Functional redundancy increases in human-modified habitats

Different species do similar things in anthropic environments.

Above: Two examples of small mammals with very distinct biologies. The Brazilian gracile opossum Gracilinanus microtarsus (left) is a generalist that forages in the ground and inhabits a broad range of habitat types — from primary to secondary forests and forest edges. Such generalists usually profit from human habitat modifications. On the other side, the ihering’s hocicudo Brucepattersonius iheringi (right) is more specialized and spends the majority of its life digging and foraging in forest undergrounds. Pictures taken by Leonardo Crestani in the Aparados da Serra National Park, Rio Grande do Sul, Brazil.

Small mammals are practically everywhere. In fact, they are so successful that they can be found in almost all kinds of habitats. A popular saying states that “if you kick a stone, a mouse appears”. The truth might be not as extreme, but we are often not aware of how many of these often shy and elusive species are actually living around us. Sometimes we can get a glimpse at their abundance and diversity, when we see large numbers of them scurrying around after crop harvests or logging in forests. If they thrive so abundantly in frequently disturbed habitats, should we actually expect differences in species richness and ecological functions between human-modified and undisturbed, natural habitats?

Editors’ choice article: (Free to read online for two years.)
Luza, A. L., Graham, C. H., Hartz, S. M., & Karger, D. N. (2021). Functional redundancy of non-volant small mammals increases in human-modified habitats. Journal of Biogeography, 48, 2967– 2980. https://doi.org/10.1111/jbi.14264 

Ecological theory and empirical data already provide evidence that small mammals are quick in recolonizing habitats after disturbances took place, cancelling out the local extinctions that happened. But simply replacing a locally extinct population with that of another species is just one side of a coin. It might keep the number of species constant, but it will not tell us much about the structural changes within these small mammal communities. A functionally unique species might simply be replaced by a generalist that does not add anything ecologically new to an ecosystem, or replaces a function that has been lost (Figure 1). To understand the structural dynamics of small mammal communities in modified habitats (Figure 2), we therefore need metrics that are sensitive to changes in species composition and ecological functions, like functional diversity, which measures the range of functional traits found into local communities.

Bearing the apparent success of small mammals in human-modified habitats in mind, I went to the literature. I searched for global, geographically replicated data on small mammal communities to understand if the ‘success’ of small mammals in human modified habitats generally translates into more or fewer ecological functions. These data already gave a good overview on the status quo of small mammal diversity in a wide variety of habitats. However, a major hurdle needed to be overcome to draw global, and general conclusions from these data. The diversity of small mammals is geographically very heterogeneous, with for example many more species in the tropics than in the subtropics. These global patterns in species richness are the effect of millions of years of biogeographic and evolutionary processes that created a geographically very distinct biodiversity of small mammals. While a success story for small mammals, it became apparent that these biogeographical differences actually pose a scientific problem in better understanding species richness and functional changes in human-modified habitats at a specific location. How could we be sure that functional diversity at one location is lower than at another, if one of them has a smaller species pool, i.e. less species around that can colonize a habitat due to different biogeographical histories? Equipped with this question I went over to the Swiss Federal Research Institute to work with Dirk Karger and Catherine Graham, to include a framework that explicitly considers the size and structure of the species pool while measuring small mammal community responses to human influence.

Human modified (top) and natural grasslands (bottom) in the municipality of Palmas, Paraná, southern Brazil. Cattle raising has been a predominant economic activity in the region for at least three centuries. Currently, these grasslands have been afforested with exotic pine or converted into crop fields, resulting in dramatic alterations in the landscape and in the economy of the region. Elusive to the human eye, these natural habitats are filled with a large number of small mammal species with a set of unique traits that provide many important ecosystem functions. Pictures taken by A.L. Luza.

The framework we used basically involved creating a spatially explicit potential species pool for all locations globally. Now we were able to see the ‘potential’ species richness and ‘potential’ functional diversity of a locality, and could compare it to the richness and functional diversity we actually observed. The first thing we saw was that most human-modified habitats are actually quite close to what they are able to achieve in terms of species numbers. While this sounded like good news in the beginning, a second pattern quickly emerged. A higher species richness in human-modified habitats comes with an increase in functional redundancy, as those species which profit from human-modification do not add new functional traits into modified habitats. This means that practically, the species profiting from human habitat modifications all do the same when it comes to their functional role in an ecosystem.

So what exactly should we take from these results? Mainly that human modifications are changing the functional roles of small mammal communities by making generalists the winners, and severely penalizing functionally distinct species. Humans are homogenizing the functions in ecosystems, which could lead to severe changes in ecosystem services and nature’s contribution to people.

Written by:
André Luza; Institution: Universidade Federal do Rio Grande do Sul

Additional information:
https://luzaandre.weebly.com/

Acknowledgements:
I would like to thank Dirk N. Karger for helping me write this blog, as well as Catherine H. Graham, and Sandra Maria Hartz for many fruitful discussions on these topics during the course of my PhD.