The diet of feral cats on islands: a review and a call for more studies
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Cats are among the most successful and damaging invaders on islands and a significant driver of extinction and endangerment. Better understanding of their ecology can improve effective management actions such as eradication. We reviewed 72 studies of insular feral cat diet from 40 islands worldwide. Cats fed on a wide range of species from large birds and medium sized mammals to small insects with at least 248 species consumed (27 mammals, 113 birds, 34 reptiles, 3 amphibians, 2 fish and 69 invertebrates). Three mammals, 29 birds and 3 reptiles recorded in the diet of cats are listed as threatened by the IUCN. However, a few species of introduced mammals were the most frequent prey, and on almost all islands mammals and birds contributed most of the daily food intake. Latitude was positively correlated with the predation of rabbits and negatively with the predation of reptiles and invertebrates. Distance from landmass was positively correlated with predation on birds and negatively correlated with the predation of reptiles. The broad range of taxa consumed by feral cats on islands suggests that they have the potential to impact almost any native species, even the smallest ones under several grams, that lack behavioral, morphological or life history adaptations to mammalian predators. Insular feral cat’s reliance on introduced mammals, which evolved with cat predation, suggests that on many islands, populations of native species have already been reduced.
KeywordsDomestic cat Felis catus Feeding behaviour Food web Island ecosystem Conservation
Invasive species, particularly mammals, have caused population declines, local extinctions and global extinctions of many island endemic species making them leading contributors to biodiversity loss (Diamond 1989; Fritts and Rodda 1998; Balmford 1996; Stattersfield and Capper 2000; Atkinson 2001; Dowding and Murphy 2001; Courchamp et al. 2003; Long 2003; Aguirre-Muñoz et al. 2008). Cats Felis catus (Driscoll et al. 2007) were first introduced to islands in the Mediterranean in 9000 BP (Vigne et al. 2004; Driscoll et al. 2007), and have since been introduced to islands worldwide from the sub Antarctic to the sub Arctic including the most arid and mesic islands (Courchamp et al. 2003). Cats are successful invaders on islands because they can survive without access to fresh water, have high fecundity, are highly adaptable to new environments, and have generalist predatory behaviours that allow them to feed on the most available prey species (Van Aarde 1986; Konecny 1987; Say et al. 2002; Fitzgerald 1988; Fitzgerald and Turner 2000; Pearre and Maass 1998; Stattersfield and Capper 2000).
Cats are one of the most widespread invasive predator on islands causing strong negative impacts on native wildlife (Fitzgerald 1988; Macdonald and Thom 2001) and are responsible, at least in part, for 8% of global bird, mammal and reptile extinctions and a significant threat to almost 10% of critically endangered birds, mammals and reptiles (Medina et al. In Review). Eradicating cats from islands can protect native species from the threat of extinction (Keitt and Tershy 2003; Nogales et al. 2004; Ratcliffe et al. 2009) and research on the ecology of insular feral cats can improve the efficacy and prioritization of cat eradications, particularly when (i) complex biotic interactions occurred on multi-invaded islands and (ii) highly vulnerable native prey are present (Fitzgerald 1988; Fitzgerald and Turner 2000; Macdonald and Thom 2001).
Here, we reviewed all available published studies of insular feral cat diet to determine: (1) what general patterns define the diet of feral cats on islands, (2) how does biogeography influence the diet of feral cats, (3) what are the taxonomic and size diversity of cat prey species, and (4) how many threatened species were found in the diet of feral cats on islands?
Materials and methods
We compiled data from published literature using electronic databases (Webspire, Web of Knowledge, Ovid SP, Inist, Blackwell Publishing, Science Direct) using the key words: cat(s), feral cat(s), Felis catus, island(s), diet, predation, and native species. Data compilation was conducted through July 2008. References cited in the resulting papers were examined for additional sources. The diet studies we analyzed used three types of samples: scats, gut contents and stomach contents. Some studies utilized more than one of these sample types. For the analyses in this study we did not differentiate between the types of samples because several studies found only minor differences between sample types (Lozano et al. 2006; Souza and Bager 2008), but see Jones et al. (2003). When studies used more than one type of sample, we analyzed each sample type as a separate study (e.g. Berruti 1986; Fitzgerald et al. 1991; Martínez-Gómez and Jacobsen 2004). Australia was excluded from our review because it is traditionally defined as a continent and cat diet in Australia has been reviewed elsewhere (Dickman 1996; Burbidge and Morris 2002). Lastly, we selected only data from feral cats, feeding on wild prey (Artois et al. 2002) and only incorporated data from papers giving detailed information about cat diet.
The following data were extracted from each paper: geography (latitude, distance from landmass, elevation and area) of each study island, study date, type of sample analyzed (guts, stomachs and scats), prey sample size and characteristics of sampling sessions (i.e. number of sessions, periods of the year).
Global patterns of feral cat diet
To describe global patterns of feral cat diet, when one study described cat diet on the same island but with two different sample types (i.e. scats and stomachs), we eliminated the less representative, i.e. the one with the smallest sample size, to avoid redundancy. We calculated the percentage of the 66 studies where the main prey categories (i.e. mammals, birds, reptiles, fishes, invertebrates) were represented by at least one species. Then we constructed a global diet pattern for the main prey categories using data from papers that gave feral cat diet in frequencies of occurrences (% FO) for all taxa (65 studies).
Biogeographical patterns of feral cat diet
For the following analyses, we only selected the most representative (the largest sample size and the most number of seasons covered) study per island, avoiding those based on excessively small sample size and which described diet with insufficient accuracy (less than 30 samples). We tested for correlations between the frequency of occurrence (%FO) of the six main prey categories (rabbits, rats, mice, birds, reptiles and invertebrates such as dependent variables) and five biogeographical variables (latitude, distance from nearest continental landmass, area, elevation, area*elevation). The dependent variables were arcsine transformed and the quantitative descriptive variables were log transformed for normality (Sokal and Rohlf 1995). Variables not normally distributed after transformation were analyzed using Spearman rank tests.
We also performed a PCA analysis, using the %FO as dependent variable, to obtain a spatial ordination of the different main prey categories and islands included in this review. For this, we applied a varimax rotation in order to identify the contributions of the different prey items. In all analyses, all data concerning %FO were arcsin transformed to better meet the assumption of normality (Tabachnick and Fidell 1996; Fowler et al. 1998).
Diet diversity and species threatened by feral cats
Levin’s index (trophic niche breadth) was applied to calculate diet diversity. This index reaches maximum values when all diet items are consumed in equal proportions, the minimum value indicating more specialized diets. The normalized Levin’s index (hereafter Levin’s-std) was used to avoid variation caused by different samples sizes (Krebs 1989). We also tested for correlations between the Levin’s-std index and the five biogeographic variables mentioned, and we performed Spearman correlation tests when variables were not normally distributed, even after transformation. Then, mean weights (from the literature) of vertebrates species preyed on by feral cats were listed. This information was not available for invertebrates.
Lastly, we compared the prey species from our review to those listed in a review of feral cat impacts on insular endangered species (Medina et al. In Review) with an emphasis on prey species that were listed by the IUCN 2008 Red List of Threatened Species in one of the five most threatened categories of the (Extinct, Extinct in the Wild, Critically Endangered, Endangered and Vulnerable).
Data compilation and global patterns of feral cat diet
Sampling description of the diet studies of feral cats: type of samples, sample size; seasons covered by sampling sessions and their number
Number of studies
Number of samplings
Several (collected occasionally)
Biogeographical patterns of feral cat diet
Spearman Rank tests describing the frequencies of occurrence of rats, rabbits, mice, birds, reptiles and invertebrates in the diet of feral cats related to biogeographic variables
Number of samples
Distance from landmass
Area * Elevation
FO of rats
FO of rabbits
FO of mice
FO of birds
FO of reptiles
FO of invertebrates
Principal component analysis of the correlation matrix between percentages of %FO (arc-sin transformed) for the main prey items included in the diet of feral cats (Felis catus) on the different islands studied
Main prey items
Diet diversity and species threatened by feral cats
Diet studies of feral cats expressed in prey number. In this study sample, Levin’s index was calculated
Distance from landmass (km)a
Number of prey categories
Karl and Best (1982)
Apps (1983) (/year)
Fitzgerald and Veitch (1985)
Van Rensburg (1985)
Santana et al. (1986)
Fitzgerald et al. (1991)
Nogales et al. (1992)
Medina and Nogales (1993)
Nogales and Medina (1996)
García-Márquez et al. (1999)
Smucker et al. (2000)
Espinosa-Gayosso et al. (2006)
Medina et al. (2006)
Bonnaud et al. (2007)
Peck et al. (2008)
Juan de Nova
Phillips et al. (2007)
Matias and Catry (2008)
Medina et al. (2008)
Kawakami and Mashiko (2008)
Faulquier et al. (2009)
Results of the Spearman Rank test describing the Levin’s index of the diet of feral cats related to biogeographical variables
Number of samples
Distance from landmass
Area * Elevation
We compiled 58 scientific papers, describing 72 studies on the diet of feral cats, in different archipelagos worldwide. Few data were available on feral cat diet from islands in the Caribbean, Indonesia or French Polynesia, three biodiversity hotspots with large numbers of threatened endemic species (Brooks and Smith 2001; Long 2003). Surprisingly, there were also only a few studies from the Mediterranean Basin, despite the presence of cats on some islands since 9000 BP (Vigne et al. 2004), and on Japanese islands. Studies in under-sampled regions will undoubtedly add new species of prey items to the nearly 250 reported here.
Scats were the most frequent type of sample analyzed (Mukherjee et al. 2004). A minimum size of ~100 scats is considered necessary to identify principal prey remains occurring in 5% of scats and ~100 samples is also required when comparing diets to distinguish moderate effects over time or between areas (Trites and Joy 2005). Less than half of the 72 studies were based on more than 100 samples, thus most diet studies reviewed here probably missed some prey items (Table 1a). Moreover, most diet studies were conducted with fewer than two sampling sessions and during just part of the year (Table 1b). This effort is likely to miss rare species, which suggests that the 36 of IUCN listed threatened species we found may be a conservative estimate.
Global patterns of feral cat diet
Feral cats feed on a wide range of prey from large birds to small insects (Langham 1990; Tidemann et al. 1994; Peck et al. 2008), yet mammals and birds were present in practically all studies. Introduced mammals (mainly rabbits, rats and mice) were the main prey and birds, reptiles and invertebrates were often consumed as secondary prey. Birds were important on islands that harbour important colonies of seabirds and, in these cases, they constitute the main source of biomass in the diet of feral cats (Jones and Barmuta 1998; Bonnaud et al. 2007; Nogales and Medina 2009, Peck et al. 2008, Faulquier et al. 2009). This observation is enhanced by cats eating bird eggs, which are undetectable in the gut, but amount to a significant loss of birdlife in certain areas (Rauzon 1983). Reptiles and invertebrates were not systematically recorded in feral cat diets but they were frequently eaten when present, particularly on tropical and subtropical islands (Konecny 1983; Nogales et al. 1990, Seabrook 1990). However, due to their small size, reptiles rarely represented a significant percentage of prey biomass (Nogales et al. 1988; Medina and Nogales 1993; Casañas-Acosta et al. 1999) but cat predation can have a high impact on them.
Large differences in the diet of cats between islands indicated considerable trophic plasticity, with feral cats eating the most available items on each island (Arnaud et al. 1993; Martínez-Gómez and Jacobsen 2004). This same pattern was also seen between habitats and seasons within an island (Clevenger 1995; Nogales and Medina 1996; Medina et al. 2006).
This ability to seasonally adjust diet preference on an island has significant ramifications for native species, especially seabirds. Both rabbits and rats were regular items in the diet of insular feral cats (Dilks 1979; Fitzgerald et al. 1991; Nogales et al. 1992; Fitzgerald and Turner 2000; Medina et al. 2006). However, during months when breeding seabirds are present on an island they can become the preferred prey item (Rauzon 1983, Bloomer and Bester 1990, Keitt et al. 2002, Peck et al. 2008). The presence of the alternative prey items, rabbits or rats, means the feral cat population can sustain itself at a larger population throughout the year and thus cat impacts on native species are likely to be exacerbated via hyperpredation (Taylor 1979; Jones 2002; Keitt et al. 2002; Oro et al. 2004). This can also facilitate the complete eradication of native seabirds because the alternative prey source allows the feral cat population to sustain itself even as the seabird resource moves to a zero population (Courchamp et al. 1999; Roemer et al. 2002). Conversely, feral cats can eradicate populations of rodents when seabirds are absent during severe climatic conditions (Rauzon et al. In Review 2011).
Biogeographical patterns of feral cat diet
The increase in rabbit consumption with latitude can be explained by their presence on most sub-Antarctic islands where this lagomorph has become a staple prey inside a low prey diversity environment. The increase of birds in the diet of cats on more geographically isolated islands probably reflected the relative absence of native mammals on these insular areas (Vitousek et al. 1995; Whittaker and Fernández-Palacios 2007). These results reinforced the opportunistic feeding behavior of feral cats. As expected by the global distribution of reptiles, higher latitudes support fewer reptiles (Schall and Pianka 1978), and pointed out by Fitzgerald (1988), the presence of reptiles in insular feral cat diet decreased with increasing latitude. The same trend was recorded for invertebrates suggesting that cats eat a wide range of prey as soon as they are available.
The PCA analysis showed that on most islands cats had primary prey like rabbits, rats, mice or birds and this preference depended on the availability and phenology of prey in the different islands. Some general trends indicated that (i) rats were the main prey especially where rabbits were absent, (ii) rabbit predation could mitigate bird predation and (iii) reptiles and invertebrate consumption was higher in tropical and subtropical islands probably due to the higher abundance of these prey.
Diet diversity and species threatened by feral cats
The 179 different vertebrate prey species we found in the diets of feral cats on islands confirmed the wide niche breadth of this efficient generalist. Introduced rodents and lagomorphs, large and small birds and small reptiles appeared more regularly in island cat diet. Data on invertebrate prey species diversity was much less consistently reported, however, cats preyed upon a large range of invertebrate prey taxa from small hymenoptera to large crustaceans. Insects were the invertebrates most frequently preyed on by feral cats (Fitzgerald and Karl 1979; Kirkpatrick and Rauzon 1986, Konecny 1987; Medina and García 2007) and most insect orders were recorded, with Orthoptera, Coleoptera and Lepidoptera most commonly recorded.
As predicted by island biogeography theory (Vitousek et al. 1995; Whittaker and Fernández-Palacios 2007), diet diversity increased with elevation and area*elevation. Surprisingly, however, diet diversity also increased with distance from nearest landmass. Perhaps this is due to confounding variables of islands size and distance from shore, because (i) more oceanic islands were more highly invaded, or (ii) more evolutionarily naïve prey species are present on oceanic islands, or (iii) more eradications were done and save island species.
Feral cats have contributed to over 8% of all bird, mammal and reptile extinctions and to the declines of almost 10% of critically endangered birds, mammals and reptiles (Medina et al. In Review). However, we found relatively few globally threatened vertebrates in the diet of feral cats (Fig. 4). Globally threatened species were underrepresented in our study because there was no study of cat diet on islands within their range and because most studies we reviewed were not designed to detect rare prey species (Blackburn et al. 2004). Threats to birds and other native species when they are the primary prey are obvious; however, there is no clear relationship between frequency of prey in the diet and impact of the predator on that prey species. Moreover, the effects of predation are usually inversely dependent on prey density when native species are not the primary prey (Sinclair et al. 1998; Cuthbert 2002). Although none of the studies in our analysis accounted for time since cat introduction, it is likely that the most vulnerable native prey were reduced in number or extirpated long before diet data were collected.
This study confirms that feral cats on islands are extremely opportunistic, generalist predators capable of consuming most available vertebrate species. This combined with their ability to survive without drinking water and their large thermal tolerance (Bradshaw 1992) explains their tremendous geographic range. Our metadata analysis also suggests that cats can negatively impact a large percentage of the native vertebrates on any island. Future studies of cat diet on islands should be, (1) conducted so as to sample during the presence of “temporary” prey such as seabirds, (2), represented by a sufficient number of samples (> 100), (3) published by indicating the results at least for the main prey categories not only in frequency of occurrence but also in prey frequencies, biomass and index of relative abundance, and (4) preferentially dedicated to under-sampled and under-studied insular regions (e.g. Caribbean, Indonesia, Japan and French Polynesia).
This contribution is dedicated to all persons who have provided information on the diet of feral cat in all islands worldwide. We are very grateful to F. Torre, P. Campagne and K. Bourgeois for helping with statistical analyses and special thanks for S. Fadda. We would like to thanks M. Fitzgerald and two anonymous referees for their helpful comments. This work has received support from the European Union and the DIREN PACA via a Life Nature project (ref. LIFE03NAT/F000105), the French National Research Agency (ALIENS project), the MEDAD (Ecotropic programme), the project CGL-2004-0161 BOS co-financed by the Spanish Ministry of Science and Education and the European Union, and a CR PACA PhD fellowship to EB.
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