Why do species interact? A test of four hypotheses using Ceropegia (Apocynaceae) as a case study

Interactions between species form the basis for community structure and ecosystem function in all terrestrial and marine habitats, via energy flow and nutrient movement between trophic levels, and top-down and bottom-up control of species’ abundances. In addition, species interactions provide the impetus for the evolution of a huge range of biological novelty. In the absence of relationships such as mutualism, predation, parasitism, competition and commensalism, biodiversity would very much simpler than it is today, and the biosphere would be very different. Despite this central importance of species interactions in relation to ecology, evolution and conservation biology, there is much that we do not understand about why certain species interact, why other species are excluded from those interactions, and how this in turn promotes organismal diversification through adaptation and co-evolution. In this study we test four non-exclusive hypotheses which relate to these questions, using the genus Ceropegia L. (Apocynaceae: Asclepiadoideae, Ceropegieae) as a case study. Ceropegia is a large Old World taxon of over 180 accepted species, with new species being regularly discovered. We tested four non-exclusive hypotheses to account for why species interact using flower visitor data for over 60 species of Ceropegia across its range in relation to a cpDNA-nrDNA molecular phylogeny of the genus. The hypotheses were: Hypothesis 1 – the coevolutionary hypothesis – this states that the pattern of species interactions that we recognise today is the result of reciprocal evolution between two unrelated clades of organisms, such that speciation in one clade results in speciation in the second clade. Hypothesis 2 – the phylogenetic hypothesis – this relates to the fact that species are the products of the evolution of their ancestors. Thus, as well as evolving adaptations that relate to their life histories and behaviour at the current time, they are influenced by a range of phylogenetically conservative traits which (presumably) evolved in the distant ancestral past of that clade. Hypothesis 3 – the biogeographic contingency hypothesis – states that species interact with one another on the basis of their current global and local distributions, i.e. they interact because, spatially, they can interact. Hypothesis 4 – the local adaptation hypothesis – states that those features of the organismal phenotype which are required adaptations for a particular interaction have generally evolved locally (i.e. within the native range of the organism) in response to natural selection imposed by the interacting partner.