Web Ecology 7: 53–62. Trophic niche partitioning between two native and two exotic carnivores in SW Portugal Maria João Santos, Bruno Miguel Pinto and Margarida Santos-Reis Santos, M. J., Pinto, B. M. and Santos-Reis, M. Trophic niche partitioning between two native and two exotic carnivores in SW Portugal. – Web Ecol. 7: 53–62. The introduction of exotic species is one of the most pervasive consequences of the increased human mobility. The most known negative effects are the decrease or extinc- tion of natives. The common-genet, Genetta genetta, and the Egyptian mongoose, Her- pestes ichneumon, were introduced in the Iberian Peninsula in the 15th and 19th centu- ries, respectively. The competitive exclusion principle defines that two ecologically simi- lar species cannot coexist. Thus, some degree of partitioning has to occur in species realized niche, which can occur at the trophic level. To test this hypothesis of partitio- ning we compared the diet of these two exotic species with that of two native species (stone marten, Martes foina, and red fox, Vulpes vulpes). The results show a high degree of overlap (>45%) between the diets of species similar in their feeding strategies (arbore- al and ground feeding). Nonetheless, at the finer scale of prey consumed at the species level some differences are found between the native and exotic species. These results suggest that if coexistence is due to trophic niche partitioning it only occurs at the level of the consumed species. However, coexistence may also be due to a combination of different strategies (home-range size, time and space use) that structured the different realized niches of each species. M. J. Santos, B. M. Pinto and M. Santos-Reis ([email protected]), Univ. de Lisboa, Centro de Biologia Ambiental, Faculdade de Ciências, Ed. C2, 3º Piso, Campo Grande, PT-1749- 016 Lisboa, Portugal. During past centuries, the increases of human mobility turbance at a site when exotics arrive, with native mam- and activities (e.g. agriculture, aquaculture, recreation and mals’ richer areas being less prompt for exotic success. transportation) have caused the introduction of species to The common genet Genetta genetta and the Egyptian different geographic areas (Sutherland 1998, Kolar and mongoose Herpestes ichneumon are two species of African Lodge 2001). Species are termed as exotic or non-native mesocarnivores, introduced in the Iberian Peninsula (Dob- when they have not evolved in a given geographic area, and son 1998). The earliest remains of common genet in Europe have been accidentally introduced or brought for a specific date to the Middle Ages (Dobson 1998) and the first records purpose (Sutherland 1998). Exotic species success is usu- of Egyptian mongoose are from the latest 1800s (Delibes ally measured by the extinction of native species and the 1982). Presently, the common genet is distributed through- potential to alter the environment or pose serious threats out the Iberian Peninsula and south of France (Livet and to human health (Primack 1998). Smallwood (1994) has Roeder 1987, Calzada 2002), whereas the Egyptian mon- shown that birds and mammals site invasibility in Califor- goose is limited to the SW of the Iberian Peninsula (De- nian habitats is directly related to the degree of human dis- libes 1982, Borralho et al. 1996, Palomares 2002). Accepted 10 May 2007 Copyright © EEF ISSN 1399-1183 WEB ECOLOGY 7, 2007 53 The competitive exclusion principle states that two spe- Sampling methods cies with similar ecology cannot live together in the same place (Hardin 1960). Considering that these two exotic For the red fox, Egyptian mongoose and stone marten, species are generalist carnivores, which coexist and poten- sampling was based on scat collection along three different tially compete with native species, the goal of this study linear pedestrian transects, with 6.5 km of length each. was to investigate if the partitioning in species realized Due to the marking behaviour of the common genet niche occurs at the trophic level. In order to test this hy- (Livet and Roeder 1987) and stone marten (Libois and pothesis, we studied the diet of two native carnivores: red Waechter 1991), radio tracked individuals of both species fox, Vulpes vulpes, and stone marten, Martes foina, and provided the location of latrines where scats were also col- compared it with the diet of that two introduced species. lected (Santos-Reis et al. 2004). The first scats found in The choice of these two native carnivores, within the local transects and latrines were destroyed, to allow for a correct community of carnivores (Santos-Reis et al. 1999), can be dating. Scat collection was conducted monthly by the justified by the fact that they are the ones that most likely same two observers, from January to November of 1997. would be affected by strong competition with the genet We collected and identified 380 scats: 195 scats of red and the mongoose because have similar body mass and siz- fox, 75 scats of common-genet, 58 scats of stone-marten es (Rosalino et al. 2005). Moreover, they share similar eco- and 52 scats of Egyptian mongoose. Scat identification ethological behaviours, with the Egyptian mongoose and was based on its characteristics (length, diameter, odour, the red fox being more terrestrial and the genet and stone and morphology; Kendall et al. 1994, Sadlier et al. 2004) marten more arboreal. It can then be predicted that in and crosschecked by other specialists. Errors in the identi- sympatric areas, the native and exotic species depend on fication of scats were minimised using more than one diag- similar prey types and have approximate diets. A high nostic feature and the exclusion of scats with doubtful trophic niche overlap between red fox/Egyptian mongoose identification (Ray and Sunquist 2001). The sample size and genet/stone marten would also be expected as a conse- was considered large enough to characterize the four spe- quence of their morphological and eco-ethological resem- cies’ diet, because the number of prey items in each catego- blance. ry reached an asymptote in the cumulative number of identified items (nred fox= 26, ncommon genet= 24, nstone marten= 30, and nEgyptian mongoose= 31). Material and methods Scats were dried in an oven at 50°C, washed in a 1 × 1 mm fine-meshed sieve, and prey items were first separated and classified in six broad categories: mammals, birds, rep- Study area tiles, arthropods (insects, chilopods, crustaceans and arachnids), fruits and other plant material. Each food item The study was carried out in a 35 km2 area located in was then dried, weighed on a scale to the nearest 0.0001 g, Grândola Mountain (Alentejo, SW Portugal). This area is and identified to the species level, whenever possible, based mostly cork oak woodland –“montado”, a typical Mediter- on previous works (Gama 1957, Keller 1978, 1980, 1981, ranean ecosystem common in southern Portugal. Cork Madureira and Ramalhinho 1981, Debrot et al. 1982, oak woodlands, Quercus suber, with holm oaks, Q. ilex, and Brom 1986, Chinery 1986), expert knowledge, and refer- Lusitanian oaks, Q. faginea, dominate the land cover of the ence collections. area. There are some patches of eucalyptus, Eucalyptus Minimum number of individuals was counted through globulus, and maritime pine, Pinus pinaster. Common al- distinctive features (e.g. jaws, bones, seeds). When non- der, Alnus glutinosa, black willow, Salix nigra, and common quantifiable remains from each food item were found, the sallow, Salix atrocinerea, are found in riparian areas. The minimum number of individuals was assumed to be equal understory is dominated by rock-roses, Cistus salvifolius, to 1. Each food item was quantified in relative frequency of and C. ladanifer, lavender, Lavandula luisieri, and straw- occurrence and of biomass. Relative frequency of occur- berry tree, Arbutus unedo, with patches of blackberry rence (RFO) is the proportion of the total number of indi- bushes, Rubus spp. Small orchards with pear, Pyrus viduals that made up each food class. Frequency of bio- bourgaeana, fig, Ficus carica, loquat, Eriobotrya japonica mass (FBiom) is the proportion of the total consumed bio- trees, and olive, Olea europaea, are found scattered mass of each class. throughout the area. The climate is Mediterranean with Biomass was assessed by multiplying the minimum some Atlantic influence, and precipitation levels assume number of individuals of each item by its correction factor mean annual values of 500 mm. The altitude ranges from (insectivores and small rodents=23, lagomorphs=50, 159 to 238 m, with few permanent or temporary streams goat=118, reptiles and amphibians=29.5, insects, crayfish (Santos-Reis and Correia 1999). Human activity occurs in and molluscs=5, fruit seed and other plant material=14; a small village and isolated farmhouses, but their effects are Jedrzejewska and Jedrzejewski 1998). We used the correc- extended to the areas where cork and livestock are pro- tion factors available in the literature for stone marten be- duced. Hunting and logging are occasional. cause all four carnivores consumed similar frequency of the 54 WEB ECOLOGY 7, 2007 different prey items (Jedrzejewska and Jedrzejewski 1998). calculated using Pianka’s index (for the food categories All the items were converted into biomass, except the plant and families), varying from 0 (exclusive food niches) to 1 (non-fruit) material, which does not contribute with si- (complete dietary overlap) (Krebs 1989). All calculations gnificant energetic value in the diet for these carnivores were performed in Microsoft Excel 2003, SPSS for Win- (Libois and Waechter 1991) and was excluded from fur- dows, release 11.5.0 (SPSS Inc.) and Statistica ver. 5 soft- ther analysis. ware. Prey availability estimation was directed towards small mammals, the main prey items identified by previous studies on the diet of these species (Palomares and Delibes Results 1991).