How generally do humans reduce connectivity? Case studies abound, and they have filled the Digests of ConservationCorridor.org. But do these individual studies add up? Do they affect a broad array of species across different ecosystems?
To answer these questions, a recent study published in Science presents a synthesis of data across an impressive range of mammal species in studies located across the globe. The analysis spans movement studies of 57 mammal species (and 803 movement paths) that have been tracked via Global Positioning Systems. Best represented in the dataset are large animals, including Roe Deer, Moose, and Wild Boar, but other, smaller species include Verreaux’s Sifaka and the Brushtail Possum. This study impressed upon me the accelerating rate of detailed movement data that is becoming available.
To test the effects of human impact, the authors brought in the Human Footprint Index, a consolidated measure of the intensity of human land use and including human population density. They controlled for factors known to affect animal movement, including ecosystem productivity (hypothesizing that lower quality would lead to longer movements) and animal body size (hypothesizing that larger animals would move further).
The findings provide the most general demonstration available on the degrading effects of people on animal movements. Across species that encompassed a range of expected movement rates (correlated with body size), urbanization, agricultural conversion, and road expansion combined to reduce by up to half the movement rates across mammals. Although these rates were measured for up to 10 days, effects became stronger and stronger over time, suggesting even higher impacts for migrating mammals over their full life cycle.
The authors identified two reasons by which a higher Human Footprint might reduce movement rates. One reason is that the footprint fragments landscapes and raises more barriers to dispersal. The other is that humans might provide more resources for some species, causing them to remain more locally. Across the mammals in this study, the study found either or both mechanisms were possible.
The general finding of this study is that the growing footprint imposed on the environment by people is reducing landscape connectivity across mammals. Lower movement can propagate to reduced population viability, less connected foodwebs, and less functional ecosystems. The generality of these results further emphasize the need for conservation plans that enhance landscape connectivity.
Tucker, M. A., et al. 2018. Moving in the Anthropocene: global reductions in terrestrial mammalian movements. Science 359: 466-469.