Up silt creek without a niche: how do stream food webs respond to sedimentation?

This blog post is provided by Francis J. Burdon, Angus McIntosh, and Jon Harding and tells the #StoryBehindThePaper for their article ”Mechanisms of landscape disturbance: evidence from landscape disturbance”.

Food webs represent a holistic systems approach to characterizing patterns of biodiversity and energy flow by describing trophic interactions between consumers and resources. However, how these ecological networks respond to natural and anthropogenic perturbations remains poorly understood.

Our research, published in the Journal of Animal Ecology earlier this year, described aquatic food webs using invertebrate and fish community composition, functional traits, and stable isotopes from twelve agricultural streams along a landscape disturbance gradient.

We found that the dominant stressor along the disturbance gradient, fine inorganic sediment, was associated with reduced trophic diversity – stream invertebrates and fish became increasingly equivalent in isotopic niche space (Figure 1A). These changes appeared to be driven largely by a shift towards a food web based on terrestrial inputs including detritus (Figure 1B,C).

Figure 1: Stable isotope analyses show evidence for changes in food webs along a sedimentation gradient in twelve agricultural streams.

Muddy waters – the stream blues

An often-used quote in introductory university courses to hydrology and stream ecology states that “Rivers are the gutters down which flow the ruins of continents”; a comment made by Luna Leopold, son of the more famous wildlife ecologist and conservationist Aldo.

Inorganic sediment transport is a natural feature of river networks, but fine sediment pollution is considered a major stressor of aquatic ecosystems and their biodiversity around the world. One only needs to go walking alongside streams draining agricultural and other human-affected lands after heavy rain to see the scale of the problem – runoff like chocolate milk from freshly tilled fields pouring into streams, changing the water from clear to different hues of greys and browns.

Combined with other problems in these impacted streams, which can include channelization, flow extractions, and excessive aquatic weed growth, fine sediment often manifests as the dominant stressor when it settles on the streambed. This sedimentation degrades and removes benthic habitat that host diverse food webs of microbes, invertebrates, and higher consumers including fish.

Sediment impacts on stream food webs

In our study, we wanted to test hypotheses regarding changes to food webs in response to disturbance. Specifically, we wanted to use advanced methods for describing stream food webs to demonstrate systematic changes in ecosystem functioning due to sedimentation. We sampled twelve agricultural streams along a landscape disturbance gradient where sedimentation was the dominant stressor. Our streams were located on the Canterbury Plains in New Zealand’s Te Waipounamu (South Island), a region known for intensive agriculture.

The method we used to describe the stream food webs involved stable isotopes: samples from basal resources, invertebrates, and fish were analysed for differences in their carbon and nitrogen isotopes. Systematic changes in the ratios of isotopes often reflect the food sources of the consumer: bringing literal meaning to the phrase “you are what you eat”.

The author returning a tuna (longfin eel) to its stream home

Niche compression: losing your Eltonian job

We found that at sites with low levels of sediment disturbance, the isotopic approach described diverse food webs with different channels reflecting “green” (autochthonous) and “brown” (allochthonous) trophic pathways to top consumers such as New Zealand’s endemic longfinned eel (Anguilla dieffenbachii) and the introduced brown trout (Salmo trutta).

Each invertebrate and fish species were spread out in isotopic niche space, reflecting different ecological roles, or “professions” in their ecosystem consistent with the Eltonian niche. However, this harmonious coexistence appeared to be disrupted by landscape disturbance – along the sedimentation gradient invertebrate and fish species become increasingly equivalent in isotopic niche space (Figure 1A), as the “green” trophic pathway collapsed and was usurped by the “brown” food web channel.

Bayesian mixing-model analyses showed that invertebrates became increasingly reliant on fine particulate organic matter of terrestrial origin (Figure 1B), and the compression of the trophic niche was indicative of reduced functional diversity. Paradoxically, the trophic position of fish became higher over the sedimentation gradient, possibly reflecting reduced competition (due to lower abundances) and a greater reliance on piscivory and inputs of terrestrial prey (Figure 1C).

Our study demonstrates that pollution and hydrogeomorphic degradation associated with human land uses may have profound consequences for stream food webs by altering the trophic basis of production and reducing realized functional diversity (Figure 2). These changes potentially undermine higher consumers such as fish by reducing ecosystem carrying capacity and endangering population persistence.

Figure 2: Disturbances, such as those involving sedimentation, may drive niche compression: here we describe how changes in realized horizontal and vertical trophic diversity lead to changes in isotopic niche space.

Read the paper

Burdon, FJ, McIntosh, AR, Harding, JS. Mechanisms of trophic niche compression: Evidence from landscape disturbance. Journal of Animal Ecology. 2020; 89: 730– 744. https://doi.org/10.1111/1365-2656.13142

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