Chapter 5 - Food Web Structure and Stability in 20 Streams Across a Wide pH Gradient

Abstract

Recent attempts to include more ecological detail in connectance food webs have revealed strong relationships between food web structure, species abundance and body size. Few studies, however, have assessed these and other macroecological patterns in food webs in order to examine how network structure, dynamics and their determinants change across environmental gradients. Here, we present 20 highly resolved, standardized stream food webs along a wide pH gradient (5.0–8.4). Our main goal of this study was to assess the influence of external environmental and internal biotic influences on community structure and stability. Many structural features of the food webs changed across the gradient, with web size, linkage density and complexity all increasing with pH. Chlorophyll- a concentrations in epilithic biofilms, as well as the biomass of macroinvertebrates and fish, were also positively correlated with pH. Directed connectance was not correlated with pH in our study, however, although some of the smallest food webs at the lowest pH displayed the highest connectance amongst the networks in this data set. The prevalence of generalism in such food webs, with many alternative food chains passing through any given species, might serve to confer a degree of stability upon these networks, if most feeding links are weak. We found clear differences between our allometrically inferred measures of per capita interaction strengths within the food web, with particularly strong links existing between herbivores and their algal resources and between fish and invertebrates, both of which also became more prevalent at high pH. Predatory interactions between invertebrates and between fishes were far weaker, with the weakest links of all being the few instances of invertebrates parasitizing fishes. Dynamic modelling simulations that ran for the equivalent of 10 years revealed that fewer species were lost from the more acid food webs than those at high pH, further confirming the suggestion of a negative relationship between stability and pH. This finding, which supports earlier ideas from empirical studies that have considered other types of stability, might account for the limited biological recovery in previously acidified freshwaters that are showing evidence of chemical recovery.