Predation of insects by feral cats (Felis silvestris catus L., 1758) on an oceanic island (La Palma, Canary Island)

J Insect Conserv (2007) 11:203 207 DOI 10.1007/s10841-006-9036-7 SHORT COMMUNICATION Predation of insects by feral cats (Felis silvestris catus L., 1758) on an oceanic island (La Palma, Canary Island) Félix M. Medina Æ Rafael García Received: 30 June 2006 / Accepted: 25 July 2006 / Published online: 1 December 2006 Ó Springer Science+Business Media B.V. 2006 Abstract Predation of insects by feral cats (Felis silvestris catus) on a heterogeneous oceanic island (La Palma, Canary Islands) was studied. A total of 127 invertebrates were identified in the analysis of 500 scats (100 from each habitat of the Island). Invertebrates appear in 18.00% of the scats, representing an insignificant percentage of the total consumed biomass by feral cats on La Palma Island (0.05%). Insects were the most common invertebrate prey both in percentage of occurrence (90.6%) and invertebrate biomass (93.53%), with a total of 115 prey items. Orthoptera, Lepidoptera and Coleoptera were the main prey groups. Among the five main habitats present in La Palma Island, the temperate forest shows the lowest consumption of invertebrates, although insect consumption did not show statistical differences. However Orthoptera and Lepidoptera were more frequently preyed on in the pine forest and in the xerophytic shrub, respectively. Moreover, applying the Simplified Morisita index, a different insect composition of the diet was observed among habitats. Although, none of insects predated by feral cats are threatened, the identification of invertebrate component of the feral F. M. Medina (&) Unidad de Medio Ambiente, Cabildo Insular de La Palma, Avenida Bajamar 20, 2, 38700 Santa Cruz de La Palma, Canary Islands, Spain e-mail: felix.medina@cablapalma.es R. García C/ San Miguel, 9, 38700 Santa Cruz de La Palma, Canary Islands, Spain F. M. Medina Island Ecology and Evolution Research Group, IPNA, CSIC, 38206 La Laguna, Tenerife, Canary Islands, Spain cats diet is an important tool for the correct understanding of predation significance and to prevent damage to endangered insect species. Keywords Arthropods Æ Felis silvestris catus Æ Invertebrates Æ Native species Æ Predation Introduction The diet of feral cats (Felis silvestris catus) is mainly composed of mammals, birds, and reptiles, and frequently including several kind of invertebrates such as insects, spiders, woodlice, crayfish or molluscs (Fitzgerald 1988). In oceanic islands where the feral cat has been introduced, the diet is similar and it is characterized by the presence of a high proportion of endemic species, including invertebrates (Fitzgerald 1988; Nogales and Medina 1996). Though insects appear frequently in the diet of feral cats, they do not contribute significantly to the total biomass consumed, and most authors have not attempted to identify all invertebrate items, but usually list just the more important ones (Fitzgerald 1988). Only Fitzgerald and Karl (1979) gave a complete list of invertebrates present in scats, studying temporal variation in their abundance and Pearre and Maass (1998) reviewed the importance of insect prey in the feral cat diet. In the Canary Archipelago, the diet of feral cats is concordant with the general pattern found in the rest of oceanic islands where studied (Nogales and Medina 1996 and references therein). In these contributions, invertebrates were differentially considered as potential prey. On La Palma Island, where the feeding ecology of feral cats was studied, the invertebrates

204 J Insect Conserv (2007) 11:203 207 were only considered from a general point of view. Orthoptera, Coleoptera, Lepidoptera and Scolopendromorpha were the best represented groups (Medina et al. 2006). Drastic impact of feral cat in island ecosystems has been described in several island ecosystems (Courchamp et al. 2003; Nogales et al. 2004) and many studies have demonstrated these effects on endemic birds (Karl and Best 1982; Pontier et al. 2002) and reptiles (Laurie 1983; Nogales et al. 2001). Nevertheless, their impact on endemic and threatened insect species has never been considered. La Palma Island shows a heterogeneous landscape due to its rough topography, exhibits the five main habitats of the Canarian Archipelago (xerophytic shrub, temperate forest, laurel forest, pine forest and high mountain shrub) and presents an important insect diversity constituted by 2072 species, 677 of them endemics (Izquierdo et al. 2001). Although on La Palma Island the general diet of feral cats has been recently studied (Medina et al. 2006), invertebrates were only taking into account within a feeding ecology approach. For these reasons, the main objective of the present contribution is to study in detail the insect composition of the feral cat diet on an oceanic island. Additional objectives are to study the differences existing in the insect predation among habitats, and to know the effect of this predator upon endangered species. Methods The study was carried out during 2000 2001 on La Palma Island, a medium size island (728 km 2 ) of the Canarian Archipelago (Fig. 1). The different bioclimatic vegetation series present in La Palma are highly influenced by the humid north-east trade winds, altitude and orientation. Thus, xerophytic scrub is present in the low parts of the island with the most common species belonging to the genus Euphorbia. The next habitat in altitude is the temperate forest, an open forest with species such us Juniperus turbinata. The laurel forest is the most humid and dense habitat and it is composed of more than 20 tree species. On the other hand, the pine forest is an open-dry woodland dominated by one endemic pine species, Pinus canariensis. At the upper part of the island, a high mountain habitat is developed, dominated by a shrubby vegetation most of them belonging to Fabaceae family. A detailed description of the five main habitats of La Palma Island is provided in Medina et al. (2006). A total of 500 scats of feral cats (100 from each habitat) were analyzed. Scats were collected from all age (fresh and old), following paths and rural roads, commonly used by cats for travelling and hunting excursions (Liberg 1980; Fitzgerald and Turner 2000). Each scat was analyzed as a single sample and invertebrate remains were separated and identified at species level wherever possible. Taking into account that indirect prey consumed primarily by reptiles can appear in the feral cat scats, only the direct consumption of those species that reached at least 0.05~g as a conservative value was considered, following the criteria of Medina et al. (2006). To compare the consumption of different prey types by feral cats, chi-square tests were performed by comparing the number of a certain prey item with respect to the total number of the remainder of preys identified. In those cases where it was necessary to use the same data set, we reduced this effect by applying a more conservative sequential Bonferroni technique proposed by Rice (1989) in assessing statistical significance. Similarity or overlap in feral cat diets among the five main habitats was evaluated applying the Simplified Morisita index of similarity (Krebs 1989) for percentage of preys. Lastly, niche-breadth index (using percentage of preys) was evaluated using the Fig. 1 Map of the Canary Islands showing the location of La Palma Island

J Insect Conserv (2007) 11:203 207 205 Table 1 Invertebrate items found in the analysis of the feral cat diet on La Palma Island Taxa No Pr FO% %B Habitat Collected CL. Gastropoda 1 0.2 0.13 Ord. Stylommatophora 1 0.2 0.13 Stylommatophora indet. 1 0.2 0.13 CL. Malacostraca 2 0.2 0.22 Ord. Isopoda 2 0.2 0.22 Fam. Armadillidiidae 2 0.2 0.22 Armadillidium vulgare (Latreille 1804) 2 0.2 0.22 TF, LF, PF LF CL. Diplopoda 1 0.2 0.26 Ord. Julida 1 0.2 0.26 Fam. Blaniulidae 1 0.2 0.26 Blaniulus guttulatus (Fabricius 1798) 1 0.2 0.26 TF, LF, PF LF CL. Chilopoda 8 1.0 5.86 Chilopoda indet. 2 0.4 1.10 Ord. Scolopendromorpha 6 0.6 4.76 Fam. Scolopendridae 6 0.6 4.76 Scolopendra valida Lucas 1840 6 0.6 4.76 XS, TF, PF, HM XS, HM CL. Insecta 115 16.4 93.53 Ord. Orthoptera 55 7.8 71.22 Orthoptera indet. 2 0.4 8.03 Acrididae indet. 22 4.2 18.78 Fam. Gryllidae 2 0.4 2.37 Gryllomorpha longicauda (Rambur 1839) 2 0.4 2.37 XS, TF XS Fam. Tettigoniidae 29 3.6 42.04 Decticus albifrons (Fabricius 1775) 1 0.2 2.03 XS, TF, LF, PF, HM HM Tettigoniidae indet. 28 3.2 40.01 Ord. Coleoptera 27 4.8 6.50 Coleoptera indet. 4 0.8 0.35 Fam. Carabidae 1 0.2 0.12 Laemostenus complanatus (Dejean 1828) 1 0.2 0.12 XS, TF TF Fam. Staphylinidae 3 0.6 0.49 Ocypus olens (Müller 1764) 3 0.6 0.49 XS, TF, LF, PF PF Fam. Scarabaeidae 6 1.2 2.26 Oryctes prolixus* Wollaston 1864 1 0.2 1.33 XS, TF XS Pachydema fuscipennis* (Brullé 1838) 4 0.8 0.70 XS, TF, PF, HM XS Tropinota squalida canariensis* Lindberg 1950 1 0.2 0.23 XS, TF, PF, HM XS Fam. Tenebrionidae 11 1.4 2.83 Hegeter glaber** Brullé 1838 4 0.8 0.43 XS, TF, PF, HM XS, HM Hegeter tristis (Fabricius 1792) 1 0.2 0.16 XS, TF, PF, HM PF Hegeter sp. 3 0.4 0.37 Pimelia laevigata laevigata** Brullé 1838 3 0.6 1.87 XS, TF, PF, HM PF, HM Fam. Cerambycidae 2 0.4 0.45 Arhopalus pinetorum (Wollaston 1864) 1 0.2 0.33 PF, HM HM Deroplia albida* (Brullé 1838) 1 0.2 0.12 XS, TF XS Ord. Lepidoptera 27 4.2 14.92 Lepidoptera indet. 16 2.4 7.23 Fam. Gracillariidae 1 0.2 0.15 Phyllocnistis canariensis* Hering 1927 1 0.2 0.15 XS, TF, LF, PF, HM HM Fam. Sphingidae 10 1.8 7.54 Hyles tithymali tithymali* (Boisduval 1832) 10 1.8 7.54 XS XS Ord. Hymenoptera 6 1.2 0.89 Fam. Anthophoridae 6 1.2 0.89 Anthophora alluaudi* Pérez 1895 4 0.8 0.57 XS, TF, LF, PF, HM HM Melecta curvispina* Lieftinck 1958 1 0.2 0.16 XS, TF, LF, PF, HM HM Anthophoridae indet. 1 0.2 0.16 FO%: frequency of occurrence, No Pr: number of prey items, %Pr: percentage of prey, %B: percentage of total invertebrate biomass consumed, *: endemic species from the Canary Islands, **: endemic species from La Palma Island, : exotic species in the Canary Islands, XS: xerophytic shrub, TF: temperate forest, LF: laurel forest, PF: pine forest, HM: high mountain shrub, Habitat: habitat where species is distributed, Collected: habitat where feral cats scat was collected

206 J Insect Conserv (2007) 11:203 207 standardized Levin s niche-breadth index (B), where a value close to 0 indicates dietary specialization, and a value close to 1 shows a broad diet (Krebs 1989). Results and discussion A total of 127 invertebrate prey items, belonging to 28 different species, were identified in the analysis of 500 feral cat scats on La Palma (Table 1). They appear in 18.0% of the scats, although their contribution to the total consumed biomass was insignificant (0.05%) (Table 2) (see Medina et al. 2006). Similar results were observed in other studies carried out in the Canary Archipelago (Nogales and Medina 1996 and references therein) and other oceanic islands (Karl and Best 1982; Kirkpatrick and Rauzon 1986). Insects (Hexapoda) were the main group preyed: 90.6% of the total prey (chi-square test v 2 = 520.13, d.f. = 4, P > 0.001), and 93.5% of the total consumed biomass (chi-square test v 2 = 161.35, df = 4, P > 0.001). Within insects, Orthoptera was the most important group preyed on, both in percentage of prey (chi-square test v 2 = 64.77, df = 3, P > 0.001) and in biomass (chi-square test v 2 = 62.91, df = 3, P > 0.001). Lepidoptera, Coleoptera, and Hymenoptera were also important in the diet (Table 1). Although the most common prey size is about 1% of their own body weight, feral cats are an exception among carnivores due to their ability to use small prey (Pearre and Maass 1998), and its generalist and opportunistic behaviour (Fitzgerald 1988) feeding on whatever food available, including insects. When insects are included in their diet, those orders represented by large size species such as Orthoptera, Lepidoptera or Coleoptera, constitute the main proportion of prey items (Tidemann et al. 1994; Nogales and Medina 1996; Tranchant et al. 2003). On the other hand, invertebrate predation by feral cats could be related with the kittens feeding behaviour (Kitchener 1991), according to the seasonal variation of prey availability (Fitzgerald and Karl 1979), or when a decrease in the abundance of other Table 2 General diet of the feral cats (Felis silvestris catus) inla Palma Island Prey type No Pr FO% %B Mammals 584 92.2 88.27 Birds 49 9.8 4.59 Reptiles 235 37.6 7.09 Invertebrates 127 18.0 0.05 FO%: frequency of occurrence, No Pr: number of prey items, %B: percentage of biomass consumed (for details see Medina et al. 2006) prey type occur (Konecny 1987). Other insect species such us ants or necrophagous insects, not included in the results due to their low weight, could be present in scats as a result of carrion consumption (Fitzgerald and Karl 1979; Bradshaw 1992). The consumption of invertebrates was different in each ecosystem, being less abundant in the temperate forest (chi-square test v 2 = 29.97, df = 4, P > 0.001) (Fig. 2). Compared with other invertebrate groups, insect consumption did not show statistical differences among habitats (chi-square test v 2 = 4.24, df = 4, P = 0.34), although Orthoptera and Lepidoptera were more frequently preyed on the pine forest (chi-square test v 2 = 23.17, df = 4, P > 0.001) and in the xerophytic shrub (chi-square test v 2 = 18.33, df = 4, P = 0.001), respectively (Fig. 2). Different distribution and population of insects in each habitat (Oromí 1982; García et al. 1992), site and seasonal variations in the proportion of prey availability (Konecny 1987) and prey catch-ability (Fitzgerald and Turner 2000) could explain the different insect composition of the diet at each habitat. Number of prey 40 35 30 25 20 15 10 5 0 XS TF LF PF HM Habitats Non-insects Orthoptera Coleoptera Lepidoptera Hymenoptera Fig. 2 Invertebrates diet composition of the feral cat in the different habitats of La Palma Island. XS: xerophytic shrub, TF: temperate forest, LF: laurel forest, PF: pine forest, HM: high mountain shrub Table 3 Niche overlap using Simplified Morisita index of similarity for the feral cat diet in the five main habitats, based on percentage of preys of each food item Habitats XS TF LF PF HM XS 0.00 0.26 0.23 0.51 TF 0.21 0.00 0.00 LF 0.10 0.39 PF 0.53 HM XS: xerophytic shrub, TF: temperate forest, LF: laurel forest, PF: pine forest, HM: high mountain shrub

J Insect Conserv (2007) 11:203 207 207 The Morisita index indicates a very low trophic overlap between most habitats (Table 3). On the other hand, Levin s niche breadth was broader in the temperate forest (0.85) and narrower in the pine forest (0.32). In the other three habitats this index reaches medium values, ranging from 0.59 in the xerophytic shrub, 0.65 in the laurel forest and 0.67 in the high mountain shrub. The low values of trophy overlap observed among habitats could reflect the different insect abundant and diversity in the distinct environments. This fact is denoted by the presence of species with a specific niche or demographic explosion such as Pachydema fuscipennis, Pimelia laevigata or Hyles tithymali. The same factors could support the different values of niche breadth as observed between habitats. Ten insect species of the 28 invertebrates preyed on by feral cats are endemic to the Canary Islands, and only two (Hegeter glaber and Pimelia laevigata laevigata, Coleoptera) are endemic to La Palma Island (Table 1). Nevertheless, none of them is threatened or included in any of the current endangered species list. Although feral cats seem to be not a current threat for the conservation of endangered insect species on La Palma Island, we consider that identification of invertebrate component of the feral cats diet is an important tool for the correct understanding of predation significance (Fitzgerald 1988) and to prevent damaged on any endangered insect species. Acknowledgements We are very grateful to Manuel Nogales, Pedro Oromí and two anonymous referees for their revision and comments made on an early draft of the manuscript. Jorge L. Pais Simón provided some data on invertebrate weights. References Bradshaw JWS (1992) The behaviour of the domestic cat. 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