Niche conservatism and evolutionary determinants of terrestrial vertebrate diversity patterns

Abstract

Large scale study of diversity patterns has the purpose of describing emergent properties of the ecosystems and biota which in occasions do not manifest in more detailscale investigations. At the biogeographic and macroecological scales, diversity patterns are the product of biota’s responses to current environmental conditions and past environmental changes that can be researched based on evolutionary phenomena (speciation, extinction) or historical events (dispersion). Works integrating macroecology and macroevolution are developing and the scientific community is still on the phase of creating and trying to reach a consensus on methodologies allowing such integration. This thesis uses four case studies with the aim of supporting the suitability of schemes incorporating contemporary, evolutionary and historical aspects of the biotas to reach more complete explanations for the mechanisms generating diversity patterns. During the development of this thesis we have created databases using empirical data from different geographical regions, which have been analyzed using geographical information systems, statistical models and phylogenetic analysis methods. Besides, all case studies share in common the assemblage approach, which uses grid cells uniformly distributed across the study area as analysis units. The choice of this approach is related to the fact that all patterns and processes studied have a geographical expression and therefore their study requires spatially explicit methods. Intending to achieve a wide range of representation, the case studies analyze three diversity patterns (i.e. species richness, body size, range size), three different taxonomic groups (i.e. reptiles, birds and mammals) and three geographical regions (i.e. East and South of Africa, the New World, the Globe). Far from trying to find methodological unification, the analyses are adapted to the idiosyncrasy of each specific question that are studied in the following cases. In the first place, we explore the species richness patterns of five reptile groups from South and East Africa and both relationships with current climate and probable ancestral relationships with palaeoclimates where the groups were originated are analyzed. Information provided by the fossil record, dated phylogenies and palaeoclimatic reconstructions. We found coincidence between climatic conditions preferred nowadays and ancestral climatic conditions for the different groups of reptiles suggesting that niche conservatism is valid to interpret African reptile richness patterns. Secondly, we investigate the extent to which evolutionary relationships between species of oscine passerine birds can explain their geographical range size patterns in the New World. Range size patterns are documented for migratory and nonmigratory species and are partitioned into the portion explained by evolutionary relationships and the portion that is independent from them by using phylogenetic methods of partition of the variance. The results indicate that range size is moderately heritable for this group of birds and hence its study needs to account for the role played by evolution. Furthermore, the patterns are different for migratory and non-migratory species, supporting the effect of mesoscale climatic gradients that are found in mountainous regions. Regional differences and phylogenetic structure found in migratory ranges, suggest the convenience of including other biological traits as dispersal ability, to reach a bettern understanding of the processes conditioning the biogeography of oscine range size. In third place, we examine the relative contributions of deep evolutionary relationships to the global bird body size gradient, distinguishing between the part of the gradient which is phylogenetically structured from the part that is independent from the phylogeny and thus might respond to adaptive phenomena under the family level. In this case, apart from the phylogenetic partition of the variance we use partial regressions to account the portions of the variance in bird body size that are explained by the phylogeny, by environmental gradients or by both simultaneously. Results show a strong effect of evolutionary relationships over the body size gradient. Also identify primary productivity either in combination with temperatura in the New World or with seasonality in the Olod World, as the primary environmental descriptor of bird body size. This supports the hypotheses of resistance to starvation and heat conservation to explain the portion of body size independent from evolutionary relationships. Finally, we inspect the effects that historical large scale dispersal events such as the biotic exchanges occurred during the Pleistocene, might have had on the mammal body size gradient in the New World. The body size patterns of the groups of mammals that participated in the biotic exchanges of and the groups that did not participate are compared, as are their phylogenetic signals and their associations with environmental gradients. Results inform of a strong, geographically localized influence of allochthon mammals over the body size gradient of all species. We also find marked differences between the sizes of autochthons and allochthons and their phylogenetic signal or their associations with climate. These differences can be interpreted based on the more recent arrival of allochthon species to North and South America in combination with niche conservatism dynamics. The findings of this thesis are relevant to macroecology and emphasize that including evolutionary and historical processes is key to understanding biogeographical diversity patterns. Phylogenetic relationships, ancestry and its associations with palaeoclimates, the fossil record and the biotic exchanges occurred due to palaeogeographic changes should be accounted for to bring us closer to the role of niche conservatism in configuring Earth´s biotas.