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Are the Combined Effects of Human Development Worse Than the Sum of Their Parts?

Decreases in river discharge can negatively affect fish like this sucker, but what happens when they’re compounded by local changes in land use? (Image Credit: Hotash, CC BY 2.0, Image Cropped)

Anthropogenic land-use change intensifies the effect of lows flows on stream fishes (2019) Walker, Girard, Alford & Walters, Journal of Applied Ecology, https://doi.org/10.1111/1365-2664.13517

The Crux

Human activity can create a lot of different problems for the world’s ecosystems. These problems can impact an ecosystem simultaneously, often in different ways. For instance, a warming climate might push some species further towards the poles, but human structures like factories or mines might impede their dispersal. It’s relatively easy to study the effect of any one stressor that we place on a species, but looking at the interaction of multiple human-caused stressors is more difficult.

Take freshwater ecosystems. A warming climate means that there’s less snow and more rain in the winter, which reduces the river’s flow (or discharge) in summer. At the same time, nearby human construction can reduce nearby plant life, which in turn increases the amount of sediment washed into a river and lowers water quality. But do the two effects combined simply equal the sum of their parts, or does that combination make the total effect on local species even worse?

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