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

Individual body size distributions (ISD) within communities are remarkably consistent across habitats and spatiotemporal scales and can be represented by size spectra, which are described by a power law. The focus of size spectra analysis is to estimate the exponent ( λ ) of the power law. A common application of size spectra studies is to detect anthropogenic pressures. Many methods have been proposed for estimating λ most of which involve binning the data, counting the abundance within bins, and then fitting an ordinary least squares regression in log-log space. However, recent work has shown that binning procedures return biased estimates of λ compared to procedures that directly estimate λ using maximum likelihood estimation (MLE). While it is clear that MLE produces less biased estimates of site-specific λ's, it is less clear how this bias affects the ability to test for changes in λ across space and time, a common question in the ecological literature. Here, we used simulation to compare the ability of two normalised binning methods (equal logarithmic and log2 bins) and MLE to (1) recapture known values of λ , and (2) recapture parameters in a linear regression measuring the change in λ across a hypothetical environmental gradient. We also compared the methods using two previously published body size datasets across a natural temperature gradient and an anthropogenic pollution gradient. Maximum likelihood methods always performed better than common binning methods, which demonstrated consistent bias depending on the simulated values of λ . This bias carried over to the regressions, which were more accurate when λ was estimated using MLE compared to the binning procedures. Additionally, the variance in estimates using MLE methods is markedly reduced when compared to binning methods. The error induced by binning methods can be of similar magnitudes as the variation previously published in experimental and observational studies, bringing into question the effect sizes of previously published results. However, while the methods produced different regression slope estimates, they were in qualitative agreement on the sign of those slopes (i.e. all negative or all positive). Our results provide further support for the direct estimation of λ and its relative variation across environmental gradients using MLE over the more common methods of binning.

Changes in vertical and horizontal diversities mediated by the size structure of introduced fish collectively shape food-web stability.

Species introductions can alter local food-web structure by changing the vertical or horizontal diversity within communities, largely driven by their body size distributions. Increasing vertical and horizontal diversities is predicted to have opposing effects on stability. However, their interactive effects remain largely overlooked. We investigated the independent and collective effects of vertical and horizontal diversities on food-web stability in alpine lakes stocked with variable body size distributions of introduced fish species. Introduced predators destabilize food-webs by increasing vertical diversity through food chain lengthening. Alternatively, increasing horizontal diversity results in more stable food-web topologies. A non-linear interaction between vertical and horizontal diversities suggests that increasing vertical diversity is most destabilizing when horizontal diversity is low. Our findings suggest that the size structure of introduced predators drives their impacts on stability by modifying the structure of food-webs, and highlights the interactive effects of vertical and horizontal diversities on stability.

Individual size distributions across North American streams vary with local temperature.

Parameters describing the negative relationship between abundance and body size within ecological communities provide a summary of many important biological processes. While it is considered to be one of the few consistent patterns in ecology, spatiotemporal variation of this relationship across continental scale temperature gradients is unknown. Using a database of stream communities collected across North America (18-68°N latitude, -4 to 25°C mean annual air temperature) over 3 years, we constructed 160 individual size distribution (ISD) relationships (i.e. abundance size spectra). The exponent parameter describing ISD's decreased (became steeper) with increasing mean annual temperature, with median slopes varying by ~0.2 units across the 29°C temperature gradient. In addition, total community biomass increased with increasing temperatures, contrary with theoretical predictions. Our study suggests conservation of ISD relationships in streams across broad natural environmental gradients. This supports the emerging use of size-spectra deviations as indicators of fundamental changes to the structure and function of ecological communities.

Habitat restoration for brown trout (Salmo trutta) has limited effects on macroinvertebrate communities in a historically metal-contaminated stream.

Quantifying the success of stream remediation or restoration projects that are designed to improve water quality or habitat, respectively, is often challenging because of insufficient posttreatment monitoring, poorly defined restoration goals, and failure to consider fundamental aspects of ecological theory. We measured the effects of habitat restoration on aquatic and terrestrial prey resources in a system recovering from the long-term effects of mining pollution. The study was conducted in the Upper Arkansas River, a Rocky Mountain stream located in central Colorado, USA. Remediation of California Gulch, a United States Environmental Protection Agency (USEPA) Superfund Site that discharged metals from past mining operations into the stream, was completed in 2000, resulting in significant improvements in water quality, benthic macroinvertebrate communities, and brown trout (Salmo trutta) populations. A large-scale restoration project designed to improve habitat and increase the density and biomass of brown trout was completed in 2014. To assess the effectiveness of these habitat improvements on invertebrate communities in this system, we sampled sites for 9 years before (2010-2014) and after (2015-2018) restoration was completed. In contrast to our expectations, we observed few changes in the abundance of aquatic or terrestrial invertebrates after restoration. The most common response was an overall reduction in abundance resulting from significant instream disturbances during and immediately after restoration, followed by a gradual return to pretreatment conditions. Despite reductions in prey abundance, the number of prey items in the diet of brown trout increased significantly after restoration. We discuss several explanations for these responses, including the effects of residual metals, increased predation by brown trout, and the recalcitrance of novel communities dominated by metal-tolerant species. Our results suggest that the effectiveness of remediation and restoration differed between macroinvertebrates and fish. Benthic macroinvertebrates were more dependent on water quality improvements at the watershed scale, whereas brown trout populations responded to both improvements in water quality and reach-scale improvements in habitat. Integr Environ Assess Manag 2022;18:1047-1055. © 2021 SETAC.

Changes in stream food-web structure across a gradient of acid mine drainage increase local community stability.

Understanding what makes food webs stable has long been a goal of ecologists. Topological structure and the distribution and magnitude of interaction strengths in food webs have been shown to confer important stabilizing properties. However, our understanding of how variable species interactions affect food-web structure and stability is still in its infancy. Anthropogenic stress, such as acid mine drainage, is likely to place severe limitations on the food-web structures availabe, due to changes in community composition and body mass distributions. Here, we used mechanistic models to infer food-web structure and quantify stability in streams across a gradient of acid mine drainage. Multiple food webs were iterated for each community based on species pairwise interaction probabilities, in order to incorporate the variability of realistic food-web structure. We found that food-web structure was altered systematically with a 32-fold decrease in the number of links and a twofold increase in connectance across the gradient. Stability generally increased sixfold with increasing acid mine drainage stress, regardless of how interaction strengths were estimated. However, the distribution of the stability measure, s, for some impacted communities separated into clusters of higher and lower magnitude depending on how interaction strengths were estimated. Management and restoration of impacted sites needs to consider their increased stability, as this may have important implications for the recolonization of desirable species. Furthermore, active species introductions may be required to overcome the internal ecological inertia of affected communities.