Our paper published in Nature Ecology and Evolution can be found here: https://rdcu.be/ZXpn

When it comes to explaining the structure of the natural world, ecologists have tended to take two quite different views. On the one hand, some ecologists think that biological communities are strongly regulated by interactions between species, with variation in community structure and diversity arising as a predictable outcome of differences in species ecological niches. Others, however, see a more capricious world, in which chance dispersal events and historical contingency dominate in explaining the distribution and diversity of life.

This debate—over whether ecological or biogeographical processes govern the assembly of species communities—led to the ‘null models wars’ of the 1970’s and remains as contentious as ever. Robert Macarthur’s ideas on both island biogeography and niche theory suggests that he simultaneously saw the world from both viewpoints, but such is the perceived gulf between these perspectives, that this has since become known as ‘Macarthur’s paradox’. Our paper in Nature Ecology and Evolution returns to this question. 

The seed of this paper was planted six years ago when I started a postdoc with Joe Tobias in the Edward Grey Institute at the University of Oxford. To some extent, Joe and I stood on either side of the debate among ecologists. While Joe believed that competition between species plays a substantial role in shaping biogeographic distributions of vertebrate animals, I felt that most of these patterns could probably be explained by simple neutral models involving geographic speciation and dispersal.

To test this, we returned to an old idea of looking at the geographic ranges of pairs of ‘sister species’. By looking at sister pairs of birds that diverged at different times in the past we could start to model the process through which species arising in different geographic places (allopatry) come back into coexistence (sympatry) and attempt to identify the factors limiting or promoting this transition. Using a handful of well studied families of South American birds, we found evidence that competition may delay sympatry—sister species with similar sized beaks and that presumably use similar resources—were less likely to make the switch to living together than species which had diverged more rapidly. However, we also found that species with more pointed wings, an indication of strong flight ability, were more likely to coexist, suggesting that dispersal may be the key factor limiting the build-up of sympatry. Whilst these results were tantalising, we lacked a global picture of how these factors fitted together and at the time we simply didn’t have the data needed to address this.

Catherine Sheard, a PhD student in Joe’s research group, made inroads into filling these gaps by spending months in museums, poring over thousands of bird skins, taking careful measurements of different traits such as the length of the beak, tarsus and wing. Together these traits would provide a detailed description of the species’ phenotype and thus a window into its ecological niche. I had also started working with Walter Jetz, who had recently assembled an evolutionary tree which included all bird species. Together, we realised that combining this phylogenetic and phenotypic data, along with information on species distributions and the environment, would provide an opportunity to re-examine the overarching question presented by Macarthur’s paradox: is ecology or biogeography the dominant factor limiting the overlap of species geographic ranges?

The answer is both complex and yet reassuringly simple. Rather than representing contradictory explanations, our results suggest that biogeography and ecology are better viewed as two complementary pieces of the same puzzle–the yin and yang of community assembly–with both sets of factors interacting in predictable ways to regulate the build-up of sympatric diversity. Our results show that different factors appear to dominate depending on the context and scale of focus, with dispersal instrumental in limiting the attainment of secondary contact and niche differences becoming progressively more important in explaining more extensive geographical overlap between related species. Thus, Macarthur’s paradox is perhaps not so paradoxical after all but instead an integral unifying model that is fundamental to understanding global patterns of coexistence.

Link to the paper: http://dx.doi.org/10.1038/s41559-018-0572-9