Land‐use intensification systematically alters the size structure of aquatic communities in the Neotropics

Abstract

Land-use and land-cover transitions can affect biodiversity and ecosystem functioning in a myriad of ways, including how energy is transferred within food-webs. Size spectra (i.e. relationships between body size and biomass or abundance) provide a means to assess how food-webs respond to environmental stressors by depicting how energy is transferred from small to larger organisms. Here, we investigated changes in the size spectrum of aquatic macroinvertebrates along a broad land-use intensification gradient (from Atlantic Forest to mechanized agriculture) in 30 Brazilian streams.We expected to find a steeper size spectrum slope and lower total biomass in more disturbed streams due to higher energetic expenditure in physiologically stressful conditions, which has a disproportionate impact on large individuals. As expected, we found that more disturbed streams had fewer small organisms than pristine forest streams, but, surprisingly, they had shallower size spectrum slopes, which indicates that energy might be transferred more efficiently in disturbed streams. Disturbed streams were also less taxonomically diverse, suggesting that the potentially higher energy transfer in these webs might be channelled via a few efficient trophic links. However, because total biomass was higher in pristine streams, these sites still supported a greater number of larger organisms and longer food chains (i.e. larger size range). Our results indicate that land-use intensification decreases ecosystem stability and enhances vulnerability to population extinctions by reducing the possible energetic pathways while enhancing efficiency between the remaining food-web linkages. Our study represents a step forward in understanding how land-use intensification affects trophic interactions and ecosystem functioning in aquatic systems.

Maximum likelihood outperforms binning methods for detecting differences in abundance size spectra across environmental gradients

“…Previous work has investigated the accuracy and inherent biases associated with different estimation methods (Edwards et al, 2017(Edwards et al, , 2020White et al, 2007). However, the extent to which these inaccuracies and biases compound across Binning methods are easy to use and interpret, which most likely accounts for their wide use in ecological studies (Collyer et al, 2023;Martínez et al, 2016;Perkins et al, 2018). However, aggregating individuals into logarithmic bins removes a large amount of information within the data by collapsing body size variation into a single value within each bin.…”

“…Binning methods are easy to use and interpret, which most likely accounts for their wide use in ecological studies (Collyer et al., 2023; Martínez et al., 2016; Perkins et al., 2018). However, aggregating individuals into logarithmic bins removes a large amount of information within the data by collapsing body size variation into a single value within each bin.…”

Scaling approaches and macroecology provide a foundation for assessing ecological resilience in the Anthropocene

“…Several studies also report results consistent with the use of size distributions to assess the impact of disturbance. Recent work by Collyer et al [137] (figure 2b) assessed the relationship between macroinvertebrate size structure along a human land-use gradient (a surrogate for increasing perturbation and disturbance) in subtropical Brazilian streams. Consistent with expectations in box 2, they found that the size distribution is characterized by an approximate inverse power law with many small individuals and few large (figure 2b).…”

“…Theory predicts that for size spectra in undisturbed trophic networks the exponent should begin to approximate −1 as is observed here. Data and figure redrawn from Collyer et al [137]. (ii) Forest tree size distribution recovering from disturbance from approximately 100 000 forests plots in North America spanning disturbance gradients.…”

Untangling the complex food webs of tropical rainforest streams

“…Among these understudied ecosystems, streams of Brazilian Atlantic Rainforest are highly threatened through human land-use intensification and the substitution of forests to pasture and monoculture plantations (Siqueira et al, 2015). Since these ecosystems provide a myriad of services to human society, from fresh water and recreation to nutrient cycling (Meyer, 1997;Palmer et al, 2014), describing their food webs in well preserved regions is crucial to define the baseline in face of human impacts (Collyer et al, 2023).…”