Recovery and nonrecovery of freshwater food webs from the effects of acidification

Many previous attempts to understand how ecological networks respond to and recover from environmental stressors have been hindered by poorly resolved and unreplicated food web data. Few studies have assessed how the topological structure of large, replicated collections of food webs recovers from perturbations. We analysed food web data taken from 23 UK freshwaters, sampled repeatedly over 24 years, yielding a collection of 442 stream and lake food webs. Our main goal was to determine the effect of acidity on food web structure and to analyse the way food web structure recovered from the effects of acidity over time. Long-term monotonic reversals of acidification were evident at many of the sites, but the ecological responses were generally far less evident than chemical changes, or absent. Across the acidity gradient, food web linkage density and network efficiency declined with increasing acidity, while node redundancy (i.e. trophic similarity among species within a web) increased. Within individual sites, connectance, linkage density, trophic height, resource vulnerability and network efficiency tended to increase over time as sites recovered from acidification, while consumer generality and node redundancy tended to decrease. There was evidence for a lag in biological recovery, as those sites showing a recovery in both their biology and their chemistry were a nested subset of those which only showed a chemistry trend. These findings support the notion that food web structure is fundamentally altered by acidity, and that inertia within the food web may be hindering biological recovery. This suggestion of lagged recovery highlights the importance of long-term monitoring when assessing the impacts of anthropogenic stressors on the natural world. This temporal dimension, and recognition that species interactions can shape community dynamics, is missing from most national biomonitoring schemes, which often rely on space-for-time proxies

Long-term demographic balance in the Broadstone stream insect community

Population models based on Lotka–Volterra-type differential equations with logistic prey were made for a simple stream community including two stonefly prey Leuctra nigra Olivier and Nemurella pictetii Klàpalek, and two predators, the caddisfly Plectrocnemia conspersa (Curtis) and the alderfly Sialis fuliginosa Pictet. In order to assess the importance of predation in this system, we constructed both an explicit four-species model and a simplified model with two functional groups which was more amenable to analytical treatment. The models were parameterized using new data on adult emergence and recruitment combined with previously published data on larval densities and prey uptake. The models were falsified if parameterizations led either to negative prey carrying capacities or to unstable dynamics. Both the functional group and four-species models predict asymptotically stable interactions, with feasible carrying capacities. The models are consistent in predicting that the observed prey are in excess of 70% of their carrying capacities. The four-species model indicates that predation impact is not evenly shared between the two prey, with L. nigra being depressed further from its carrying capacity than N. pictetii. Sensitivity analysis shows that the results of the full four-species model remain very robust to realistic levels of stochastic variation in the input data. The four-species model is used to predict the outcome of an ongoing large-scale field experiment involving the transfer of all S. fuliginosa eggs from one stretch of the stream to another. Although the equilibrial prey populations are barely affected by the manipulation, the model predicts marked transient prey-release and prey-depression of L. nigra in the predator addition and removal areas, respectively

Delivering on a promise: integrating species traits to transform descriptive community ecology into a predictive science

Contains fulltext : 111477.pdf (publisher's version ) (Open Access

Body size and diversity in marine systems

Much has been written concerning the relationship between body size and biological traits, mostly concerning the terrestrial situation. There is no reason to suppose that many of these relationships will be different in the sea; for example quarter-power scaling with body mass applies to virtually all organisms (West, Brown & Enquist, 1999). For marine animals, metabolic rate and production scales at three-quarters power (e.g. Brey, 1990; Warwick & Price, 1979), while it is likely that life span increases in proportion to body mass raised to the power of one quarter, although so little is known about the natural history of marine animals that this latter relationship cannot yet be established. On the other hand, the very different phyletic composition of terrestrial and marine faunas, and the big differences in life-history characteristics, suggest that relationships between body size and diversity will differ between these two realms. The relationship between body size and diversity is fraught with uncertainties and inconsistencies. Hutchinson (1959) suggested that ‘… small size, by permitting animals to become specialised to the conditions offered by small diversified elements of the environmental mosaic, clearly makes possible a degree of diversity quite unknown among groups of larger organisms’. However, it is now suggested that the spatial and temporal structure of the physical environment is fractal (Bell et al., 1993 and references therein; see Schmid & Schmid-Araya, this volume), and if habitat complexity largely determines species diversity this leads to the prediction (for a single perfect fractal) that all organisms, regardless of size, will perceive the environment as equally complex and should have equivalent diversity