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How do multiple stressors affect food webs in South Wales streams?

December 21, 2017
Welsh stream

A Welsh stream. Image: Kev Lewis | Flickr Creative Commons

Human activities are altering freshwater ecosystems and biodiversity across the world at an unprecedented rate. However, predicting how freshwater species populations are affected by multiple stressors is often difficult, as a result of the complex and interactive ecological processes such as trophic links, competition and mutualism that take place across an ecosystem.

In turn, changes in species composition can have unexpected or emergent effects on ecosystem processes and dynamics. For example, changes in food availability and competition for predator species can lead to ‘trophic cascades’ throughout an ecosystem. As a result, predicting and mitigating the ecosystem-level effects of human stressors requires a detailed understanding of such ‘inter-specific’ interactions between species.

Stream ecosystems are often extremely sensitive to the impacts of human disturbance. The intensification of catchment land use – often agriculture and urbanisation –  is a major driver of biodiversity loss in streams. Bottom-dwelling aquatic insects (or ‘benthic macroinvertebrates’) are often abundant in stream ecosystems, and their communities play key roles in a wide range of ecosystem processes.

However, despite increasing evidence that macroinvertebrate communities are being altered by human pressures, there has been relatively little research on the effects such alterations have on wider freshwater food webs.


Stonefly. Image: Wlodzimierz | Wikipedia Creative Commons

A new open-access study published in Molecular Ecology aims to address this shortfall. A research team led by Caitlin Pearson from the Cardiff School of Biosciences, Cardiff University used a DNA technique called ‘next generation sequencing‘ to investigate the feeding habits of two macroinvertebrates – the Caddisfly, Rhyacophila dorsalis and the Stonefly, Dinocras cephalotesin streams affected by different intensities of livestock farming.

The study – carried out along ten upland streams in South Wales – allowed the research team to analyse how predatory macroinvertebrate diets varied along a land-use intensity gradient. As a result, it offers new insights into how human stressors affect ecological communities in streams.

Lead author of the study, Caitlin Pearson – who is now heavily involved in river conservation with the UK’s West Cumbrian Rivers Trust –  summarises the key findings:

“We know that agricultural land use can affect streams and the organisms that live there, but know less of the consequences for stream food webs.  In part this is because of the difficulties of resolving feeding interactions, particularly among small organisms or where predators eat soft-bodied prey that leave no remains.  We tried to overcome these problem here by probing the guts of predators simultaneously for the DNA of many possible prey types using next generation sequencing.

Both of the predators we investigated were generalists, consuming overall 30 different prey taxa in proportion to their availability.  This included greater use at agricultural sites of the prey that were tolerant of agricultural conditions.  These are important new insights that tell us something of the mechanisms through which how land use affects stream communities, and potentially stream ecosystem functioning.”


Caddisfly. Image: Janet Graham | Flickr Creative Commons

Cardiff University Professor of Ecology and MARS researcher Steve Ormerod continues:

“Modern molecular methods – such as next generation sequencing – are adding very powerful tools to the work of freshwater ecologists.  In this example, we’ve not only gained a wholly new view of the way invertebrate predators feed, but also we have a new perspective of the way in which agricultural intensification affects stream ecosystems.

Predators such as the large and beautiful stonefly Dinocras cepahalotes are being lost from these sites more likely because of physical stressors such as sedimentation rather than because their prey are depleted.  This is in line with similar effects we’ve seen previously (Larsen & Ormerod 2010), and illustrates how classical and modern ecological methods can work together for river management.”

Pearson CE et al (2017) “The effects of pastoral intensification on the feeding interactions of generalist predators in streams” Molecular Ecology, doi: 10.1111/mec.14459. (open-access pre-publication)

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