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

Abstract

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.

Bayesian hierarchical modeling of size spectra

“…is typically <<0.01, this implies that a ߣ values of ~-1.75 represent a reasonable first guess of expected ISD exponents, with values of ranging from -1.2 to -2 often appearing in the literature (Andersen and Beyer 2006, Blanchard et al 2009, Pomeranz et al 2020b.…”

“…It is the reciprocal of scaling coefficient of metabolic rate and mass (0.75) (Brown et al 2004) and as a result, values of λ + 1 have been used to estimate metabolic scaling across ecosystems (Reuman et al 2008, Perkins et al 2018, 2019). Because is typically <<0.01, this implies that a λ values of ~−1.75 represent a reasonable first guess of expected ISD exponents, with values of ranging from −1.2 to −2 often appearing in the literature (Andersen and Beyer 2006, Blanchard et al 2009, Pomeranz et al 2020b).…”

“…Whether λ represents a fixed or variable value is debated, but it often varies among samples and ecosystems (Blanchard et al 2009, Perkins et al 2018, Pomeranz et al 2020b). It is often described by its “steepness”, with more negative values (i.e., “steeper”) indicating lower abundance of large relative to small individuals, and vice versa.…”

“…For example, overfishing in marine communities has been detected using size spectra in which λ was steeper than expected, indicating fewer large fish than expected (Jennings and Blanchard 2004). Shifts in λ have also been used to document responses to acid mine drainage in streams (Pomeranz et al 2019, 2020a), land use (Martínez et al 2016), resource subsidies (Perkins et al 2018), and temperature (O’Gorman et al 2017, Pomeranz et al 2022).…”

Integrative analysis of stressor gradients reveals multiple discrete trait‐defined axes underlie community assembly

“…A gradient of fine sediment was defined by visually assessed reach scale (30 m) estimates of fine (<2 mm) inorganic particle cover (Burdon et al, 2013). Finally, an AMD gradient was defined by a PCA axis associated with pH, specific conductance, and concentrations of dissolved metals (Pomeranz, Warburton, et al, 2019; Pomeranz, Wesner, et al, 2020).…”