We’re crossing these bridges (and streams)

Crossing a stream on a drive may feel like a treat, but would bugs residing in the stream feel the same way? The development and use of roads and bridges across streams may not be so harmless. Through changes to the local environment during and after construction, the physical characteristics of the crossed stream can undergo substantial change, leading to decreased native bug biodiversity and increased abundance of nonnative species in streams.

Citation: Gál, B., Weiperth, A., Farkas, J. et al. The effects of road crossings on stream macro-invertebrate diversity. Biodiversity and Conservation 29, 729–745 (2020). https://doi.org/10.1007/s10531-019-01907-4

Hit the road and the road hits back

Humans have a rich and diverse history of causing biodiversity decline through habitat degradation, spreading nonnative species, and much more. Roads, rails, and pipelines are among the most widespread alterations to the face of the Earth. As landscapes are often naturally full of streams, where the natural and unnatural routes intersect can be additional sites of degradation in the environment and biodiversity.

A trafficked bridge crossing a stream. Developing and using bridges like this one can have negative impacts on the streams below. Photo provided by G. Frandson.

These intersections, especially of roads and streams (“crossings”), can prevent rainfall from properly soaking into the ground or running into water bodies; clear vegetation; lead to increased runoff of sediments, heavy metals, salts, and other pollutants; and lead to modifications of the stream channel itself due to differences in flow velocity, turbulence, and extra sediment. All of these changes and more still can lead the stream downstream of a crossing to potentially look and behave, both physically and biologically, significantly differently to the upstream section of stream. Macroinvertebrates (“bugs,” like insects, worms, crustaceans, and mollusks) are greatly important residents of streams, and many are picky about their living conditions. A healthy bug community is key for a healthy stream system in large part due to their linking of primary production, like algae, to higher levels in the food web. Crossings might make healthy conditions harder to obtain.

Drs. Gál, Weiperth, Farkas, and Schmera set out to better understand the effects of such a common yet understudied human alteration to the landscape on some of the organisms who call it home. Given the increasing abundance of crossings, the sooner we can build a better picture of their impacts, the better!

Hungry for data

Bugs from a stream. Building a picture of a macroinvertebrate community involves identifying its members under a microscope. Photo provided by G. Frandson.

The team of scientists conducted their study in nine similar streams all located in rural Hungary. The streams were each separated into three components: upstream, crossing, and downstream. A host of parameters, including water temperature, substrate composition, pH, conductivity (a measure of dissolved ions, or charged particles), and salinity, were taken from each component three times over the course of nine months. Bugs were also collected from the stream during each of these sampling events, with multiple samples taken each time to better represent all the microhabitats present in the streams: if you just sample a few times, you might miss a small patch of leaves or a tree’s roots that act as their own unique environments within the stream. These bugs were then identified and categorized as being native or nonnative. By combining locational, physical/environmental, and biological data, the team of scientists were able to assemble a picture of what relationships, if any, existed and informed on how crossing ultimately affected bugs’ richness, abundance, and other metrics of community structure.

It turns out that bugs hit the road

Drs. Gál, Weiperth, Farkas, and Schmera set out to answer a few questions: were upstream and downstream sections of stream indeed different? Did the crossing decrease native bug biodiversity and perhaps lead to increased nonnative species? and did the bug community change in composition between up- and downstream sections of stream? Due to the known effects of crossings’ development and use and the tolerances of various types of bugs to certain environmental conditions, the team of scientists had some expectations for the results. These expectations were met!

Gabion, wire-encased stone used for stabilizing stream banks. It is often found around stream-crossing bridges. From the Wikimedia Commons.

Whereas upstream stretches of stream were close to if not entirely natural, the crossing and downstream sections were markedly altered, most notably due to the presence of concrete stream beds, riprap, and gabion, a wire netting holding stones, at the crossings. That these alterations at the crossings were felt downstream was not surprising; water erodes material, even concrete, and will transport it downstream, and the presence alone of a foreign material is enough to throw off the natural flow of things downstream. The simplified habitat created by these alterations is a known stressor to bugs who typically do better with more complex habitats available to them.

Both the raw numbers of bugs and the numbers of different types of bugs both decreased downstream of the crossings, and there were more nonnative species found in the stretch of stream closest to the crossing than in the up- or downstream stretches. All the data they collected on bugs pointed in the same direction: crossings negatively impact the biodiversity of native bugs in no small part because of altered physical habitat and flow conditions. That some of the natives could be replaced by nonnatives in the crossing sections is not good news, either; while total numbers of organisms might not be as different because of their introduction, nonnative species are problematic, sometimes easily outcompeting native species or introducing themselves as a foreign food resource, sending ripples through the broader food web.

Though there were some safe predictions made about what effect roads crossing over streams may have on the bugs residing in the streams, it is great practice, especially when many studies have not already been done, to verify or counter these predictions. Drs. Gál, Weiperth, Farkas, and Schmera demonstrated that, despite road crossing seeming to be only a relatively moderate impact on stream ecosystems, the sum of their changes on the biodiversity of stream bugs can be quite significant, encouraging others to conduct similar studies across their world. Perhaps it’s a rural Hungarian phenomenon, or perhaps we need to reevaluate the impacts of our actions!

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Garrett Frandson

I'm a master's student at the University of Missouri studying stream macroinvertebrates and the substrate they inhabit. I'm broadly interested in bugs, streams, drivers of change including climate change and other anthropogenic disturbances, and communicating the value and beauty of natural systems and the need to protect them to the general public and those who can enact policy changes. When not working, I'm probably poking around and photographing these systems and their inhabitants! IG: @frandsong

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