 F. catus (60%). Based on mass, principal components of the diet were: O. cuniculus 37%, R. rattus 22%, M. musculus 16%, P. tenuirostris 12% and E. minor 4% (Table 1). We differentiated the diet into two periods: when P. tenuirostris were present (20 September to 19 May) and when they were absent (20 May to 19 September). When P. tenuirostris were present, they represented 17% of the diet, and when absent, higher proportions of rats and E. minor were eaten (Fig. 1). A total of 147 V. vulpes were killed, 94% (138) during night hunts, 5% in cage traps, one in a snare and one was road-killed. The most commonly occurring dietary components in the 115 stomachs that contained recognisable food were P. tenuirostris, O. cuniculus, plant fragments, insects and M. musculus (Table 1). Greater than 20 g of food matter was taken from 109 (74%) stomachs. Based on mass, principal components of the diet were: P. tenuirostris 34%, O. cuniculus 21%, E. minor 13%, M. musculus 10%, O. aries 7% and insects 5% (Table 1). When P. tenuirostris were present, they represented 48% of the diet, when absent, higher proportions of O. cuniculus, O. aries and E. minor were eaten (Fig. 1). Interestingly, when live P. tenuirostris were absent from the island, they were still eaten (also by F. catus, Fig. 1). This probably indicates consumption of carcasses cached by V. vulpes (see Kirkwood et al. 2000). 100 80 60 % 40 20 0 Cat Shearwaters Present Absent N = 98 N = 67 Sheep Mouse Rat Rabbit 100 Fig. 1. Principal diet components of F. catus and V. vulpes on Phillip Island, Victoria, based on the contribution by mass to stomach contents (includes only stomachs that contained > 20 g). Diets are distinguished between when live shearwaters were present on the island (20 Sept - 19 May) and when they were absent (20 May - 19 Sept). Comparing the diets based on FOO, F. catus ate a greater range of prey and more rats, reptiles and plant matter, while V. vulpes ate more P. tenuirostris, O. aries and insects (Table 1). Based on mass, both predators fed to varying degrees on introduced mammals (O. cuniculus, M. musculus, R. rattus and O. aries) and burrow-nesting seabirds (P. tenuirostris and E. minor), with other foods being minor components in the diets. F. catus ate more introduced mammals than seabirds (77% of diet by mass c.f. 16%) whereas V. vulpes ate more seabirds than introduced mammals (47% c.f. 41%). The derived PSI between the predators diets was 56%, 80 60 40 20 0 Other bird Shearwater Penguin Fox Shearwaters Present Absent N = 74 N = 35 Mammal Bird Other Refuse Plant Insect Reptile & Amphibian 90
KIRKWOOD ET AL.: CAT & FOX DIET ON PHILLIP ISLAND 91 Name Species F. catus V. vulpes Difference in FOO FOO % FOO % χ 2 P N total 277 147 N with contents 239 166 115 109 European rabbit Oryctolagus cuniculus 84 37 41 21 0.021 0.926 house mouse Mus musculus 76 16 27 10 2.680 0.106 black rat Rattus rattus 57 22 10 4 11.855 0.001 sheep Ovis aries 4 2 12 7 35.994 0.000 goat Capra hircus 1 1 dog Canis (lupus) familiaris 1 <1 short-tailed shearwater Puffinus tenuirostris 49 12 58 34 32.803 0.000 little penguin Eudyptula minor 25 4 20 13 3.227 0.067 rock dove Columba livia 2 <1 barn owl Tyto alba 1 <1 chestnut teal Anas castanea 1 <1 New Holland honeyeater Phylidonyris novaehollandiae 1 <1 masked lapwing Vanellus miles 1 1 Australian magpie Gymnorhina tibicen 1 1 Aves unid 14 1 10 2 2.236* 0.122 grass skink Pseudemoia entrecasteauxii 7 <1 metallic skink Niveoscincus metallicus 5 <1 eastern three-lined skink Bassiana duperreyi 2 <1 blotched blue-tongue Tiliqua nigrolutea 1 <1 reptilia unid 6 3 brown tree frog Litoria ewingii 2 <1 common froglet Ranidella signifera 2 <1 banjo frog Limnodynastes dumerilii 1 <1 insect 20 1 28 5 16.594 0.000 plant 119 3 35 2 11.884 0.001 table scraps 1 <1 1 1 cat food 4 2 Mammalia 176 77 79 41 0.920 0.332 Aves 86 18 83 51 40.750 0.000 Reptilia 20 1 10.748 0.001 Amphibia 2 <1 Table 1. Assessment of the diets of feral cats (Felis catus) and foxes (Vulpes vulpes) on Phillip Island, Victoria, based on the frequency of occurrence (FOO) and the percentage contribution by mass in stomachs of individuals killed between 1983 and 1994. χ 2 tests assessed the degree of difference in FOO. * This χ 2 test is on FOO of all non-seabirds. demonstrating a moderate level of overlap. The diets were more similar when P. tenuirostris were absent (PSI = 65%) than when present (PSI = 54%). The consumption by F. catus of more groundmammals than seabirds is in accord with the findings of other studies where both prey types are abundant (Jones 1977; Dilks 1979). Stomach content analysis may even exaggerate the impact of F. catus on the seabirds. Observations of tracks around seabird carcases and patterns of carcase damage suggest some of the seabirds eaten may be scavenged from V. vulpes surplus kills and caches, rather than being directly killed by F. catus. Even when seabirds are a minor component of the diet, however, F. catus may still threaten their long-term population viability (Brothers 1984; McChesney and Tershy 1998; Lanyon 2000). Accordingly, a reduction in the number of feral F. catus on Phillip Island is likely to increase the viability of the resident seabirds. F. catus also appears to threaten a greater range of species than do V. vulpes, particularly skinks and amphibians. We did not investigate population sizes and consumption rates of the predators, which will influence their respective impacts on prey populations. Even so, given that both were obviously common in the study area, that V. vulpes generally are larger in size than F. catus (e.g., mean 4.4 kg cf. 2.9 kg; Risbey et al. 1999) and V. vulpes ate considerably more P. tenuirostris and E. minor, V. vulpes appears to be a greater threat to seabirds than F. catus. The threat is bigger than is revealed from stomach contents analysis alone as, V. vulpes, but not F. catus, surplus kills seabirds (Kruuk 1964). The substantial incorporation of P. tenuirostris into the V. vulpes diet, suggests they may actively select this
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