J. Iwan Jones

Warming alters the metabolic balance of ecosystems

The carbon cycle modulates climate change, via the regulation of atmospheric CO2, and it represents one of the most important services provided by ecosystems. However, considerable uncertainties remain concerning potential feedback between the biota and the climate. In particular, it is unclear how global warming will affect the metabolic balance between the photosynthetic fixation and respiratory release of CO2 at the ecosystem scale. Here, we present a combination of experimental field data from freshwater mesocosms, and theoretical predictions derived from the metabolic theory of ecology to investigate whether warming will alter the capacity of ecosystems to absorb CO2. Our manipulative experiment simulated the temperature increases predicted for the end of the century and revealed that ecosystem respiration increased at a faster rate than primary production, reducing carbon sequestration by 13 per cent. These results confirmed our theoretical predictions based on the differential activation energies of these two processes. Using only the activation energies for whole ecosystem photosynthesis and respiration we provide a theoretical prediction that accurately quantified the precise magnitude of the reduction in carbon sequestration observed experimentally. We suggest the combination of whole-ecosystem manipulative experiments and ecological theory is one of the most promising and fruitful research areas to predict the impacts of climate change on key ecosystem services.

DOES THE FISH-INVERTEBRATE-PERIPHYTON CASCADE PRECIPITATE PLANT LOSS IN SHALLOW LAKES?

Alternative equilibria dominated by either submerged plants or phytoplankton have been described for communities in shallow lakes. A nutrient-mediated increase in periphyton (algae attached to plant surfaces) is often described as being responsible for the loss of plants from shallow lakes, yet this violates the stochastic assumptions of alternative equilibria. To determine if periphyton is capable of forcing a switch between these communities, the factors governing the success of aquatic plants were surveyed in 17 plant-dominated shallow lakes in Norfolk (United Kingdom) that varied in nutrient concentration and fish biomass. In these lakes, plant biomass was negatively related to the density of periphyton. However, the density of periphyton on the plants was correlated with the density of grazing invertebrates, not nutrient concentration. In turn, the biomass of fish determined the density of invertebrates. This cascade from fish to periphyton via invertebrates appeared to be evident even though plant-dominated lakes are heterogeneous and complex. Under conditions of plant dominance, periphyton appeared to have a stronger influence on plant growth than phytoplankton. Our data support a model where, within the range of nutrients where alternative equilibria are possible, fish are the prime determinants of community structure in shallow lakes, through a cascading effect of predation on grazing invertebrates influencing the biomass of periphyton and hence, plants. We suggest that the stochasticity required for alternative equilibria in shallow lake communities is often derived from the vagaries of fish colonization and reproduction.

The British river of the future: How climate change and human activity might affect two contrasting river ecosystems in England

The possible effects of changing climate on a southern and a north-eastern English river (the Thames and the Yorkshire Ouse, respectively) were examined in relation to water and ecological quality throughout the food web. The CLASSIC hydrological model, driven by output from the Hadley Centre climate model (HadCM3), based on IPCC low and high CO(2) emission scenarios for 2080 were used as the basis for the analysis. Compared to current conditions, the CLASSIC model predicted lower flows for both rivers, in all seasons except winter. Such an outcome would lead to longer residence times (by up to a month in the Thames), with nutrient, organic and biological contaminant concentrations elevated by 70-100% pro-rata, assuming sewage treatment effectiveness remains unchanged. Greater opportunities for phytoplankton growth will arise, and this may be significant in the Thames. Warmer winters and milder springs will favour riverine birds and increase the recruitment of many coarse fish species. However, warm, slow-flowing, shallower water would increase the incidence of fish diseases. These changing conditions would make southern UK rivers in general a less favourable habitat for some species of fish, such as the Atlantic salmon (Salmo salar). Accidental or deliberate, introductions of alien macrophytes and fish may change the range of species in the rivers. In some areas, it is possible that a concurrence of different pressures may give rise to the temporary loss of ecosystem services, such as providing acceptable quality water for humans and industry. An increasing demand for water in southern England due to an expanding population, a possibly reduced flow due to climate change, together with the Water Framework Directive obligation to maintain water quality, will put extreme pressure on river ecosystems, such as the Thames.

Individual-based food webs: species identity, body size and sampling effects

The study of food webs has been a central theme within ecology for decades, and their structure and dynamics have been used to assess a range of key properties of communities (e.g. complexity–stability relationships) and ecosystems (e.g. fluxes of energy and nutrients). However, many food web parameters are sensitive to sampling effort, which is rarely considered, and further, most studies have used either species- or size-averaged data for both nodes and links, rather than individual-based data, which is the level of organisation at which trophic interactions occur. This practice of aggregating data hides a considerable amount of biologically meaningful variation and could, together with potential sampling effects, create methodological artefacts. New individual-based approaches could improve our understanding of, and ability to predict, food web structure and dynamics, particularly if they are derived from simple metabolic and foraging constraints. We explored the effect of species-averaging in four highly-resolved individual-based aquatic food webs (Broadstone Stream, the Afon Hirnant, Tadnoll Brook and the Celtic Sea) and found that it obscured structural regularities resulting from intraspecific size variation. The individual-based approach provided clearer insights into seasonal and ontogenetic shifts, highlighting the importance of the temporal component of size-structuring in ecological networks. An extension of the Allometric Diet Breadth Model predicted the structure of the empirical food webs almost twice as accurately as the equivalent species-based webs, with the best-fitting model predicting 83% of the links correctly in the Broadstone Stream size-based web, and the few mismatches between the model and data were explained largely by sampling effects. Our results highlight the need for theoretical explanations to correspond closely with methods of data collection and aggregation, which is the exception rather than the rule at present. We suggest how this situation can be improved by including individual-level data and more explicit information on sampling effort when constructing food webs in future studies.

Mobility of stream invertebrates in relation to disturbance and refugia: a test of habitat templet theory

The rate of recolonization of artificially denuded substrata by benthic macroinvertebrates was investigated in 7 southeastern England streams that differed in disturbance frequency and refugium availability. Disturbance frequency and refugia were estimated from observation of substratum movements and shear stress distributions, respectively. Thus, high-refugium sites had a high proportion of low shear stress spots at high discharge, whereas low-refugium sites had a low proportion. Colonization was estimated experimentally using wide-meshed cages containing washed substratum, which remained in the stream for 2 or 7 d. Natural and recolonized benthic assemblages were ordinated, and colonization rate was estimated from Euclidean distances. Macroinvertebrate colonization rate (i.e., mobility) increased with disturbance frequency, but only where refugium availability was low. High-refugium sites had faster colonization than low-refugium sites, irrespective of disturbance regime. Higher mobility in high-refugium sites may reflect lower mortality risk from flow disturbance when the physical habitat is heterogeneous. Our study suggests that high refugium availability may reduce the effect of flow disturbance in streams.

Chapter 6 - Individual-Based Food Webs: Species Identity, Body Size and Sampling Effects

The study of food webs has been a central theme within ecology for decades, and their structure and dynamics have been used to assess a range of key properties of communities (e.g. complexity–stability relationships) and ecosystems (e.g. fluxes of energy and nutrients). However, many food web parameters are sensitive to sampling effort, which is rarely considered, and further, most studies have used either species- or size-averaged data for both nodes and links, rather than individual-based data, which is the level of organisation at which trophic interactions occur. This practice of aggregating data hides a considerable amount of biologically meaningful variation and could, together with potential sampling effects, create methodological artefacts. New individual-based approaches could improve our understanding of, and ability to predict, food web structure and dynamics, particularly if they are derived from simple metabolic and foraging constraints. We explored the effect of species-averaging in four highly-resolved individual-based aquatic food webs (Broadstone Stream, the Afon Hirnant, Tadnoll Brook and the Celtic Sea) and found that it obscured structural regularities resulting from intraspecific size variation. The individual-based approach provided clearer insights into seasonal and ontogenetic shifts, highlighting the importance of the temporal component of size-structuring in ecological networks. An extension of the Allometric Diet Breadth Model predicted the structure of the empirical food webs almost twice as accurately as the equivalent species-based webs, with the best-fitting model predicting 83% of the links correctly in the Broadstone Stream size-based web, and the few mismatches between the model and data were explained largely by sampling effects. Our results highlight the need for theoretical explanations to correspond closely with methods of data collection and aggregation, which is the exception rather than the rule at present. We suggest how this situation can be improved by including individual-level data and more explicit information on sampling effort when constructing food webs in future studies.

Seeing Double: Size-Based and Taxonomic Views of Food Web Structure

Abstract Here, we investigate patterns in the size structure of one marine and six freshwater food webs: that is, how the trophic structure of such ecological networks is governed by the body size of its interacting entities. The data for these food webs are interactions between individuals, including the taxonomic identity and body mass of the prey and the predator. Using these detailed data, we describe how patterns grouped into three sets of response variables: (i) trophic orderings; (ii) diet variation; and (iii) predator variation, scales with the body mass of predators or prey, using both a species- and a size-class-based approach. We also compare patterns of size structure derived from analysis of individual-based data with those patterns that result when data are “aggregated” into species (or size class-based) averages. This comparison shows that analysis based on species averaging can obscure interesting patterns in the size structure of ecological communities. Specifically, we found that the slope of prey body mass as a function of predator body mass was consistently underestimated and the slope of predator–prey body mass ratio (PPMR) as a function of predator body mass was overestimated, when species averages were used instead of the individual-level data. In some cases, no relationship was found when species averages were used, but when individual-level data were used instead, clear and significant patterns were revealed. Further, when data were grouped into size classes, the slope of the prey body mass as a function of predator body mass was smaller and the slope of the PPMR relationship was greater compared to what was found using species-aggregated data. We also discuss potential sampling effects arising from size-class-based approaches, which are not always seen in taxonomical approaches. These results have potentially important implications for parameterisation of models of ecological communities and hence for predictions concerning the dynamics of ecological communities and their response to different kinds of disturbances.

Diurnal carbon restrictions on the photosynthesis of dense stands of Elodea nuttallii (Planch.) St. John

In slow-moving and static eutrophic waters, submerged macrophytes growing in dense stands produce a highly structured environment, with reduced internal water flow. An afternoon lull in the net photosynthesis of such stands has been reported from a number of previous studies. This has been attributed to increased photorespiration caused by an accumulation of photosynthetically-derived, dissolved oxygen in the surrounding water. Results here demonstrate that even in a water quite rich in dissolved inorganic carbon (2.5 mmol l−1), limitations on the supply of inorganic carbon will normally be more important in curtailing photosynthesis, with photorespiration playing only a minor role.

Ecology of Industrial Pollution: Ecological monitoring and assessment of pollution in rivers

Introduction Many organisms respond to pollution in a predictable way, and it has long been realised that the biota can be used to determine the extent of pollution at a site, a technique termed biomonitoring. Much of the science of biomonitoring developed in aquatic systems, driven by concerns about the impact of industrial and domestic pollution on potable water resources. Over the past century, aquatic biomonitoring has travelled a long way from the early methodologies, and much about the pitfalls and benefits of using biota to assess pollution or other stressors has been discovered. Here we describe the history of biomonitoring and how our understanding has developed, with particular focus on RIVPACS (River InVertebrate Prediction And Classification System). This system marked a major advance in biomonitoring techniques, introducing the reference condition approach, where the physical and geographical characteristics of the river were taken into account when determining what taxa would be expected to be present if the site were not polluted. Assessment of a site was then based on a comparison of the observed community and derived scores, to that expected if the site were not polluted. RIVPACS was also the first biomonitoring tool to incorporate a measure of uncertainty; any assessment is based on spatially and temporally variable samples and it is necessary to calculate the confidence associated with the quality class derived using these samples.

Do agri-environment schemes result in improved water quality?

1. Improved water quality, through a reduction in diffuse pollution from agricultural sources, is an expected benefit of agri-environment schemes, but this has yet to be demonstrated in practice. Here, we evaluate the impact of Welsh agri-environment schemes on water quality and freshwater ecosystem condition through a combined monitoring and modelling framework. 2. To determine the influence of the agri-environment schemes on ecosystem condition, spatially independent catchments dominated by a single scheme (>40% of catchment) were compared to control catchments dominated (>70%) by agricultural land that was not part of any scheme. Biological indicators of water quality were monitored at the outfall of each catchment and a spatially explicit modelling framework of diffuse pollutant emissions applied to each of the 80 catchments. 3. Direct comparison (scheme/non-scheme) was unable to identify any significant effect of agrienvironment scheme participation. However, derived biological indicators that reflected organic pollution, eutrophication and pesticide run-off were strongly correlated with modelled concentrations of corresponding diffuse pollutants, thus providing a ground-truth for the models. Scenarios that assessed the correct counterfactuals (i.e. the influence of scheme entry on pollutant output) were developed for the whole of Wales. The models indicated an important effect of scheme entry on water quality, but this effect was not evenly distributed across the landscape. 4. Synthesis and applications. Our results indicate that agri-environment schemes can deliver improvements in water quality, through a reduction in diffuse pollution from agricultural sources. However, it is not easy to demonstrate scheme effectiveness; the combination of field survey and modelling used here provides a framework for addressing these difficulties. A spatially targeted approach for agri-environment scheme options to protect water resources from diffuse pollution is likely to be most effective at delivering water quality improvements.