Recovery and Nonrecovery of Freshwater Food Webs from the Effects of Acidification

Abstract

We analysed food web data taken from 23 UK freshwaters, sampled repeatedly over 37 24 years, yielding a collection of 442 stream and lake food webs. Our main goal was 38 to determine the effect of acidity on food web structure and to analyse the way food 39 web structure recovered from the effects of acidity over time. 40Long-term monotonic reversals of acidification were evident at many of the sites, but 41 the ecological responses were generally far less evident than chemical changes, or 42 absent. Across the acidity gradient, food web linkage density and network efficiency 43 declined with increasing acidity, while node redundancy (i.e. trophic similarity 44 among species within a web) decreased. Within individual sites, connectance, 45 linkage density, trophic height, resource vulnerability and network efficiency tended 46 to increase over time as sites recovered from acidification, while consumer generality 47 and node redundancy tended to decrease. There was evidence for a lag in biological 48 recovery, as those sites showing a recovery in both their biology and their chemistry 49 were a nested subset of those which only showed a chemistry trend. 50These findings support the notion that food web structure is fundamentally 51 altered by acidity, and that inertia within the food web may be hindering biological 52 recovery. This suggestion of lagged recovery highlights the importance of long-term 53 monitoring when assessing the impacts of anthropogenic stressors on the natural 54 world. This temporal dimension, and recognition that species interactions can shape 55 community dynamics, is missing from most national biomonitoring schemes, which 56 often rely on space-for-time proxies. 57 3

Inferring predator–prey interactions in food webs

“…Currently, WebBuilder is likely to only be capable of detecting changes in food web structure between sites when there is a large concomitant species turnover rate. This method has been used to infer changes in food web structure across a changing pH gradient through space and time in the United Kingdom (Gray et al., ). However, in that case, community composition altered through time with increasing pH due to recolonization by large bodied, acid intolerant predators (Gray et al., ; Layer et al., ).…”

“…This method has been used to infer changes in food web structure across a changing pH gradient through space and time in the United Kingdom (Gray et al., ). However, in that case, community composition altered through time with increasing pH due to recolonization by large bodied, acid intolerant predators (Gray et al., ; Layer et al., ). Since large taxa are generally more well connected due to their wide diet breadth (Brose et al., ), inferring food web responses where they have recolonized may be an ideal use of WebBuilder.…”

“…WebBuilder allows altering the level of relationship per taxa individually, and independently for their role as consumers and resources. This method allows for the transparent, reproducible construction of food webs, and only requires a list of taxa for a given site together with the registry of feeding interactions (Gray et al., , ). A potential limitation of the WebBuilder method is that it assumes that if species have ever been documented to interact, they will always interact whenever they co‐occur (Gray et al., ).…”

Anthropogenic mining alters macroinvertebrate size spectra in streams

“…Changes in food web structure also have implications for the restoration of sites impacted by environmental perturbations. Biotic responses in systems that are recovering from physical or chemical impacts can be delayed or patchy in nature, are modulated by the species and interactions present, and do not follow a reverse trajectory of the response to the original impact (Scheffer, Carpenter, Foley, Folke, & Walker, 2001), thereby causing the community to appear to be impacted for long after the stressor has been removed (Gray et al, 2014(Gray et al, , 2016Layer et al, 2011). One explanation for this is that impacted communities have found an alternative stable state, and that this state has internal ecological inertia (Layer et al, 2011), or negative resilience (Lake, 2013), which impedes the community returning to a non-impacted state.…”

Ecological network assembly: how the regional metaweb influences local food webs

“…It is noteworthy to mention that the small-world pattern and modularity act in opposite directions. Whereas a small-world topology favours the spread of perturbations through its rapid dissipation (Gray et al 2016), the presence of high modularity prevents the dispersal of perturbations (Stouffer and Bascompte 2011, Krause et al 2003).…”