Showing papers in "Advances in Ecological Research in 2011"

Biomonitoring of Human Impacts in Freshwater Ecosystems: The Good, the Bad and the Ugly

Abstract: Summary 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 EU's 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.

From Natural to Degraded Rivers and Back Again: A Test of Restoration Ecology Theory and Practice

Abstract: Summary Extensive degradation of ecosystems, combined with the increasing demands placed on the goods and services they provide, is a major driver of biodiversity loss on a global scale. In particular, the severe degradation of large rivers, their catchments, floodplains and lower estuarine reaches has been ongoing for many centuries, and the consequences are evident across Europe. River restoration is a relatively recent tool that has been brought to bear in attempts to reverse the effects of habitat simplification and ecosystem degradation, with a surge of projects undertaken in the 1990s in Europe and elsewhere, mainly North America. Here, we focus on restoration of the physical properties (e.g. substrate composition, bank and bed structure) of river ecosystems to ascertain what has, and what has not, been learned over the last 20 years. First, we focus on three common types of restoration measures—riparian buffer management, instream mesohabitat enhancement and the removal of weirs and small dams—to provide a structured overview of the literature. We distinguish between abiotic effects of restoration (e.g. increasing habitat diversity) and biological recovery (e.g. responses of algae, macrophytes, macroinvertebrates and fishes). We then addressed four major questions: (i) Which organisms show clear recovery after restoration? (ii) Is there evidence for qualitative linkages between restoration and recovery? (iii) What is the timescale of recovery? and (iv) What are the reasons, if restoration fails? Overall, riparian buffer zones reduced fine sediment entry, and nutrient and pesticide inflows, and positive effects on stream organisms were evident. Buffer width and length were key: 5–30 m width and > 1 km length were most effective. The introduction of large woody debris, boulders and gravel were the most commonly used restoration measures, but the potential positive effects of such local habitat enhancement schemes were often likely to be swamped by larger-scale geomorphological and physico-chemical effects. Studies demonstrating long-term biological recovery due to habitat enhancement were notable by their absence. In contrast, weir removal can have clear beneficial effects, although biological recovery might lag behind for several years, as huge amounts of fine sediment may have accumulated upstream of the former barrier. Three Danish restoration schemes are provided as focal case studies to supplement the literature review and largely supported our findings. While the large-scale re-meandering and re-establishment of water levels at River Skjern resulted in significant recovery of riverine biota, habitat enhancement schemes at smaller-scales in other rivers were largely ineffective and failed to show long-term recovery. The general lack of knowledge derived from integrated, well-designed and long-term restoration schemes is striking, and we present a conceptual framework to help address this problem. The framework was applied to the three restoration types included in our study and highlights recurrent cause–effect chains, that is, commonly observed relationships of restoration measures (cause) and their effects on abiotic and biotic conditions (effect). Such conceptual models can provide useful new tools for devising more effective river restoration, and for identifying avenues for future research in restoration ecology in general.

A Belowground Perspective on Dutch Agroecosystems: How Soil Organisms Interact to Support Ecosystem Services

Abstract: Summary 1. New patterns and trends in land use are becoming increasingly evident in Europe's heavily modified landscape and else whereas sustainable agriculture and nature restoration are developed as viable long-term alternatives to intensively farmed arable land. The success of these changes depends on how soil biodiversity and processes respond to changes in management. To improve our understanding of the community structure and ecosystem functioning of the soil biota, we analyzed abiotic variables across 200 sites, and biological variables across 170 sites in The Netherlands, one of the most intensively farmed countries. The data were derived from the Dutch Soil Quality Network (DSQN), a long-term monitoring framework designed to obtain ecological insight into soil types ( STs ) and ecosystem types ( ETs ). 2. At the outset we describe ST s and biota, and we estimate the contribution of various groups to the provision of ecosystem services. We focused on interactive effects of soil properties on community patterns and ecosystem functioning using food web models. Ecologists analyze soil food webs by means of mechanistic and statistical modelling, linking network structure to energy flow and elemental dynamics commonly based on allometric scaling. 3. We also explored how predatory and metabolic processes are constrained by body size, diet and metabolic type, and how these constraints govern the interactions within and between trophic groups. In particular, we focused on how elemental fluxes determine the strengths of ecological interactions, and the resulting ecosystem services, in terms of sustenance of soil fertility. 4. We discuss data mining, food web visualizations, and an appropriate categorical way to capture subtle interrelationships within the DSQN dataset. Sampled metazoans were used to provide an overview of belowground processes and influences of land use. Unlike most studies to date we used data from the entire size spectrum, across 15 orders of magnitude, using body size as a continuous trait crucial for understanding ecological services. 5. Multimodality in the frequency distributions of body size represents a performance filter that acts as a buffer to environmental change. Large differences in the body-size distributions across ET s and ST s were evident. Most observed trends support the hypothesis that the direct influence of ecological stoichiometry on the soil biota as an independent predictor (e.g. in the form of nutrient to carbon ratios), and consequently on the allometric scaling, is more dominant than either ET or ST . This provides opportunities to develop a mechanistic and physiologically oriented model for the distribution of species’ body sizes, where responses of invertebrates can be predicted. 6. Our results highlight the different roles that organisms play in a number of key ecosystem services. Such a trait-based research has unique strengths in its rigorous formulation of fundamental scaling rules, as well as in its verifiability by empirical data. Nonetheless, it still has weaknesses that remain to be addressed, like the consequences of intraspecific size variation, the high degree of omnivory, and a possibly inaccurate assignment to trophic groups. 7. Studying the extent to which nutrient levels influence multitrophic interactions and how different land-use regimes affect soil biodiversity is clearly a fruitful area for future research to develop predictive models for soil ecosystem services under different management regimes. No similar efforts have been attempted previously for soil food webs, and our dataset has the potential to test and further verify its usefulness at an unprecedented space scale.

The Role of Body Size in Complex Food Webs: A Cold Case

Abstract: Human-induced habitat destruction, overexploitation, introduction of alien species and climate change are causing species to go extinct at unprecedented rates, from local to global scales. There are growing concerns that these kinds of disturbances alter important functions of ecosystems. Our current understanding is that key parameters of a community (e.g. its functional diversity, species composition, and presence/absence of vulnerable species) reflect an ecological network’s ability to resist or rebound from change in response to pressures and disturbances, such as species loss. If the food web structure is relatively simple, we can analyse the roles of different species interactions in determining how environmental impacts translate into species loss. However, when ecosystems harbour species-rich communities, as is the case in most natural systems, then the complex network of ecological interactions makes it a far more challenging task to perceive how species’ functional roles influence the consequences of species loss. One approach to deal with such complexity is to focus on the functional traits of species in order to identify their respective roles: for instance, large species seem to be more susceptible to extinction than smaller species. Here, we introduce and analyse the marine food web from the high Antarctic Weddell Sea Shelf to illustrate the role of species traits in relation to network robustness of this complex food web. Our approach was threefold: firstly, we applied a new classification system to all species, grouping them by traits other than body size; secondly, we tested the relationship between body size and food web parameters within and across these groups and finally, we calculated food web robustness. We addressed questions regarding (i) patterns of species functional/trophic roles, (ii) relationships between species functional roles and body size and (iii) the role of species body size in terms of network robustness. Our results show that when analyzing relationships between trophic structure, body size and network structure, the diversity of predatory species types needs to be considered in future studies.

Long-Term Dynamics of a Well-Characterised Food Web: Four Decades of Acidification and Recovery in the Broadstone Stream Model System

Abstract: Summary An understanding of the consequences of long-term environmental change for higher levels of biological organisation is essential for both theoretical and applied ecology. Here, we present four decades of data from the well-characterised Broadstone Stream community, detailing biological responses to amelioration of acidification and the recent invasion of a top predator (brown trout, Salmo trutta L.) that was previously excluded by low pH. After several decades of reductions in acidifying emissions, species characteristic of less acid conditions have started to invade or recolonise Broadstone and other European freshwaters, but these signs of biological recovery are still patchy and have lagged behind chemical recovery. One possible explanation for slow recovery is ecological inertia arising from the internal dynamics of the food web, a hypothesis we investigate here using a combination of surveys, experiments and mathematical modelling. The invasion of this hitherto invertebrate-dominated system by a large, generalist vertebrate predator could be expected to alter the structure and stability of the food web. Long-term survey data revealed that the community has experienced waves of invasions or irruptions of progressively larger predators since the 1970s, as pH has risen. Intra-annual fluctuations in prey populations have become increasingly damped and the mean abundance of many species has declined, although none of the previously common taxa have been lost. This suggests that predation, rather than simple chemical tolerance, plays a key role in determining the trajectory of recovery, as the top-down effects of the generalist predators spread diffusely through the reticulate food web. Dynamical simulations indicate that the food web may have become less robust over time as pH has risen and larger predators have become dominant. These results suggest that, though none of the original suite of large invertebrate predators has been driven to local extinction, such an eventual outcome is feasible.

Stream ecosystem functioning in an agricultural landscape: The importance of terrestrial-aquatic linkages

Abstract: The loss of native riparian vegetation and its replacement with non-native species or grazing land for agriculture is a worldwide phenomenon, but one that is prevalent in Europe, reflecting the heavily-modified nature of the continent's landscape. The consequences of these riparian alterations for freshwater ecosystems remain largely unknown, largely because bioassessment has traditionally focused on the impacts of organic pollution on community structure. We addressed the need for a broader perspective, which encompasses changes at the catchment scale, by comparing ecosystem processes in woodland reference sites with those with altered riparian zones. We assessed a range of riparian modifications, including clearance for pasture and replacement of woodland with a range of low diversity plantations, in 100 streams to obtain a continental-scale perspective of the major types of alterations across Europe. Subsequently, we focused on pasture streams, as an especially prevalent widespread riparian alteration, by characterising their structural (e.g. invertebrate and fish communities) and functional (e.g. litter decomposition, algal production, herbivory) attributes in a country (Ireland) dominated by this type of landscape modification, via field and laboratory experiments. We found that microbes became increasingly important as agents of decomposition relative to macrofauna (invertebrates) in impacted sites in general and in pasture streams in particular. Resource quality of grass litter (e.g., carbon : nutrient ratios, lignin and cellulose content) was a key driver of decomposition rates in pasture streams. These systems also relied more heavily on autochthonous algal production than was the case in woodland streams, which were more detrital based. These findings suggest that these pasture streams might be fundamentally different from their native, ancestral woodland state, with a shift towards greater reliance on autochthonous-based processes. This could have a destabilizing effect on the dynamics of the food web relative to the slower, detrital-based pathways that dominate in woodland streams.

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.

Scale Dependence of Predator–Prey Mass Ratio: Determinants and Applications

Abstract: Body size exerts a critical influence on predator–prey interactions and is therefore crucial for understanding the structure and dynamics of food webs. Currently, predator–prey mass ratio (PPMR) is regarded as the most promising modelling parameter for capturing the complex patterns of feeding links among species and individuals in a simplified way. While PPMR has been widely used in food-web modelling, its empirical estimation is more difficult, with the methodology remaining controversial. This is because PPMR (i) may be defined at different biological scales, such as from individuals to communities, and (ii) may also vary with biological factors, such as species identity and body mass, both of which conflict with the conventional model assumptions. In this chapter, we analyse recently compiled gut content data of marine food webs to address the two fundamental issues of scale-dependence and determinants of PPMR. We consider four definitions of PPMR: (i) species-averaged PPMR, (ii) link-averaged PPMR, (iii) individual-predator PPMR, and (iv) individual-link PPMR. First, we show that PPMR values have a complicated scale-dependence characterised by data elements, such as body mass and sample counts of predators and prey, due to averaging and sampling effects. We subsequently used AIC to systematically evaluate how the four types of PPMR are related to predator species identity and body mass. The results indicate that the model providing the best explanation for individual-predator and individual-link PPMRs incorporates both species identity and body mass. Meanwhile, the best model for species-averaged and link-averaged PPMRs was unclear, with different models being selected across sampling sites. These results imply that the size-based community-spectrum models describing individual-level interactions should include taxonomic dissimilarities. Based on the present study, we suggest that future research regarding PPMR must account for scale dependence and associated determinants to improve its utility as a widely applicable tool.

Body Size, Life History and the Structure of Host–Parasitoid Networks

Abstract: Recent studies of the allometric scaling of metabolism, resource handling and space use have provided a mechanistic understanding of how interactions within ecological networks are arranged. Especially, the ‘allometric diet breadth model’ (ADBM), which considers the association between consumer size, resource availability and handling costs, has shown that food webs are predictably shaped according to the body-size relationships of the organisms within them. However, size-based models of network structure are more applicable to predator–prey webs than to insect host–parasitoid networks because the relationship between body size and host use appears to be less straightforward in host–parasitoid interactions. Herein, we describe the structuring of host–parasitoid networks using frameworks that are based not only upon parasitoid body-size considerations but also upon the life-history characteristics that are commonly used to describe variation among hymenopteran parasitoids: the degree of ovigeny, idio/koinobosis and endo/ectoparasitism. We compare these frameworks with those suggested by the ADBM and elucidate upon why it has been unable to successfully predict host–parasitoid network structure. For instance, body-size constraints upon foraging capability are a stronger determinant of whether an interaction is possible in predator–prey webs than they are in host–parasitoid networks because the ultimate determinant of host suitability is its phylogeny. Further, the degree to which the taxonomic host range of a parasitoid is constrained by phylogeny is largely determined by parasitoid life history, for example, whether the larva develops as an endo- or ectobiont. In addition, we describe how parasitoid life history influences host-choice decisions, which are expected to be tailored towards the optimal allocation of scarce resources, through the determination of how species are limited in their reproductive success. To conclude, we describe some fruitful avenues for future research and highlight the importance of considering how temporal or spatial variation in the characteristics of parasitoids or their hosts affects how networks are structured.

Eco-evolutionary Dynamics of Individual-Based Food Webs

Abstract: The past decade has seen the rise of high resolution datasets. One of the main surprises of analysing such data has been the discovery of a large genetic, phenotypic and behavioural variation and heterogeneous metabolic rates among individuals within natural populations. A parallel discovery from theory and experiments has shown a strong temporal convergence between evolutionary and ecological dynamics, but a general framework to analyse from individual-level processes the convergence between ecological and evolutionary dynamics and its implications for patterns of biodiversity in food webs has been particularly lacking. Here, as a first approximation to take into account intraspecific variability and the convergence between the ecological and evolutionary dynamics in large food webs, we develop a model from population genomics and microevolutionary processes that uses sexual reproduction, genetic-distance-based speciation and trophic interactions. We confront the model with the prey consumption per individual predator, species-level connectance and prey–predator diversity in several environmental situations using a large food web with approximately 25,000 sampled prey and predator individuals. We show higher than expected diversity of abundant species in heterogeneous environmental conditions and strong deviations from the observed distribution of individual prey consumption (i.e. individual connectivity per predator) in all the environmental conditions. The observed large variance in individual prey consumption regardless of the environmental variability collapsed species-level connectance after small increases in sampling effort. These results suggest (1) intraspecific variance in prey–predator interactions has a strong effect on the macroscopic properties of food webs and (2) intraspecific variance is a potential driver regulating the speed of the convergence between ecological and evolutionary dynamics in species-rich food webs. These results also suggest that genetic–ecological drift driven by sexual reproduction, equal feeding rate among predator individuals, mutations and genetic-distance-based speciation can be used as a neutral food web dynamics test to detect the ecological and microevolutionary processes underlying the observed patterns of individual and species-based food webs at local and macroecological scales.

Determinants of Density–Body Size Scaling Within Food Webs and Tools for Their Detection

Abstract: The density mass–relationship (DMR) between abundance and body size is a key attribute of biodiversity organisation. The identification of the determinants of the DMR has consolidated as a major research area, focused on both statistical and ecological issues. Here, we advance the connection between food webs and DMR, by showing how gape limitation could determine the amount of resources available and consumption by enemies, the number of modes, scaling exponents, and intercepts of the DMR. The widely used statistical approach of applying ordinary least squares (OLS) regressions to log-transformed data of recorded densities—or histogram frequencies—and mass has been shown to be biased and to present statistical problems. Improvements have been suggested for all these methods, with the maximum likelihood (ML) approach emerging as the best one for both frequency distributions and fits to untransformed data in bivariate relationships. The combination of these methods with tools to detect more than one scaling in a dataset, such as segmented regressions, could detect more complex patterns, to test and validate theoretical expectations. At least five different DMRs have been reported in the literature to date, but it is not evident whether variations in the reported patterns originate from attributes of the studied systems or if they are determined by properties of particular DMR used. We analysed these five DMRs and related statistical tools in a metacommunity composed of 18 local communities of temporary ponds. DMRs presented steeper slopes than those usually reported, with evidence for changes in the scaling regime across size classes. Evaluation of the performance of alternative statistics confirmed ML estimates as the best method available, even with small sample sizes. To understand DMR, it is clear that explicit attention should be paid to the ecological mechanisms involved in each one of the alternative approaches, and to the statistical tools that can be used for its detection.

Predicted Effects of Behavioural Movement and Passive Transport on Individual Growth and Community Size Structure in Marine Ecosystems

Abstract: We develop a spatially explicit, continuous, time-dependent model of size spectra to predict how the active movement and passive transport of individuals can influence individual growth and size spectra. Active movements are ‘prey-seeking’ behaviour, with individuals moving locally towards areas with high concentrations of favoured prey, and ‘predator-avoiding’ behaviour, with prey moving away from areas of high predator density. Passive transport represents the effects of turbulent mixing on small individuals. The model was used to explore the individual and community effects of these biotic and abiotic processes and their interactions, and to predict how energy from local sources of primary production is propagated through the food web. Prey-seeking and predator-avoiding behaviour led to systematic changes in the relative abundance of different-sized individuals in relation to centres of primary production and associated changes in size-spectra slopes. In areas of high phytoplankton abundance, community size-spectrum slopes were shallower and larger individuals were present, whereas in low production areas, slopes were steeper and size spectra truncated. Variations in size-spectra slopes were much reduced by spatial aggregation across the gradient of phytoplankton abundance, and regional slopes most closely approximated the slopes close to centres of high primary production. Individual growth was faster when closer to centres of production. The extent to which stability is apparent in size spectra depended on the scale of aggregation. This implied that sampling at relatively large space and time scales in relation to those of phytoplankton ‘blooms’ was necessary to compare emergent properties, such as size spectra, among regions or ecosystems. Further, at larger scales, responses to human impacts will be clearer and less likely to be masked by variability induced by smaller scale processes.

Preface: Editorial Commentary: Body Size and the (Re)unification of Ecology

Abstract: Publisher Summary This section provides the preface of Volume 45 of the book Advances in Ecological Research . The seven papers in this volume are the product of collaborations resulting from the activities of Sizemic, and the evolution of the ideas that are presented in the chapters. Sizemic has been an important venue for this initial stage of cross-fertilization to occur, and its inception may be seen in hindsight as one of the decisive moments when fisheries science and other branches of ecology became engaged anew.

Preface: Linking Ecological Theory to Reality: Measuring and Mitigating Human Impacts in Europe's Heavily Modified Ecosystems

Abstract: Publisher Summary This section provides the preface of Volume 44 of the book Advances in Ecological Research . This thematic volume is devoted to the applied ecology of European ecosystems, with a particular emphasis on the higher (i.e., multispecies) levels of organization. The book is focused primarily on freshwater and soil ecosystems, which are both highly susceptible to human perturbations and where biomonitoring approaches and food web, community, and ecosystem ecology are well-established traditions. From the onset of the extensive Neolithic forest clearances, through to the rise of the intensively farmed and urban landscapes that now dominate much of the continent, the human footprint has become ever more prominent.