A generalist species of highly specialized individuals?

Abstract

The ecological niche and the species-environment relationship are both cornerstones of contemporary ecological science. The realized habitat niche defines the conditions in which a species occurs, is adapted and can thrive, and quantification of the species-environment relationship is a means to describe the realized habitat niche. A frequent, if unspoken, assumption in analyses of species-environment relationships and the ecological niche is that there is a common, stationary and stable relationship between a species and its environment. This implies, additionally, that this relationship applies to the species as a whole, or rather to all individuals of the species. However, another cornerstone of ecological science is that populations, and even individuals, differ in their genetic characteristics and the environmental influences that shape their behavior. Therefore, the species-environment relationship and the realized habitat niche are likely to vary intra-specifically. Uniformity in behaviour under different ecological circumstances or genetic homogeneity in response to spatially varying limiting factors are assumptions that should be investigated and tested. In this paper, using European wildcat (Felis silvestris) x domestic cat (F.catus) hybrids in Scotland as a policy-relevant exemplar, we explore what ecological modellers call nonstationary habitat use, and what field ecologists call individual variation, or, with an evolutionary perspective, intra-specific variation. We analyze the occurrence patterns and ecological response curves of 14 individual wildcat hybrids distributed across Scotland to assess how much individual variation there is in expressed patterns of habitat association across multiple environmental variables. We propose three conceptual models corresponding to three divergent patterns of habitat association for the sampled population: stationary generalist, stationary specialist, and nonstationary specialist. Each of these alternative hypotheses of habitat selection for wildcat hybrids had unique expectations for the shape and overlap of response curves along environmental variables, and for the degree of overlap between used and available habitat among individuals. We were able to show a high degree of individual heterogeneity and specialization across our small but geographically widespread sample. Our results support the hypothesis that wildcat hybrids in Scotland are nonstationary habitat specialists. That is, the habitat associations of wildcat hybrids are highly heterogeneous at an individual level, and that pooled analyses across individuals fails to completely represent the range or variation of individual responses, and also fails to represent the actual habitat selection response curves of any individual. This provides a compelling example of the highly variable and heterogenous nature of habitat association within a single species. Our results support other recent studies where species demonstrate local adaptations to available conditions.