Anthropogenic mining alters macroinvertebrate size spectra in streams

Abstract

Food web properties can be used in bioassessment as indicators of ecosystem stress, although logistical constraints restrict their widespread use. Size spectra (body mass–abundance relationships) are easier to produce, still incorporate much of the variation in feeding interactions and indicate the strength of the energy transfer efficiency. Here we examined the effect of acid mine drainage on the size spectra of stream macroinvertebrate communities in 25 New Zealand streams with a comparative survey. We predicted that the largest organisms would be most susceptible to acid mine drainage, leading to a reduction in their abundances and associated decrease in the range of body sizes present across the gradient, as well as a reduction in total community abundance. The largest organisms were more sensitive to inputs of acid mine drainage and were absent at the most affected sites. Surprisingly, the smallest body sizes were also removed by acid mine drainage. This led to a reduction of up to two orders of magnitude in the range of body sizes present in mine impacted sites. Total community abundance decreased along the impact gradient. The changes in size spectra were also associated with changes in the proportion of functional feeding groups, suggesting concomitant changes in food web structure. Specifically, communities became dominated by collector browsers and small bodied predators across the gradient. The simplification of the food web structure suggests that communities may be dominated by a few strong energy pathways, lowering their functionality and stability. However, the loss of large bodied predators also reduces top down pressure, probably increasing community stability. Further research is needed to elucidate the cumulative effects of these interacting processes.

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

“…Species richness declined across the AMD gradient. Likewise, community structure was simplified, because of the loss of the largest‐sized taxa (e.g., fish, large‐bodied invertebrates; Pomeranz et al 2019 b ). This is consistent with the findings of several studies showing a decline in species richness and trophic levels in response to AMD inputs (reviewed in Hogsden and Harding 2012 b ).…”

“…These variables were analyzed using principal components to generate an AMD gradient (Appendix : Table S1, Fig. S1; Pomeranz et al 2019 b ). Principal component (PC) axis 1 explained 78% of the variation among sites, and was strongly correlated with pH and dissolved metal concentrations.…”

“…This study uses data originally published in Pomeranz et al (2019 b ). Twenty‐five streams were sampled in the Buller‐Grey region in the northwest of the South Island, New Zealand.…”

“…We used a data set of stream communities across an AMD stress gradient. AMD stress is known to reduce species richness (Hogsden and Harding 2012 a , b ), and alter local population densities, and biomass distributions (Pomeranz et al 2019 b ). We expected food webs to become simpler (e.g., fewer links), and more connected (e.g., high proportion of potential links realized) with increasing AMD impact.…”

“…AMD significantly impacts abiotic conditions with severely lowered pH (e.g., <3) and high concentrations of dissolved trace metals (e.g., >20 mg/L; Hogsden and Harding 2012 b ). AMD is known to decrease community diversity by removing sensitive taxa (Hogsden and Harding 2012 b ), alter size spectra (Pomeranz et al 2019 b ), and simplify food‐web structure (Hogsden and Harding 2012 a ). The restoration of AMD‐impacted streams remains an important goal for ecologists.…”