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“Nonlinear effects of intraspecific competition alter landscape-wide scaling up of ecosystem function”

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Chelsea Jean Little, Emanuel A. Fronhofer, and Florian Altermatt (March 2020)

Macroinvertebrates process leaf litter in streams in nonlinear relation to their population density

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Forested headwater streams such as this one in Switzerland receive substantial inputs of leaf litter, which are processed by shredding macroinvertebrates.<br />(Credit: Chelsea J. Little)
Forested headwater streams such as this one in Switzerland receive substantial inputs of leaf litter, which are processed by shredding macroinvertebrates.
(Credit: Chelsea J. Little)

Scientists have always been interested in understanding populations and processes at large scales. How many of this kind of animal are there in the world? How much plant growth is produced globally each year? What’s the biomass of trees in this state or province? In an era of global change, interest in predicting these kinds of measures is growing, because it’s apparent that nature contributes a lot to human wellbeing.

But getting answers to these questions is not always easy. In this research, Little et al. asked how much leaf litter was processed by invertebrates in entire stream catchments. Leaves and other dead material from trees are an important resource input to freshwater ecosystems, which don’t produce as much of their own plant material as you can find in a forest. That dead material is often processed by invertebrates (but also microbes and sometimes even fish), which incorporates it into the freshwater food web.

In a lab experiment, Little et al. showed that the rate at which invertebrates consume leaf litter depends on the density of invertebrates: the more neighbors they have, the less they eat. This turned out to be very important in predicting how much leaf litter might be processed in whole stream catchments. Because the relationship was nonlinear, simply extrapolating the leaf processing rate of the average density of invertebrates would not produce an accurate prediction. Instead, the authors used geostatistical modeling to estimate the distribution of invertebrates in the stream catchments based on field survey data, and then to map the predicted the variation in leaf litter processing in space through the catchments.

Little et al. worked in ten streams in eastern Switzerland which were roughly matched for stream length and catchment size. The nonlinear density dependence of leaf consumption, in combination with differences in invertebrate distributions between the catchments, led to a 40-fold difference in how much leaf litter was predicted to be processed between the lowest- and highest-processing rate streams. This is potentially important because it determines how much terrestrial material is available to the aquatic food web. It also suggests that depending on the stream catchment, some dead terrestrial material may be exported downstream and end up in a lake to decompose, rather than being processed in the stream.


Abstract

A major focus of ecology is to understand and predict ecosystem function across scales. Many ecosystem functions are only measured at local scales, while their effects occur at a landscape level. Here, we investigate how landscape-scale predictions of ecosystem function depend on intraspecific competition, a fine-scale process, by manipulating intraspecific density of shredding macroinvertebrates and examining effects on leaf litter decomposition, a key function in freshwater ecosystems. For two species, we found that per-capita leaf processing rates declined with increasing density following power functions with negative exponents, likely due to interference competition. To demonstrate consequences of this nonlinearity, we scaled up estimates of leaf litter processing from shredder abundance surveys in 10 replicated headwater streams. In accordance with Jensen’s inequality, applying density-dependent consumption rates reduced estimates of catchment-scale leaf consumption by an order of magnitude relative to density-independent rates. Density-dependent consumption estimates aligned closely with metabolic requirements in catchments with large, but not small, shredder populations. Importantly, shredder abundance was not limited by leaf litter availability and catchment-level leaf litter supply was much higher than estimated consumption. Thus leaf litter processing was not limited by resource supply. Our work highlights the need for scaling-up which accounts for intraspecific interactions.