ECR feature: Brooks Kohli on the role of abiotic filters and competition assembling small mammals communities

Brooks Kohli is a postdoc at Ohio State University in the USA. He is an ecologist interested in understanding small mammal diversity and distribution. Here, Brooks shares his recent work on small mammal trait-base tests to reveal the importance of abiotic filters.

Brooks Kohli, from the summit of Bald Mountain (in Great Basin National Park, Snake Range, Nevada, USA), with Wheeler Peak, the highest peak of the interior Great Basin, in the background (Photo by Brooks Kohli).

Personal links. Twitter | Personal Website

Institute. Ohio State University

Academic life stage. Postdoc

Major research themes. Community assembly, biogeography, functional diversity, especially of small mammals 🐀🐿

Current study system. Rather than focusing on individual species, I study entire assemblages of small mammals (roughly rabbit- or squirrel-sized and smaller) and how their diversity and community composition change along environmental gradients. The best part about it is I get to work with all the amazing diversity of small mammals in a region, everything from kangaroo rats to water shrews to bushy-tailed woodrats!  I currently use mountains as study systems because of the rapid turnover in temperature, precipitation, and habitats tied to elevation that occurs over relatively short distances, making it easier to span a wide gradient that is essentially replicated across independent mountains. 

A water shrew (Sorex navigator), one of the 34 species studied in this paper, captured during small mammal surveys in the mountains across the Great Basin, Nevada, USA.  The large hind feet have stiff, bristly hairs on the outer margin that help this small semi-aquatic mammal swim in fast-slowing mountain streams to catch insects to eat (Photo by Brooks Kohli).

Recent JBI paper. Kohli, B. A., Stevens, R. D., Rickart, E. A., & Rowe, R. J. (2021). Mammals on mountainsides revisited: Trait‐based tests of assembly reveal the importance of abiotic filters. Journal of Biogeography, 48(7), 1606-1621

Motivation behind this paper. For decades, small mammals have been an important group for studying how species richness patterns change with elevation, as well as for developing and testing community assembly theory, which seeks to explain why certain species coexist where they do and why others do not, due to factors such as abiotic conditions or competition for resources. Still, these perspectives have rarely been combined, despite the recent development of functional and phylogenetic diversity methods that make it possible. For my dissertation, I wanted to take advantage of the excellent study system mammals on mountainsides provide and harness the power of the trait-based approach to infer assembly processes. Specifically, we wanted to test classic community assembly hypotheses about the relative importance of competition versus environmental filtering in structuring small mammal assemblages. We were also curious about whether functional and phylogenetic diversity patterns would differ from the typical mid-elevational peak in species richness observed for small mammals, as a growing body of evidence suggests that species richness is not a good proxy for other aspects of diversity.

Variety of habitat types typical of the mid-high elevations in Great Basin mountain ranges (Photo by Brooks Kohli).

Key methodologies. With the objective to quantify multiple dimensions of small mammal diversity across elevations, this work combined data from many years of intensive field surveys (led by myself, some of my co-authors, and involving many other colleagues, students, and volunteers), trait data collection directly from museum specimens, and compilation of other trait data and phylogenetic relationships from existing resources. The key to our analysis was identifying alternative hypotheses and using only those functional traits most tightly linked to the process of interest to test each of them separately. For example, testing abiotic filtering hypotheses only based on traits that most directly reflect environmental tolerances and limits of species. Parsing the data is critical because multiple processes likely act on different traits simultaneously to structure assemblages and may produce opposing patterns that cannot be detected when all are analysed together.

Creekside riparian habitat in upper elevations of the Snake Range (Photo: Brooks Kohli).

Unexpected challenges. One of the biggest challenges was a seemingly simple task: choosing which functional traits to include and determining which ones should be used to test abiotic filtering or competition. This is a persistent challenge in trait-based ecology, and a critical one to grapple with to ensure the most meaningful interpretation of results from a study such as this. We weren’t satisfied with just using traits readily available in existing global mammal trait databases because they provide few continuous traits with strong ties to environmental tolerances. To remedy this shortcoming, we used several continuous traits that I had previously identified as reliable indicators of water and temperature requirements (e.g. relative medullary thickness of the kidney, fur density), and combined our knowledge of the species’ natural history with literature searches to construct additional finer resolution traits such as detailed habitat use and nest location.

A Great Basin pocket mouse (Perognathus mollipilosus), these mice have fur-lined cheek pouches that they gather seeds and other food in to store in their underground burrows (Photo: Jessica Taylor).

Major results. Our paper has two major results. The first is that abiotic filtering seems to be the most important thing driving community assembly for small mammal assemblages. In the three dry mountains we studied in the Great Basin of the Western US, this is especially true in the desert lowlands but also towards mountaintops, to a lesser degree. Second, even though species richness decreased substantially at high elevations, overall functional and phylogenetic diversity generally increased in all three mountains. This finding contradicts the general paradigm of community assembly patterns along elevational gradients, in which functional and phylogenetic diversity is expected to decrease as conditions become harsher at higher elevations. However, little previous work was done in dry regions, where the harshest conditions may not be on the mountaintops. Thus, our work emphasizes that more comparative work is needed on functional and phylogenetic diversity patterns on mountainsides.

Brooks Kohli, checking small mammal traps during field surveys (Photo: Jessica Taylor).

Next steps for this research. The next step is to jump to a macroecological scale to determine whether there are, in fact, consistent differences between wet versus dry mountains or tropical versus temperate mountains, or if the patterns we see in the Great Basin are an exception to a global rule. I am currently looking at this question during my postdoc along with my advisor, Marta Jarzyna. I’m very excited about this work because there have been no large-scale comparative analyses of mammal functional or phylogenetic diversity along elevational gradients to date.

If you could study any organism on Earth, what would it be? If I had to pick one, it would probably be the Inyo shrew (Sorex tenellus). This tiny insectivore only weighs 2-4g and was thought to be rare throughout its limited distribution in the western US until recently. Our recent surveys of mountains across the Great Basin and earlier ones led by my colleague, Eric Rickart, substantially increased the number of voucher specimens of the species and greatly expanded its known range. We encountered these shrews primarily on rocky talus slopes at high elevations, where they coexist with pikas and some other, more “charismatic” montane mammals.  It’s really exciting to contribute new knowledge about the distribution and natural history of a mammal species in such a well-studied region!

Profile of the Snake Range, Nevada, USA, from desert valley to mountain peaks (Photo: Brooks Kohli).

Anything else to add? One final note is that many people cringe at the thought of small mammals, especially rodents, which have been cast as vermin.  And while I understand not wanting a rat living in my walls, the diversity of wild rodents (mice, rats, squirrels, etc.) is astounding and there are more species than in any other group of mammals!  They are found all over the world in virtually all types of habitats and environments, and are key components of many ecological processes, including food webs, soil perturbation, pathogen transmission dynamics, seed and spore dispersal, and more! We have so much to learn from studying them!

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