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?

What They Did

This week’s researchers looked at 2 fish species, the Mottled Sculpin, which has a low stress tolerance and poor ability to disperse, and the Mountain Sucker, which is more tolerant and more mobile. They looked at 64 different sites across 2 stream drainages for seven years, testing for land use change and average river discharge for the year. They then compared these metrics to the abundance and presence of the two species.

The study is particularly novel as it specifically looks at the effect of oil and natural gas development (ONGD). One stream network had significantly more ONGD along its course, and total land use change was measured as percentage of nearby land surface area that was occupied by ONGD structures.

Did You Know: Measuring Discharge

Measuring the amount of water that flows through a river is a complicated business. There are a huge variety of ways to do it, from complicated machinery to simply measuring width and depth at certain points, to seeing how long it takes a tennis ball to float down a river (not kidding). Techniques vary in accuracy. The technical equipment used here is generally on the more accurate side, but it is possible to get reasonable reading from much cheaper methods.

What They Found

As the researchers had guessed, both species were positively affected by discharge. This makes sense, as the more water you have flowing through a river, the happier fish will be. But whereas the Sucker didn’t seem bothered by changes in land use, it made the effect of discharge on the Sculpin even more pronounced. This essentially means that in areas where land use was higher, a lower discharge was likely to have a much worse effect on the Sculpin’s ability to persist.

Southdown Power Station

The construction of natural gas plants like this one mean that nearby vegetation needs to be cleared, which reduces local water quality (Image Credit: russellstreet, CC BY-SA 2.0)

Problems

This is probably inherent bias as we’ve recently interviewed Jane Reid on the value of long term studies. I can’t help but think how useful it would be to have data from before the ONGD started. Using a nearby system which has been subject to less human development is of course a pretty good substitute. There were also several environmental factors known to have an effect on Mountain Sucker populations which were not measured here, which could have made the statistical modelling a bit more comprehensive (although adding even more stressors to the situation would have potentially made it needlessly complicated).

So What?

In areas where land use has intensified close to rivers, knowing when local populations will be hit hardest is an important conservation tool. The knowledge that Sculpin will be hit hard during periods of low discharge in areas where human land use is high means we can take steps to ensure their persistence. Discharge in this river system was highly related to how much snow had formed during the preceding winter, which means that action can be taken well ahead of time to ensure the species’ local persistence.

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