How do the geographical distributions of species drive patterns of biodiversity?

The rate of species formation is conditional on the relatedness of co-occurring species, with packing and competition apparently driving maximum diversity in single areas.

Above: The high diversity of species in the tropics, as seen here in Costa Rica, mean that many more species are found within smaller geographic areas

I have always been fascinated by the processes driving the vast variation in species we see around us today. A rich history of research suggests that this diversity should be regulated by whether species are found in the same geographic area as those closely related to them, which both compete for shared resources, as well as preventing the processes that lead to the formation of new species. Researchers are constantly collecting new detailed information about where specific species are found and how they are related. Using these large datasets covering thousands of species I tested firstly, whether there was support for previous hypotheses and secondly, whether groups with vastly different ecologies, e.g. flying versus land-based species, would show similar or divergent patterns. Quantifying these patterns across this number of species and different groups will hopefully allow insight into whether there are any general rules driving species diversity..

Editors’ Choice article: (Free to read online for a year.)
Crouch, N.M.A. (2021), Shared patterns of spatial accumulation of lineages across terrestrial vertebrates. J Biogeogr. 48

In this work I primarily use a term called “allopatry” which describes whether a population or species is geographically separated from other such groups. When I quantified allopatry using only species and their closest living relative I find, as predicted, new species form fastest when they are physically separate. This separation prevents exchange of genetic material while species adapt to different environments, causing populations to become distinct. However, when I defined allopatry for species using all other members of its family, regardless of how they are related, I find higher speciation rates when species co-occur. This suggests that for the greatest number of species to accumulate they must pack within a single area. This work therefore shows how quantifying biological patterns using different numbers of species can produce different results, and therefore further insight into evolutionary processes.

Red billed quelea (Quelea quelea) in Tsavo National Park, Kenya.

The most surprising aspect of the results for me was additional finding: a negative relationship between one definition of species co-occurrence and the number of species in a family. I had predicted a positive relationship because coexisting with fewer species should provide more opportunities for more species to form. Interpreting this result is challenging; one possibility is that, rather than new ecological opportunities determining how many species can be supported within a family, it is competition for shared resources within a single area or region that dictates species richness.

One of the most challenging aspects of this work is how to use information on species distributions to define allopatry. Although the overall distributions of species may overlap, individual species may exploit different features of the environment. For example, some species may live high in the treetops while others may spend most of their time on the ground. Integrating this information into an analysis like this is extremely challenging, but by repeating the analysis over such a large number it appears possible to identify general biological patterns.

This work certainly creates many exciting questions. For example, do these results translate to the marine environment where the barriers to species movements contrast dramatically to those on land, including long-distance dispersal of larval offspring. For land-based species, how do other aspects of their biology relate to these results? In particular, species’ morphology – which influences how they interact with the environment and resources – has long been thought to influence how species coexist, and it will be exciting to integrate those data in combination with the results of this study.

Written by:
Dr. Nicholas Crouch
Department of the Geophysical Sciences, The University of Chicago

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Published by jbiogeography

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