To avoid competition, coexisting species use various resources that usually require different morphological adaptations (reviewed by Begon et al., 1990; Dayan & Simberloff, 1998; Moulton et al., 2001). For example, arboreal mammals need curved and sharp claws, while fossorial mammals require claws that are straight and blunt. Two animal species with a similar body form but different body sizes may avoid competition by using food items (Gittleman, 1985; Dayan & Simberloff, 1994) and shelters (burrows, rock crevices, tree hollows etc.) of different dimensions.
Several authors (Grant, 1972; Strong et al., 1979; Simberloff & Boecklen, 1981; Duncan & Blackburn, 2002) criticized the importance of interspecific competition. Begon et al. (1990, p. 738) concluded that even when interspecific competition is important, it affects only interactions between members of the same guild and even within a guild only those species closest together are likely to compete. Therefore, it is not always certain that the morphological overlap between two coexisting species leads to interspecific competition, but we can assume that morphologically different species are very unlikely to compete. These considerations were taken into account here to estimate the probability of interspecific competition between introduced and native rodents in Madagascar.
There are 9 genera and 24 species of native rodents in Madagascar (Goodman & Soarimalala, 2005; Goodman et al., 2005; Musser & Carleton, 2005), all belonging to the same endemic subfamily of morid rodents: the Nesomyinae. Their monophyletic origin, however, is a matter for discussion and was neither clearly confirmed nor rejected (Jansa et al., 1999; Jansa & Carleton, 2003a; Musser & Carleton, 2005).
The number of introduced species of rodents in Madagascar is three: the black rat Rattus rattus, the brown rat R. norvegicus, and the house mouse Mus musculus (Garbutt, 1999); all three are synanthropic species with worldwide distribution. The brown rat in Madagascar is restricted to urban environments, while the black rat is found throughout the island, occupying a variety of habitats from human dwellings to pristine rainforests (Goodman, 1995). However, the abundance of R. rattus increases with the level of habitat disturbance (Lehtonen et al., 2001). The oldest record of R. rattus in Madagascar is from an 11th-14th century Islamic archaeological site in the northern part of the island (Rakotozafy, 1996; Radimilahy, 1997). An allozymic study demonstrated that specimens of R. rattus collected in different habitats and altitudes in Madagascar all belong to the same species and present the same diploid number (2n = 38) (Duplantier et al., 2003).
The house mouse is found in houses, rice fields, savannas, and marshes, but never in closed forests (Langrand & Goodman, 1997; Rakotondravony & Randrianjafy, 1998; Lehtonen et al., 2001). A mitochondrial study showed that the house mouse in Madagascar originated from the Arabian Peninsula in a single colonization (Duplantier et al., 2002).
The introduction of alien rodents may threaten native rodent species of Madagascar (Goodman, 1995). The more so as in certain intact areas of primary forest 96% of the rodents captured were R. rattus (Goodman et a1., 1997). It was suggested that R. rattus competes with nesomyine rodents as a result of overlap in their food preferences (Goodman & Sterling, 1996). However, more detailed field studies are needed to clear up this competition hypothesis. Steps in this direction were taken by Ramanamanjato & Ganzhorn (2001) and Ganzhorn (2003). Ganzhorn (2003) stated that for the time being it is impossible to evaluate the effects of introduced rats on the native mammal fauna of Madagascar.
The main goal of this work is to provide a preliminary estimation of the potential for interspecific competition between introduced and native rodents in Madagascar. …