Michael Krabbe Borregaard:
Geographical ranges in macroecology: Processes, patterns and implications

Date: 15-05-2010    Supervisor: Carsten Rahbek

This thesis investigates how ecological patterns at large scale are shaped by the geographic ranges of species. A species’ range is the geographic area in which an animal lives and breeds. The size of such ranges vary tremendously: some species only exist in a tiny area, whereas others, like humans, are distributed over the entire Earth. Species’ ranges are one of the basic units of the science of macroecology, which deals with patterns in the distribution of life on Earth. An example of such patterns is the large geographic variation in species richness between areas. These patterns are closely linked to the ranges of individual species, in two distinct ways: Ecology and evolution determine the ranges of species; and at the same time the ranges of species shape ecological patterns. This link between geographical ranges and macroecological patterns is the subject of the present thesis. To investigate the link, I draw upon a wide range of approaches, including statistical comparative analysis, computer simulations and null models. The core of the thesis is constituted by five independent scientific articles. These chapters fall naturally within two thematic groups:

The first group consists of articles that investigate how ecology and evolution determine species’ ranges. The central paper in this group is a large review article about one of the best described patterns in ecology: That species with large ranges tend to also be very locally abundant within their range. In the article I review the potential causes for this relationship. In going through the mechanisms, I distinguish between ‘structural’ causes, such as differences between the niches of species; and ‘dynamic’ causes, such as dispersal of individuals among populations. A central conclusion is that both of these types of mechanisms contribute to creating the relationship, although the causalities of the two types follow disparate pathways. A second paper addresses how the sizes of geographical ranges could be affected by evolution. Here I used a computer simulation to investigate the possibility that ranges are ‘inherited’ between species at speciation, constituting a species-level parallel of inheritance in individuals. Such inheritance is theoretically possible, though highly controversial. Nevertheless, a simulation model demonstrated that species-level heritability of range sizes is consistent with observed range-size distributions.

Finally, this thematic group includes a popularly written book chapter, where the causes and consequences of the spatial distribution of organisms are introduced more generally.

The second group consists of several papers investigating the link between ranges and richness patterns. Variation in species richness is probably a result of geographical variations in climatic conditions: humid and warm places are home to many more species and dry and cold areas. However, by investigating the coincidence of distributions of different species, I demonstrate that the regional fauna of areas is determined by the configuration of biomes; and that the size of this regional fauna plays an important role for creating patterns of species richness. A related approach to investigating the link between ranges and richness is to use so-called range-diversity plots, which are tools for describing covariance in species’ distributions. In the final paper, I consider the applicability of this approach, and define a set of null models for the interpretation of range-diversity plots.