Food habits of feral cats (Felis silvestris catus L.) in insular semiarid environments (Fuerteventura, Canary Islands)

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1 Wildlife Research, 2008, 35, CSIRO PUBLISHING Food habits of feral cats (Felis silvestris catus L.) in insular semiarid environments (Fuerteventura, Canary Islands) Félix M. Medina A,C,E, Marta López-Darias B,C, Manuel Nogales C and Rafael García D A Unidad de Medio Ambiente, Cabildo Insular de La Palma, Avenida Los Indianos 20, 2, Santa Cruz de La Palma, Canary Islands, Spain. B Department of Applied Biology (EBD-CSIC), Pabellón del Perú, Avenida María Luisa s/n, Sevilla, Spain. C Island Ecology and Evolution Research Group (IPNA-CSIC), Astrofísico Francisco Sánchez 3, La Laguna, Tenerife, Canary Islands, Spain. D C/ San Miguel 9, Santa Cruz de La Palma, Canary Islands, Spain. E Corresponding author. felix.medina@cablapalma.es Abstract. In this study, we present the first data on diet and impacts of feral cats on a semiarid island (Fuerteventura, Canary Islands). A total of 614 prey was identified in the 209 scats analysed. Introduced mammals, especially rabbits and mice, were the most consumed vertebrate prey and constituted more than 90% of biomass. Barbary ground squirrels, Algerian hedgehogs, and rats were preyed upon less even though they were abundant on the island. Invertebrates, mainly Orthoptera, Coleoptera, Hymenoptera and Odonata, were the second most important prey items (in terms of actual numbers) but they contributed only minimally with respect to biomass (<1.1%). The presence of terrestrial molluscs in the diet was interesting because they are a rare prey in an insular context. Birds and reptiles occurred at a low frequency. A total of 677 seeds was counted, mainly belonging to Lycium intricatum (Solanaceae) and two unidentified plant species. Levin s niche breadth was narrow due to the high consumption of mammals. Morisita s index showed a similar trophic overlap in diet with respect to the other xeric habitats of the Canarian archipelago. Considering that more than 90% of biomass corresponded to introduced mammals, we conclude that feral cats are not having a large direct impact on the native prey species. Introduction Since domestication of feral cats (Felis silvestris catus) ~4000 islands (Medina et al. 2006; Nogales et al. 2006). The distriyears ago (Randi and Ragni 1991; Serpell 2000; Driscoll et al. bution and high diversity of habitats present in the different 2007), a close association with humans has been maintained, islands of the archipelago depends on their altitude and orientaresulting in the transport and introduction of the species world- tion (del Arco et al. 1999). An exhaustive study of the diet of wide, including to many isolated islands (Fitzgerald 1988; Long feral cats in these different habitats has been made in the Canary 2003). Present on more than 200 islands (Long 2003), feral cats Islands (Nogales and Medina 1996; Medina et al. 2006). This are commonly considered responsible for the extinction of archipelago thus probably constitutes the insular area of the numerous species across multiple taxa (Lever 1994; Mellink world where the diet of this introduced predator has been most et al. 2002; Tershy et al. 2002). For a complete understanding extensively studied (Medina et al. 2006). of the potential impact of feral cats on prey species, data on Of all the habitats present in the Canarian archipelago, the prey availability are required (Paltridge et al. 1997). xerophytic shrub is the most broadly represented across the Nevertheless, the first step to interpret and quantify their islands. Owing to the great heterogeneity of xeric habitats in the impacts on wildlife is to carry out ongoing studies on their diet eastern (more xeric) and western (more humid) islands, studies (Paltridge et al. 1997). For this reason numerous contributions of the diet of feral cats in this environment have been carried out have been performed, showing that feral cats can prey on on three islands (Alegranza islet: Nogales et al. 1992; Casañasendemic and endangered species (see reviews of Nogales and Acosta et al. 1999; Tenerife: Medina and Nogales 1993; La Medina 1996 and Courchamp et al. 2003). The opportunistic Palma: Medina et al. 2006). On each of these, introduced feeding behaviour of feral cats lead them to prey on a wide mammals (basically rabbits and mice) represented the most variety of prey species on islands, including mammals, reptiles, important part of the biomass. Although xerophytic shrub is birds, and insects (Kirkpatrick and Rauzon 1986; Konecny present on many of the Canary Islands, it predominates on the 1987; Fitzgerald 1988; Nogales and Medina 1996; Fitzgerald main dry eastern islands of Fuerteventura and Lanzarote where, and Turner 2000). to date, no study of the diet of feral cats has been carried out. For Feral cats were introduced to the Canarian archipelago about this reason, and considering that Fuerteventura harbours a disthe 15th century and occupy all habitat types on all the main tinctive fauna of introduced mammals and native birds, the CSIRO /WR /08/020162

2 Diet of feral cats in insular semiarid environments Wildlife Research 163 study of the diet of feral cats on this island is of particular interest. Among all the mammals present there is only one endemic species, the Canarian shrew (Crocidura canariensis), which is absent from the central and western Canary Islands (Nogales and González 2005), and one native bat (Pipistrellus kuhli). The rest are introduced species such as mice, two rat species, rabbits, squirrels, hedgehogs, cats, dogs and goats. Of significance is the relatively high abundance of two of these: the Barbary ground squirrel (Atlantoxerus getulus), a rodent that has become widely distributed across the island since its introduction in 1965 (Machado 1979; Machado and Domínguez 1982; López-Darias and Lobo 2008), and the Algerian hedgehog (Ateleryx algirus), introduced to the Canaries in 1892 and particularly abundant on Lanzarote and Fuerteventura (Nogales et al. 2006). Apart from the mammalian fauna, the birds are highly significant in Fuerteventura Island in the context of the Macaronesian archipelagos (Azores, Madeira, Salvages, Canaries and Cape Verde) where both endemic species such as the Canary Island stonechat (Saxicola dacotiae), and endemic subspecies such as the houbara bustard (Chlamydotis undulata fuertaventurae) and the stone curlew (Burhinus oedicnemus insularum) are distributed. Moreover, other birds, such as the cream-coloured courser (Cursorius cursor), have important populations on this island (Nogales and González 2005). Although feral cats have probably been involved in the decline and local extinction of small bird populations in the main islands of the Canaries, probably limiting their current distribution, no species has yet been declared extinct in this archipelago or on Fuerteventura Island because of feral cats (Nogales et al. 2006). For this reason, and as the first step to interpret their impact on prey species (Paltridge et al. 1997), the aim of this research was to study the diet of the feral cat and its potential impact on the endangered birds and the particular mammalian fauna of Fuerteventura, an island that harbours the primary and well conserved arid ecosystems of the Canarian archipelago. Considering the opportunistic feeding habits of feral cats (Andersson and Erlinge 1977; Veitch 1985; Kirkpatrick and Rauzon 1986; Fitzgerald 1988; Fitzgerald and Turner 2000), we would expect a similar consumption of introduced mammals to those found in other Canarian habitats. However, the inclusion of abundant and widely distributed new mammal species (such as the Barbary ground squirrel and the Algerian hedgehog) across the island, and the facultative specialist behaviour of feral cats, by which they adapt their trophic behaviour to variation in the availability of prey in time and space (Malo et al. 2004; Harper 2005), could decrease the presence of other common prey, such as rabbits or mice, in other Canarian habitats. Material and methods Study area The Canarian archipelago consists of seven main volcanic islands situated between N and W off the Atlantic coast of north-west Africa (Fig. 1). Fuerteventura is the second largest island of the archipelago (~1660 km 2 ), the second lowest in altitude (807 m above sea level) and the closest to the African continent (~115 km) (Fig. 1). The climate is arid, and habitats are semiarid, with mean annual temperatures of ~20 C (Dorta 2005) and annual precipitation below 100 mm m 2. Fuerteventura is also the oldest island of the archipelago (20.4 ± 0.4 million years: Carracedo et al. 2005; Criado 2005). This island has high erosion due to climatic conditions (frequent strong winds and torrential rain), producing a landscape of tapering cliffs (usually no more than 600 m above sea level), surrounding an extensive valley. Large extensions of biogenic sand, called jable, and a few malpaís (or badlands) characterise the landscape (Criado 2005). The island vegetation has been drastically altered by the exploitation of wood resources, intensive livestock grazing and the introduction of exotic herbivores (Rodríguez 2005). Owing to the presence of introduced herbivore species on the island, natural plant communities occur in inaccessible areas, far from herbivores, and in the areas that the herbivores can access, the vegetation is composed of pioneer native plant species (Rodríguez 2005). Despite this fact, Fuerteventura supports a wide variety of endemic flora and fauna species (Izquierdo et al. 2004) and harbours some of the most well preserved Canarian arid ecosystems. Canary Islands Lanzarote La Palma El Hierro La Gomera Tenerife Gran Canaria Fuerteventura Fig. 1. Map of the Canary Islands showing the 19 localities where scats of feral cats were collected on Fuerteventura Island. N Kilometres

3 164 Wildlife Research F. M. Medina et al. Diet and data analysis The diet of feral cats was determined from scats, scat analysis being one of the most commonly used methods for researching the diet of mammalian carnivores (Delibes 1980; Trites and Joy 2005). In order to minimise the chance of sampling domestic cats, which are likely to differ substantially in their diet, sampling localities were placed as far as possible from areas of human settlement. In total, 209 scat groups were collected during 2003 at 19 different well distributed and representative localities on the island (Fig. 1). Since scats remain unaltered over a long period before their disintegration, the collected material represented all seasons. In contrast to other mammal predators, feral cats have specific defaecation behaviour. They frequently bury their faeces (Bradshaw 1992) so it is very difficult to assign an exact date to when faeces were deposited. This complicates inclusion of information about prey availability in studies of the feeding ecology of feral cats. Scats were broken up at the laboratory after prior saturation in water. Prey items were identified at a magnification of 16 to species level when possible using hairs, bones, feathers, and arthropod exoskeletons, and reference collections. Quantifying the number of individuals from scats is difficult because the same prey may appear in more than one scat (Delibes 1980). When it was possible, particularly with mammals, the minimum number of individuals was estimated by bone remains, especially jaws and teeth. The ingested biomass was estimated from values obtained from bibliographic resources and those used in the previous studies conducted in the Canary Islands (Nogales and Medina 1996, and references therein). In the case of invertebrates, only prey weighing more than 0.05 g were considered in order to avoid the presence of other indirect prey previously ingested by lizards (Medina et al. 2006; Medina and García 2007). For large prey such as rabbits (Oryctolagus cuniculus), Algerian hedgehogs and Barbary ground squirrels, which constitute more than the daily food intake by cats, the total biomass applied was 170 g (Fitzgerald and Karl 1979). Number of prey, frequency of occurrence, relative frequency (number of individuals from the same species or taxonomic group 100 divided by the total number of prey), and percentage of biomass (biomass of the same species or taxonomic group 100 divided by the total consumed biomass) were the measures used for expressing diet composition. In order to overcome possible bias from these measures (Fitzgerald 1988), an index of relative importance (thereafter IRI) was calculated because it is suitable for ranking the relative importance of food types in the diet (Short et al. 1999; Hart et al. 2002). The IRI was calculated for each food category, except food scraps and plant material, using the formula IRI = F(N+W) where N is the numerical percentage, W is the biomass percentage, and F is the percentage frequency of occurrence (Pinkas 1971; see Martin et al for derivation of formula substituting mass per volume). Statistical analysis involved a Chi-square test to compare the consumption of the different prey types, using the number of prey items appearing in the scats. Analysis consisted of comparing a certain prey item with respect to the total number of the remainder of the prey identified (Medina et al. 2006). Contingency tables were constructed to assess association between seeds and vertebrate prey remains in scats. Spearman rank correlation coefficients were used to compare relationships between the components of the different measures of diet. Standardised Levin s niche-breadth index (B) was applied to evaluate the niche-breadth of the prey consumed by feral cats; values close to 0 indicate dietary specialisation while those close to 1 show a broad diet (Krebs 1989). Morisita s index of similarity (Krebs 1989), using the percentage of prey, was calculated to evaluate the overlap in the diet of the feral cats on Fuerteventura and on other xerophytic islands. Results A total of 614 prey was identified in the 209 scats analysed (Table 1). Introduced mammals were the main group preyed on (χ 2 = , P < 0.001), as indicated by each of number of 3 prey, frequency of occurrence and biomass consumed. Mice were the most common mammal prey (χ 2 4 = , P < 0.001), although rabbits contributed ~58% to the total biomass consumed by feral cats on the island. Rats, Barbary ground squirrels and Algerian hedgehogs were preyed on to a lesser degree. Although birds and reptiles were consumed at a lower proportion than invertebrates, their contribution to the total biomass was higher (Fig. 2). Invertebrates were the second prey group in importance, but their contribution to the total biomass was low (Fig. 2). Within the insects, the main groups preyed on (χ 2 = , P = 0.013) were the large ones: Orthoptera, Coleoptera, Hymenoptera and Odonata (Table 1). An interesting invertebrate group preyed on by feral cats on Fuerteventura was Gastropoda (mainly Theba geminata, Helicidae), the first record of a high consumption of molluscs in the Canary Islands. Plant material and seeds from at least three species (Lycium intricatum, Solanaceae and two unidentified taxa) were found in the scats. The presence of seeds in droppings was associated with bird remains (χ 2 = 5.24, P = 0.022) rather than lizards 1 (χ 2 = 0.052, P = 0.82) and mammals (Likelihood test, G = , d.f. = 1, P = 0.79). A low percentage of both carrion and rubbish was found in the scats (Table 1). The five different measures considered in this study produced similar rankings of the importance of prey taxa, although the lowest correlation between measures was observed for the importance of the biomass supported by each prey type (Table 2). Finally, we found a narrow value of niche breadth for the diet of feral cats in semiarid environments on Fuerteventura Island (0.38). Of all the identified species preyed on by feral cats in this study (Table 1), none are listed as threatened on the IUCN Red List of Threatened Species (IUCN 2007) or any current official endangered species acts. Discussion Introduced mammals constituted the basis of the feral cat s diet on Fuerteventura Island, as has been recorded in the main habitats of the Canarian archipelago (Nogales and Medina 1996), and in other insular (Jones 1977; Dilks 1979; Karl and Best 1982; Fitzgerald 1988) and arid environments (Short et al. 1999; Read and Bowen 2001). Mice were the most frequent prey, which is a result in accordance with records from the same habitat (xerophytic shrubs) on

4 Diet of feral cats in insular semiarid environments Wildlife Research 165 other Canarian islands where the diet of feral cats has been preyed upon; in particular, the squirrel is scarcely exploited at studied (Nogales et al. 1992; Medina and Nogales 1993; all. This may be due to the fact that cats are mainly crepuscular Casañas-Acosta et al. 1999). Nevertheless, rabbits constituted and nocturnal hunters while squirrels are strictly diurnal, shelthe main proportion of consumed biomass and are the most tering at night in burrows that provide protection from predators important prey in most Canarian habitats (Nogales and Medina (Machado and Domínguez 1982). In the case of the Algerian 1996). Despite the high abundance of Barbary ground squirrels hedgehog, the low predation rate could be due to its protective and Algerian hedgehogs on this island (Nogales and González quills. In fact, the presence of hedgehogs in the diet of feral cats 2005; López-Darias and Lobo, 2008), they were clearly less may be due to the consumption of roadkill carcasses (Félix M. Table 1. Composition of the diet of feral cats (Felis silvestris catus) in semiarid environments of Fuerteventura (Canary Islands) No. Pr, number of prey; % Pr, percentage of prey; % FO, frequency of occurrence; % B, percentage of biomass; IRI, Index of Relative Importance; BGR, biogeographic range; Int, introduced species; Nat, native species; End, endemic species Prey No. Pr % Pr % FO % B IRI BGR Mammals Atelerix algirus Int Mus domesticus Int Rattus spp Int Atlantoxerus getulus Int Oryctolagus cuniculus Int Birds Passerines Nat Non-passerines Nat Reptiles Tarentola angustimentalis End Gallotia atlantica End Invertebrates Mollusca Gastropoda Theba geminata End Rumia decollate Nat Arachnida Araneae indet Hexapoda Odonata Anax imperator Nat Odonata indet Orthoptera Acrididae Dericorys lobata Nat Acrididae indet Gryllidae Tettigonidae Coleoptera Curculionidae Coniocleonus excoriatus Nat Conorhynchus conicirostris Nat Herpisticus calvus End Scarabeidae Phyllognathus excavatus Nat Tenebrionidae Paivaea hispida End Tenebrionidae indet Coleoptera indet Hymenoptera Anthophoridae indet Hymenoptera indet Plant material Seeds Carrion 4.83 Rubbish 5.80

5 166 Wildlife Research F. M. Medina et al. Alegranza La Palma Fig. 2. Biomass recorded in the diet of feral cats from xerophytic shrubs studied in the Canary Tenerife Fuerteventura Islands Alegranza (Nogales et al. 1992), La Palma (Medina et al. 2006), Tenerife (Medina and Nogales 1993) and Fuerteventura (present study). Mammals Birds Reptiles Invertebrates Medina, pers. obs.). Predators are known to change their feeding behaviour from generalist to specialised according on the availability of their principal prey (Malo et al. 2004). On Fuerteventura all introduced mammals are abundant and widely distributed (Nogales et al. 2006), ensuring ready availability of prey for feral cats. Because of this, the presence of new introduced mammal prey (Barbary ground squirrel and Algerian hedgehog) did not provoke a major shift in the general dietary pattern or a decrease in consumption of species that are frequently preyed upon by feral cats in other habitats of the Canary Islands. Despite the relative abundance of birds in Fuerteventura (Martín and Lorenzo 2001; Nogales and González 2005), predation upon birds was low, although higher than has been recorded in the other studies of xeric habitats of the Canary Islands (Nogales et al. 1992; Medina and Nogales 1993). Predation upon reptiles was low, which agrees with data obtained in other Canarian habitats (Nogales and Medina 1996). Though reptiles are frequently present in the diet of feral cats in tropical and subtropical islands (Laurie 1983; Konecny 1987; Fitzgerald 1988), as well as in arid and semiarid zones of Australia (Bayly 1976; Catling 1988; Paltridge et al. 1997), they rarely contributed much in biomass on Fuerteventura. The small size of the endemic subspecies of lizard (Gallotia atlantica mahoratae) on this island (snout vent length cm: Castroviejo et al. 1985) probably contributes little to the total biomass consumed. On Tenerife Island, where the highest levels of reptile consumption have been documented (Nogales et al. 1990), the lizards (Gallotia galloti) are larger (SVL cm: Hernández et al. 2000), contribute substantially more to the biomass consumed (Nogales et al. 1990). With regard to predation on invertebrates, terrestrial molluscs were more frequently consumed on Fuerteventura than on other xeric Canarian habitats, where groups such as Orthoptera and Coleoptera were most important, as is typical for the species (Pearre and Maass 1998). Although snails have been reported in low numbers in the feral cat s diet on other islands (Apps 1983; Bloomer and Bester 1990; Langham 1990; Nogales et al. 1992; Medina and García 2007), their high abundance in Fuerteventura could represent an important feeding resource when other prey types become scarce (Paltridge et al. 1997). Table 2. Results of Spearman rank correlation coefficients comparing the different measures considered as indices of prey importance in the diet of feral cats in Fuerteventura Island No. Pr, number of prey; % Pr, percentage of prey; % FO, frequency of occurrence; % B, percentage of biomass; IRI, Index of Relative Importance. Bilateral significance levels are set at 0.01 % Pr % FO % B IRI No. Pr % Pr % FO % B r s = 1.00, P < r s = 0.92, P < r s = 0.92, P < r s = 0.69, P < r s = 0.69, P < r s = 0.71, P < r s = 0.96, P < r s = 0.96, P < r s = 0.96, P < r s = 0.80, P < 0.001

6 Diet of feral cats in insular semiarid environments Wildlife Research 167 Table 3. Morisita s index values reflecting niche overlap in the diet of feral cats in all xerophytic shrub habitats studied in the Canarian archipelago Islands Tenerife Alegranza Fuerteventura La Palma Tenerife Alegranza 0.64 In contrast to previous studies of the diet of feral cats in the Canaries (Nogales et al. 1996), in the present study seeds did not co-occur with lizards in scats, but did occur in scats containing bird remains. This result agrees with the finding of low predation upon lizards, the lizards omnivorous insectivorous diet on Fuerteventura and the abundance of granivorous and frugivorous birds on Fuerteventura (Nogales and González 2005). The presence of carrion and rubbish in the faeces was very low, as was the case in other habitats of the Canary Islands where the diet of feral cats has been studied (Nogales and Medina 1996), indicating that these faeces could be from domestic or stray cats and even from feral ones that move within human settlements (Newsome 1991). Nevertheless, the proportion of this food type is very low in practically all the studies carried out in the Canary Islands (Nogales and Medina 1996), demonstrating that the great majority of faeces collected were from feral cats. Although the IRI is an appropriate measure for the simple description of diet (Hart et al. 2002), the ranking of each prey type obtained was similar to those obtained from the other measures of diet (number of prey, percentage of prey, frequency of occurrence, and biomass), probably because these variables were highly correlated (MacDonald and Green 1983). Low values of Levin s niche breadth index obtained for the feral cat s diet on Fuerteventura was due to the high consumption of introduced mammals, mainly mice and rabbits. Values of this index were very similar in other Canarian xeric habitats (Medina et al. 2006). On Alegranza, feral cats also predominantly feed on rabbits and mice (Nogales et al. 1992), while on Tenerife the diet is more specialised, with high predation on mammals and some reptiles (Medina and Nogales 1993). In contrast, cats on La Palma have the most diverse diet, including a high proportion of introduced mammals, native birds and invertebrates (Medina et al. 2006). Values of Morisita s index of trophic niche overlap indicate similarity in diets of feral cats in the different islands (Table 3), except in Alegranza where the diet was narrower owing to the lower diversity in available prey (Nogales et al. 1992). In summary, dietary patterns found in the diet of feral cats in Fuerteventura coincided with those found in other arid environments where mammals compose the greatest percentage of prey and biomass consumed (Bayly 1976; Molsher et al. 1999; Short et al. 1999; Read and Bowen 2001). Nevertheless, terrestrial molluscs and birds were also preyed upon much more frequently than in other Canarian xeric habitats. No native species consumed by feral cats on Fuerteventura is threatened. Nevertheless, Illera and Díaz (2006) showed that feral cats prey heavily on nests of the Canary Island stonechat (Saxicola dacotiae), an endemic species from Fuerteventura that is listed as endangered on the IUCN Red List of Threatened Species. Moreover it is well known that the feral cat is responsible for the decline and extinction of a large number of native and endemic species in the world (Lever 1994; Moors and Atkinson 1984; Veitch 1985; Fitzgerald 1988). Although dietary studies could be a poor indicator of impacts on populations (Towns et al. 2006), they represent the first step to interpret the impact of feral cats on endangered species (Paltridge et al. 1997). Considering that more than 90% of prey biomass was composed of introduced mammals, we can conclude that this predator is not having a significant direct impact on the native prey species. Nevertheless, it is very difficult to conclude that there is a direct relationship between introduced species and the decline of indigenous species (Towns et al. 2006), even when the presence of only one predator has been considered sufficient to induce the extinction of the endemic prey (Courchamp et al. 1999). 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