Exploring changes in mammalian population densities in response to humans

Submitted by editor on 28 September 2020.

By Marlee Tucker

The world is changing. Humans have modified a large proportion of the Earth’s land surface through a range of activities such as urbanisation and the expansion of agriculture. In addition to landscape changes, the presence of humans through activities, such as recreation and hunting can also have an impact. Human activities have altered species behaviours, distributions and abundances (Torres-Romero and Olalla-Tárraga 2015, Tucker et al. 2018, Bernardo-Madrid et al. 2019), it is likely that these developments have also altered mammal population densities. Population density patterns are important for understanding population dynamics and they are an important measure in conservation. However, collecting population density data is an intensive process and data are generally only collected at small scales, such as a single population, making it difficult to examine these patterns across multiple species or countries. 

In this study, we examine global patterns of mammalian population density to see whether we can detect any general influence of human impacts.

We used a global database of mammalian population densities including 468 species and 6729 density estimates. For each population density estimate, we attached additional information about the environment, such as human impacts (e.g., human footprint, night-time lights, human population density) and productivity. We also attached species information, such as body mass and trophic level. We then ran models examining how mammalian population density is influenced by human impacts, accounting for the productivity and species traits.

Our results suggest that on average, mammalian population densities tend to be higher in human-modified areas. For some species, human-modified areas are attractive due to the presence of anthropogenic resources (e.g., crops, human waste, supplemental feeding) and the potential reduction in competition and predation (e.g., human shield effect), supporting a larger number of individuals. While this might sound like good news, our results may not reflect a win for all species. First, only some species can adapt to living close to humans (species filtering), where some species are probably disappearing (e.g., large carnivores), while others are “thriving” (e.g., raccoons). So, while we might be seeing an increase in density on average, there is likely a corresponding decrease in species richness (the number of species) in human-modified areas. Our exploration of species richness indicated that indeed shifts in species composition could be occurring. Second, human-modified landscapes are often simplified compared to their “natural” state, and this reduction in complexity can reduce the number of available niches. This means that the few species that can fill these niches will be abundant, but again, there will be fewer species than in a more complex environment.

Our findings provide additional evidence that large-scale ecological patterns are being altered by human impacts, where some species will benefit, and others will be negatively impacted or even be lost.

 

References

Bernardo‐Madrid, R. et al. 2019. Human activity is altering the world’s zoogeographical regions. - Ecol. Lett. 22: 1297–1305.

Torres‐Romero, E. J. and Olalla‐Tárraga, M. Á. 2015. Untangling human and environmental effects on geographical gradients of mammal species richness: a global and regional evaluation. - J. Anim. Ecol. 84: 851–860.

Tucker, M. A. et al. 2018. Moving in the Anthropocene: Global reductions in terrestrial mammalian movements. - Science. 359: 466–469.

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