Francois Edwards

Biodiversity, Species Interactions and Ecological Networks in a Fragmented World

Biodiversity is organised into complex ecological networks of interacting species in local ecosystems, but our knowledge about the effects of habitat fragmentation on such systems remains limited. We consider the effects of this key driver of both local and global change on both mutualistic and antagonistic systems at different levels of biological organisation and spatiotemporal scales. There is a complex interplay of patterns and processes related to the variation and influence of spatial, temporal and biotic drivers in ecological networks. Species traits (e.g. body size, dispersal ability) play an important role in determining how networks respond to fragment size and isolation, edge shape and permeability, and the quality of the surrounding landscape matrix. Furthermore, the perception of spatial scale (e.g. environmental grain) and temporal effects (time lags, extinction debts) can differ markedly among species, network modules and trophic levels, highlighting the need to develop a more integrated perspective that considers not just nodes, but the structural role and strength of species interactions (e.g. as hubs, spatial couplers and determinants of connectance, nestedness and modularity) in response to habitat fragmentation. Many challenges remain for improving our understanding: the likely importance of specialisation, functional redundancy and trait matching has been largely overlooked. The potentially critical effects of apex consumers, abundant species and super-generalists on network changes and evolutionary dynamics also need to be addressed in future research. Ultimately, spatial and ecological networks need to be combined to explore the effects of dispersal, colonisation, extinction and habitat fragmentation on network structure and coevolutionary dynamics. Finally, we need to embed network approaches more explicitly within applied ecology in general, because they offer great potential for improving on the current species-based or habitat-centric approaches to our management and conservation of biodiversity in the face of environmental change.

Biomonitoring of human impacts in freshwater ecosystems:the good, the bad and the ugly

It is critical that the impacts of environmental stressors on natural systems are detected, monitored and assessed accurately in order to legislate effectively and to protect and restore ecosystems. Biomonitoring underpins much of modern resource management, especially in fresh waters, and has received significant sums of money and research effort during its development. Despite this, the incorporation of science has not been effective and the management tools developed are sometimes inappropriate and poorly designed. Much biomonitoring has developed largely in isolation from general ecological theory, despite the fact that many of its fundamental principles ultimately stem from basic concepts, such as niche theory, the habitat template and the r–K continuum. Consequently, biomonitoring has not kept pace with scientific advances, which has compromised its ability to deal with emerging environmental stressors such as climate change and habitat degradation. A reconnection with its ecological roots and the incorporation of robust statistical frameworks are key to progress and meeting future challenges. The vast amount of information already collected represents a potentially valuable, and largely untapped, resource that could be used more effectively in protecting ecosystems and in advancing general ecology. Biomonitoring programmes have often accumulated valuable long-term data series, which could be useful outside the scope of the original aims. However, it is timely to assess critically existing biomonitoring approaches to help ensure future programmes operate within a sound scientific framework and cost-effectively. Investing a small proportion of available budgets to review effectiveness would pay considerable dividends. Increasing activity has been stimulated by new legislation that carries the threat of penalties for non-compliance with environmental targets, as is proposed, for example, in the EUs Water Framework Directive. If biomonitoring produces poor-quality data and has a weak scientific basis, it may lead either to unjustified burdens placed on the users of water resources, or to undetected environmental damage. We present some examples of good practice and suggest new ways to strengthen the scientific rigour that underpins biomonitoring programmes, as well as highlighting potentially rewarding new approaches and technologies that could complement existing methods.

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.

Horizon scanning for invasive alien species with the potential to threaten biodiversity in Great Britain

Invasive alien species (IAS) are considered one of the greatest threats to biodiversity, particularly through their interactions with other drivers of change. Horizon scanning, the systematic examination of future potential threats and opportunities, leading to prioritization of IAS threats is seen as an essential component of IAS management. Our aim was to consider IAS that were likely to impact on native biodiversity but were not yet established in the wild in Great Britain. To achieve this, we developed an approach which coupled consensus methods (which have previously been used for collaboratively identifying priorities in other contexts) with rapid risk assessment. The process involved two distinct phases: Preliminary consultation with experts within five groups (plants, terrestrial invertebrates, freshwater invertebrates, vertebrates and marine species) to derive ranked lists of potential IAS. Consensus-building across expert groups to compile and rank the entire list of potential IAS. Five hundred and ninety-one species not native to Great Britain were considered. Ninety-three of these species were agreed to constitute at least a medium risk (based on score and consensus) with respect to them arriving, establishing and posing a threat to native biodiversity. The quagga mussel, Dreissena rostriformis bugensis, received maximum scores for risk of arrival, establishment and impact; following discussions the unanimous consensus was to rank it in the top position. A further 29 species were considered to constitute a high risk and were grouped according to their ranked risk. The remaining 63 species were considered as medium risk, and included in an unranked long list. The information collated through this novel extension of the consensus method for horizon scanning provides evidence for underpinning and prioritizing management both for the species and, perhaps more importantly, their pathways of arrival. Although our study focused on Great Britain, we suggest that the methods adopted are applicable globally.

Invasive alien Crustacea: dispersal, establishment, impact and control

The subphylum Crustacea includes the most successful species among aquatic alien invaders. The impacts of invasive alien crustaceans (IAC) are often substantial, due to the complex trophic role of most of these species leading to cascading effects throughout the invaded ecosystems. IAC also have the potential to cause a shift in the ‘keystone’ ecosystem functions, changing energy flux and nutrient cycles which together affect critical ecosystem services such as biodiversity, fisheries yield and water quality. Although no individual trait appears to be a good predictor of invasion success, a combination of some characteristics such as eurytolerance, omnivory and certain r-selected life-history traits results in a high probability of alien crustacean species becoming invasive. Both environmental factors, such as habitat heterogeneity in the invaded ecosystems, and evolutionary factors, such as adaptations to new environmental conditions, also play important roles during establishment. Therefore, individual environmental niche models, including genetic algorithm, have the highest likelihood of providing useful predictive information about invasion success and spread of alien Crustacea. Attempts to control IAC through biocides or mechanical removal have had mixed success in the past but a strategic combination of different methods may lead to some success in the future.

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.

Climate change impacts in multispecies systems: drought alters food web size structure in a field experiment

Experimental data from intergenerational field manipulations of entire food webs are scarce, yet such approaches are essential for gauging impacts of environmental change in natural systems. We imposed 2 years of intermittent drought on stream channels in a replicated field trial, to measure food web responses to simulated climate change. Drought triggered widespread losses of species and links, with larger taxa and those that were rare for their size, many of which were predatory, being especially vulnerable. Many network properties, including size–scaling relationships within food chains, changed in response to drought. Other properties, such as connectance, were unaffected. These findings highlight the need for detailed experimental data from different organizational levels, from pairwise links to the entire food web. The loss of not only large species, but also those that were rare for their size, provides a newly refined way to gauge likely impacts that may be applied more generally to other systems and/or impacts.

The relationship between length, mass and preservation time for three species of freshwater leeches (Hirudinea)

We present body length to wet mass, dry mass and ash-free dry mass equations for three common species of British leeches. We quantified the effects of preservation in alcohol on specimens of these species and found important reductions in both body length and wet mass. However, effects on length-mass relationships were minimal. We found no clear pattern across species between the rates of loss of length and mass, and either water or inorganic content, though larger specimens of all three species were less affected by these losses. We recommend specimens are preserved in alcohol for a least a month for the rate of loss to stabilise.

Extreme Climatic Events Alter Aquatic Food Webs. A Synthesis of Evidence from a Mesocosm Drought Experiment

Extreme climatic events are expected to increase in frequency and intensity under climate change. Climate models predict shifts in rainfall patterns that will exacerbate drought, with potentially devastating effects on freshwater ecosystems. Experimental approaches are now advocated to explore the impact of extreme events on natural systems: here, we synthesise research conducted in a stream mesocosms experiment to simulate the effect of prolonged drought on the structure and functioning of complex food webs in a 2-year manipulation of flow regimes. Drought triggered the losses of species and trophic interactions, especially among rare predators, leading to the partial collapse of the food webs. Drying caused marked taxonomic and functional turnover in algal primary producers, from encrusting greens to diatoms, whereas the total number of algal taxa in the food webs remained unchanged. The recurrent drying disturbances generated transient macroinvertebrate communities dominated by relatively few, r-selected, species and compensatory dynamics sustained total macroinvertebrate densities. However, the standing biomass and secondary production of the food webs were more than halved by the droughts. Consumer-resource biomass flux was also strongly suppressed by disturbance, yet several network-level properties (such as connectance and interaction diversity) were conserved, driven by consumer-resource fidelity and a reconfiguration of fluxes within the webs, as production shifted down the size spectrum towards the smaller species. Our research demonstrates that flow extremes could have far-reaching consequences for the structure and functioning of complex freshwater communities.