Food webs: reconciling the structure and function of biodiversity
2012; Elsevier BV; Volume: 27; Issue: 12 Linguagem: Inglês
10.1016/j.tree.2012.08.005
ISSN1872-8383
AutoresRoss M. Thompson, Ulrich Brose, Jennifer A. Dunne, Robert O. Hall, Sally Hladyz, R. L. Kitching, Neo D. Martinez, Heidi M. Rantala, Tamara N. Romanuk, Daniel B. Stouffer, Jason M. Tylianakis,
Tópico(s)Ecology and Vegetation Dynamics Studies
ResumoThe global biodiversity crisis concerns not only unprecedented loss of species within communities, but also related consequences for ecosystem function. Community ecology focuses on patterns of species richness and community composition, whereas ecosystem ecology focuses on fluxes of energy and materials. Food webs provide a quantitative framework to combine these approaches and unify the study of biodiversity and ecosystem function. We summarise the progression of food-web ecology and the challenges in using the food-web approach. We identify five areas of research where these advances can continue, and be applied to global challenges. Finally, we describe what data are needed in the next generation of food-web studies to reconcile the structure and function of biodiversity. The global biodiversity crisis concerns not only unprecedented loss of species within communities, but also related consequences for ecosystem function. Community ecology focuses on patterns of species richness and community composition, whereas ecosystem ecology focuses on fluxes of energy and materials. Food webs provide a quantitative framework to combine these approaches and unify the study of biodiversity and ecosystem function. We summarise the progression of food-web ecology and the challenges in using the food-web approach. We identify five areas of research where these advances can continue, and be applied to global challenges. Finally, we describe what data are needed in the next generation of food-web studies to reconcile the structure and function of biodiversity. We are experiencing two interrelated global ecological crises. One is in biodiversity, with unprecedented rates of species loss across all major ecosystems, combined with greatly accelerated biotic exchange between landmasses [1MEA Millennium Ecosystem Assessment, Synthesis Report.2005Google Scholar]. Consequently, spatial and temporal patterns of species occurrence are being fundamentally altered by extinction and invasion. The second crisis concerns the regulation of ecological processes and the ecosystem services they provide. Processes such as primary production and nutrient cycling have been severely altered by human activities [1MEA Millennium Ecosystem Assessment, Synthesis Report.2005Google Scholar]. Our understanding of biodiversity comes largely from a sound theoretical and empirical basis provided by community ecology, including core concepts such as niche segregation [2Hutchinson G.E. Population studies - animal ecology and demography - concluding remarks.Cold Spring Harb. Sym. Quant. Biol. 1957; 22: 415-427Crossref Google Scholar] and Island Biogeography [3Macarthur R.H. Wilson E.O. Equilibrium-theory of insular zoogeography.Evolution. 1963; 17: 373-387Crossref Google Scholar, 4Hubbell S.P. The Unified Neutral Theory of Biodiversity and Biogeography. Princeton University Press, 2001Google Scholar]. Early studies of individual species’ habitat preferences and physiological tolerances have been complemented by studies of inter-specific interactions from field surveys and experiments. Applications such as conservation management depend largely on mapping of community patterns and studies of habitat occupancy and preferences. More recently, habitat models have been applied, and the advent of simple and cheap molecular markers has allowed quantification of important community assembly (see Glossary) processes such as dispersal [5Shea K. et al.Management of populations in conservation, harvesting and control.Trends Ecol. Evol. 1998; 13: 371-374Abstract Full Text Full Text PDF PubMed Scopus (123) Google Scholar]. In community ecology the unit of study is the individual, population, or species, consistent with other sub-disciplines such as behavioural and population biology. Ecosystem ecology focuses on the fluxes of energy and nutrients through ecological systems [6Lindeman R.L. The trophic-dynamic aspect of ecology.Ecology. 1942; 23: 399-418Crossref Google Scholar, 7Pace M.L. Groffman P. Successes, Limitations, and Frontiers in Ecosystem Science. Springer, 1998Crossref Google Scholar]. Whereas community ecology tends to be reductionist, concentrating largely on processes driven by individuals, populations, or species, ecosystem research often takes a more holistic, systems approach. It remains tractable by aggregating species into broad functional compartments such as primary producers, herbivores, and carnivores, with fluxes of energy and materials between them. This approach has allowed development of flux models, which have been applied widely in forestry, fisheries, and agriculture. However these system-based approaches lack the detail to detect changes in single species, and the compartments used cannot be readily reconciled with other areas of biology [8Loeuille N. Loreau M. Evolutionary emergence of size-structured food webs.Prco. Natl. Acad. Sci. U.S.A. 2005; 102: 5761-5766Crossref PubMed Scopus (259) Google Scholar]. A gap exists between community ecology, which incorporates species diversity, and ecosystem ecology, which can describe changes in function but does not incorporate diversity (Table 1). In the 1980s biodiversity–ecosystem function (BEF) studies attempted to bridge this gap. These studies initially focused on concerns over the consequences of species loss for ecosystem productivity and stability [9Hooper D.U. et al.Effects of biodiversity on ecosystem functioning: a consensus of current knowledge.Ecol. Monogr. 2005; 75: 3-35Crossref Scopus (5207) Google Scholar]. Early studies relating biodiversity to functions such as primary productivity, decomposition, pollination, and fisheries production were necessarily simple and primarily focused on single trophic levels ([9Hooper D.U. et al.Effects of biodiversity on ecosystem functioning: a consensus of current knowledge.Ecol. Monogr. 2005; 75: 3-35Crossref Scopus (5207) Google Scholar] for a review). The limitations of this approach have been widely described [9Hooper D.U. et al.Effects of biodiversity on ecosystem functioning: a consensus of current knowledge.Ecol. Monogr. 2005; 75: 3-35Crossref Scopus (5207) Google Scholar, 10Montoya J.M. et al.Food web complexity and higher-level ecosystem services.Ecol. Lett. 2003; 6: 587-593Crossref Scopus (84) Google Scholar], and there is a growing recognition that BEF studies would benefit from a framework that considers effects of changes in biodiversity across trophic levels on multiple ecosystem processes [9Hooper D.U. et al.Effects of biodiversity on ecosystem functioning: a consensus of current knowledge.Ecol. Monogr. 2005; 75: 3-35Crossref Scopus (5207) Google Scholar, 10Montoya J.M. et al.Food web complexity and higher-level ecosystem services.Ecol. Lett. 2003; 6: 587-593Crossref Scopus (84) Google Scholar, 11Cardinale B.J. et al.Separating the influence of resource ‘availability’ from resource ‘imbalance’ on productivity-diversity relationships.Ecol. Lett. 2009; 12: 475-487Crossref PubMed Scopus (174) Google Scholar].Table 1An overview of community ecology and ecosystem science, and the potential for food-web ecology to integrate across disciplinesCommunity ecologyEcosystem ecologyFood web ecologyPrimary interestSpatial and temporal variability of individuals, populations, and species.Stocks, flows, and dynamics of energy, biomass, and nutrients.Structure and dynamics of species’ feeding relationships and abundance.Usual approachAnalyse patterns of individuals across relatively few of many co-occurring species in time and space.Analyse fluxes of energy and material within ecosystems containing few functional groups and their environment.Analyse biomass distributions and energetic flows within systems containing many species.Units of studyIndividuals, populations, and species.Biomass, energy, and nutrients.Species, populations, and bioenergetics.Underlying processesIndividual behaviour, dispersal.Niche occupancy.Competition/consumption.Invasion, extinction, and speciation.Chemical processes.Thermodynamics.Mass conservation.Ecological stoichiometry.Feeding behaviour and metabolism.Consumer–resource interactions.Community assembly and disassembly.Diversity, complexity, and productivity.Main applicationsConservation science.Biomonitoring, invasion biology.Sustainable agriculture.Resource management (e.g., fisheries, forestry, and agriculture).Global carbon accounting.Unification of ecological sub-disciplines.Managing the effects of biodiversity on ecosystem function and services.StrengthsDetailed mechanistic understanding of forces affecting existence and health of basic ecological units, for example, dispersal.Relatively simple models provide information of element flow and transformation at very large scales, for example, global carbon fluxes.Integrates species-based and function-based approaches. Large potential for better understanding biodiversity–ecosystem function relationships.WeaknessesKnowledge of small parts of systems cannot be scaled up to whole systems.Often ignores species composition and interactions, unable to address effects of species loss.Construction and analysis can be extremely labour-intensive.Need for further development of testable models with clear mechanistic basis.Conceptual diagram Open table in a new tab We propose that reconciling biodiversity and ecosystem function in a single conceptual framework is best achieved through application of a food-web approach. Food webs are maps of the trophic interactions between species, usually simplified into networks of species and the energy links between them. These networks have a suite of attributes which can be calculated to describe food web structure (Box 1). Because this approach includes both species and energy flows among species, food webs provide a natural framework for understanding species’ ecological roles and the mechanisms through which biodiversity influences ecosystem function [12Thebault E. Loreau M. Food-web constraints on biodiversity-ecosystem functioning relationships.Proc. Natl. Acad. Sci. U.S.A. 2003; 100: 14949-14954Crossref PubMed Scopus (203) Google Scholar]. Recognising that the relationship between biodiversity and ecosystem function is a reciprocal one, we have taken an inclusive approach, and consider mechanisms whereby biodiversity can influence function, and function can influence biodiversity. We have deliberately not considered stability as a function, as this has been the subject of a recent major review [13Rooney N. McCann K. Integrating diversity, food web structure and stability.Trends Ecol. Evol. 2011; 27: 40-46Abstract Full Text Full Text PDF PubMed Scopus (294) Google Scholar]. We use five core challenges for the application of food-web approaches to biodiversity and ecosystem function studies to organise our discussion of emerging trends in this area. We finish by summarising those trends into an overview of what the next generation of food-web studies needs to include in order to be applied to the study of biodiversity and ecosystem function.Box 1Food webs and food-web attributesFood webs characterise the networks of trophic interactions that occur among species within ecological communities. We focus here on biodiversity and ecosystem function of whole communities of species rather than more narrowly defined networks such as host–parasitoid and mutualistic networks5. Individual species, aggregates of species, life-stages of species, or non-taxonomic groups (e.g., detritus) form nodes within food webs. Flows of energy form links via transfers of live or dead biomass between nodes. Food webs are usually characterised as binary networks where links are either present or absent, although webs with weighted links that quantify energy flows (‘weighted networks’) are becoming increasingly common [18Bersier L.F. et al.Quantitative descriptors of food-web matrices.Ecology. 2002; 83: 2394-2407Crossref Scopus (375) Google Scholar, 90Tylianakis J. et al.Habitat modification alters the structure of tropical host-parasitoid food webs.Nature. 2007; 455: 202-205Crossref Scopus (642) Google Scholar]. Many attributes are used to describe aspects of food-web structure (Table I).Table IFood websFood-web attributeBiological meaningTaxa richness (S)Number of taxa (nodes) in the food web.Number of trophic links (L)Number of directed feeding links (edges) between taxa.Linkage density (= L/S)Number of links per taxon. A measure of mean dietary specialisation across the food web 90Tylianakis J. et al.Habitat modification alters the structure of tropical host-parasitoid food webs.Nature. 2007; 455: 202-205Crossref Scopus (642) Google Scholar.Connectance (C) (= L/{S2})Proportion of potential trophic links that do occur. An indication of degree of inter-connectivity in a food web, typically 0.05–0.30 91Warren P.H. Making connections in food webs.Trends Ecol. Evol. 1994; 9: 136-141Abstract Full Text PDF PubMed Scopus (89) Google Scholar, 92Martinez N.D. Constant connectance in community food webs.Am. Nat. 1992; 139: 1208-1218Crossref Scopus (264) Google Scholar.Generality (G)The mean number of prey per consumer 93Schoener T.W. Food webs from the small to the large.Ecology. 1989; 70: 1559-1589Crossref Scopus (443) Google Scholar.Vulnerability (V)Mean number of consumers per prey 93Schoener T.W. Food webs from the small to the large.Ecology. 1989; 70: 1559-1589Crossref Scopus (443) Google Scholar.Food chainA distinct path within the food-web matrix from any taxon down to a basal taxon (a taxon which feeds on no other taxa) 18Bersier L.F. et al.Quantitative descriptors of food-web matrices.Ecology. 2002; 83: 2394-2407Crossref Scopus (375) Google Scholar.Mean chain length (mean FCL)Average number of links found in a food chain across a food web 94Williams R.J. Martinez N.D. Simple rules yield complex food webs.Nature. 2000; 404: 180-183Crossref PubMed Scopus (1015) Google Scholar. Food-chain length appears to be reduced by disturbance and increased by higher energy supply and increased ecosystem size 21Thompson R.M. Townsend C.R. Energy availability, spatial heterogeneity and ecosystem size predict food-web structure in streams.Oikos. 2005; 108: 137-148Crossref Scopus (133) Google Scholar, 22McHugh P.A. et al.Dual influences of ecosystem size and disturbance on food chain length in streams.Ecol. Lett. 2010; 13: 881-890Crossref PubMed Scopus (139) Google Scholar, 23Post D.M. et al.Ecosystem size determines food-chain length in lakes.Nature. 2000; 405: 1047-1049Crossref PubMed Scopus (540) Google Scholar.Maximum chain length (max FCL)The maximum number of links found in any food chain in a food web 94Williams R.J. Martinez N.D. Simple rules yield complex food webs.Nature. 2000; 404: 180-183Crossref PubMed Scopus (1015) Google Scholar.Number of basal taxa (b)The number of taxa which do not consume any other taxa, by definition autotrophs.Number of intermediate taxa (i)The number of taxa which are both consumed by, and consume, other taxa.Number of top taxa (t)The number of taxa which are not consumed by any other taxa.Prey:predator (= {b + i}/{t + i})A measure of food-web ‘shape’; high values are more triangular, low values are more ‘square’ in shape. When <1 the food web has an inverted structure that might indicate instability. Note criticisms of this attribute 95Closs G. et al.Constant predator prey ratios - an arithmetical artifact.Ecology. 1993; 74: 238-243Crossref Scopus (14) Google Scholar and its sensitivity to the common practice of aggregating of low trophic level taxa.RobustnessThe minimum level of secondary extinction that occurs in response to a particular perturbation (species removal) 96Dunne J.A. et al.Network structure and biodiversity loss in food webs: robustness increases with connectance.Ecol. Lett. 2002; 5: 558-567Crossref Scopus (1125) Google Scholar.Food-web motifsThe set of unique connected parts of a food web containing n species. Can be thought of as the fundamental building blocks of complex networks. Most studies focus on triplets of species, for which there are 13 possible combinations.Degree distributionThe frequency distribution of the number of interactions per taxa (termed its ‘degree’). Can identify important interactors such as keystone species.IntervalityThe degree to which the prey in a food web can be ordered so that the diets of all species are placed contiguously within a single dimension 82Stouffer D.B. et al.A robust measure of food web intervality.Proc. Natl. Acad. Sci. U.S.A. 2006; 103: 19015-19020Crossref PubMed Scopus (103) Google Scholar. Open table in a new tab Food webs characterise the networks of trophic interactions that occur among species within ecological communities. We focus here on biodiversity and ecosystem function of whole communities of species rather than more narrowly defined networks such as host–parasitoid and mutualistic networks5. Individual species, aggregates of species, life-stages of species, or non-taxonomic groups (e.g., detritus) form nodes within food webs. Flows of energy form links via transfers of live or dead biomass between nodes. Food webs are usually characterised as binary networks where links are either present or absent, although webs with weighted links that quantify energy flows (‘weighted networks’) are becoming increasingly common [18Bersier L.F. et al.Quantitative descriptors of food-web matrices.Ecology. 2002; 83: 2394-2407Crossref Scopus (375) Google Scholar, 90Tylianakis J. et al.Habitat modification alters the structure of tropical host-parasitoid food webs.Nature. 2007; 455: 202-205Crossref Scopus (642) Google Scholar]. Many attributes are used to describe aspects of food-web structure (Table I).Table IFood websFood-web attributeBiological meaningTaxa richness (S)Number of taxa (nodes) in the food web.Number of trophic links (L)Number of directed feeding links (edges) between taxa.Linkage density (= L/S)Number of links per taxon. A measure of mean dietary specialisation across the food web 90Tylianakis J. et al.Habitat modification alters the structure of tropical host-parasitoid food webs.Nature. 2007; 455: 202-205Crossref Scopus (642) Google Scholar.Connectance (C) (= L/{S2})Proportion of potential trophic links that do occur. An indication of degree of inter-connectivity in a food web, typically 0.05–0.30 91Warren P.H. Making connections in food webs.Trends Ecol. Evol. 1994; 9: 136-141Abstract Full Text PDF PubMed Scopus (89) Google Scholar, 92Martinez N.D. Constant connectance in community food webs.Am. Nat. 1992; 139: 1208-1218Crossref Scopus (264) Google Scholar.Generality (G)The mean number of prey per consumer 93Schoener T.W. Food webs from the small to the large.Ecology. 1989; 70: 1559-1589Crossref Scopus (443) Google Scholar.Vulnerability (V)Mean number of consumers per prey 93Schoener T.W. Food webs from the small to the large.Ecology. 1989; 70: 1559-1589Crossref Scopus (443) Google Scholar.Food chainA distinct path within the food-web matrix from any taxon down to a basal taxon (a taxon which feeds on no other taxa) 18Bersier L.F. et al.Quantitative descriptors of food-web matrices.Ecology. 2002; 83: 2394-2407Crossref Scopus (375) Google Scholar.Mean chain length (mean FCL)Average number of links found in a food chain across a food web 94Williams R.J. Martinez N.D. Simple rules yield complex food webs.Nature. 2000; 404: 180-183Crossref PubMed Scopus (1015) Google Scholar. Food-chain length appears to be reduced by disturbance and increased by higher energy supply and increased ecosystem size 21Thompson R.M. Townsend C.R. Energy availability, spatial heterogeneity and ecosystem size predict food-web structure in streams.Oikos. 2005; 108: 137-148Crossref Scopus (133) Google Scholar, 22McHugh P.A. et al.Dual influences of ecosystem size and disturbance on food chain length in streams.Ecol. Lett. 2010; 13: 881-890Crossref PubMed Scopus (139) Google Scholar, 23Post D.M. et al.Ecosystem size determines food-chain length in lakes.Nature. 2000; 405: 1047-1049Crossref PubMed Scopus (540) Google Scholar.Maximum chain length (max FCL)The maximum number of links found in any food chain in a food web 94Williams R.J. Martinez N.D. Simple rules yield complex food webs.Nature. 2000; 404: 180-183Crossref PubMed Scopus (1015) Google Scholar.Number of basal taxa (b)The number of taxa which do not consume any other taxa, by definition autotrophs.Number of intermediate taxa (i)The number of taxa which are both consumed by, and consume, other taxa.Number of top taxa (t)The number of taxa which are not consumed by any other taxa.Prey:predator (= {b + i}/{t + i})A measure of food-web ‘shape’; high values are more triangular, low values are more ‘square’ in shape. When <1 the food web has an inverted structure that might indicate instability. Note criticisms of this attribute 95Closs G. et al.Constant predator prey ratios - an arithmetical artifact.Ecology. 1993; 74: 238-243Crossref Scopus (14) Google Scholar and its sensitivity to the common practice of aggregating of low trophic level taxa.RobustnessThe minimum level of secondary extinction that occurs in response to a particular perturbation (species removal) 96Dunne J.A. et al.Network structure and biodiversity loss in food webs: robustness increases with connectance.Ecol. Lett. 2002; 5: 558-567Crossref Scopus (1125) Google Scholar.Food-web motifsThe set of unique connected parts of a food web containing n species. Can be thought of as the fundamental building blocks of complex networks. Most studies focus on triplets of species, for which there are 13 possible combinations.Degree distributionThe frequency distribution of the number of interactions per taxa (termed its ‘degree’). Can identify important interactors such as keystone species.IntervalityThe degree to which the prey in a food web can be ordered so that the diets of all species are placed contiguously within a single dimension 82Stouffer D.B. et al.A robust measure of food web intervality.Proc. Natl. Acad. Sci. U.S.A. 2006; 103: 19015-19020Crossref PubMed Scopus (103) Google Scholar. Open table in a new tab The use of food webs to describe fluxes of energy between species was mired for many years in debate over inclusiveness, approaches to sampling, and the meaning of some of the food-web attributes that can be calculated [13Rooney N. McCann K. Integrating diversity, food web structure and stability.Trends Ecol. Evol. 2011; 27: 40-46Abstract Full Text Full Text PDF PubMed Scopus (294) Google Scholar, 14Dunne J.A. The network structure of food webs.in: Pascual M. Dunne J.A. Ecological Networks: Linking Structure to Dynamics in Food Webs. Oxford University Press, 2006: 27-86Google Scholar, 15Ings T.C. et al.Ecological networks - beyond food webs.J. Anim. Ecol. 2009; 78: 253-269Crossref PubMed Scopus (640) Google Scholar]. There is now a clear understanding of the limitations of older data and the need for methodological rigour in describing food webs [14Dunne J.A. The network structure of food webs.in: Pascual M. Dunne J.A. Ecological Networks: Linking Structure to Dynamics in Food Webs. Oxford University Press, 2006: 27-86Google Scholar, 15Ings T.C. et al.Ecological networks - beyond food webs.J. Anim. Ecol. 2009; 78: 253-269Crossref PubMed Scopus (640) Google Scholar]. With those concerns dealt with, food-web ecology can be placed in the context of other major sub-disciplines of ecology. Table 1 shows the potential for food-web ecology to act as an underlying conceptual and analytical framework for studying biodiversity and ecosystem function. However, doing so requires addressing a series of key challenges. There is now an established suite of attributes that describe food-web structure, and it is known that food-web structure can influence function [16Strogatz S.H. Exploring complex networks.Nature. 2001; 410: 268-276Crossref PubMed Scopus (6277) Google Scholar, 17Pascual M. Dunne J.A. From small to large ecological networks in a dynamic world.in: Pascual M. Dunne J.A. Ecological Networks: Linking Structure to Dynamics in Food Webs. Oxford University Press, 2006: 3-24Google Scholar]. The central challenge is determining which aspects of structure are related to which aspects of function. Because many food-web attributes systematically vary with the size and complexity of webs (Box 1, [18Bersier L.F. et al.Quantitative descriptors of food-web matrices.Ecology. 2002; 83: 2394-2407Crossref Scopus (375) Google Scholar, 19Montoya J.M. Sole R.V. Topological properties of food webs: from real data to community assembly models.Oikos. 2003; 102: 614-622Crossref Scopus (117) Google Scholar]) understanding these underlying patterns is critical in determining relationships between attributes and ecosystem functions. Traditionally, most food webs have been described as ‘binary’ networks, with trophic links between taxa identified as either present or absent. These relatively simple representations of food webs allow calculation of a suite of attributes that can be correlated with measures of ecosystem function. The basic attribute of food-web size is measured as the number of species (S). Complexity measures incorporate the number of trophic links (L) in terms of link density (L/S) or connectance, the proportion of potential links that actually occur (L/S2). A central measure of all networks’ structure is the variability of links among nodes or ‘degree distributions’ that, in food webs, describe the balance among trophic specialists and generalists. When normalized by L/S, degree distributions have a general exponential-type shape [20Stouffer D.B. et al.Quantitative patterns in the structure of model and empirical food webs.Ecology. 2005; 86: 1301-1311Crossref Scopus (144) Google Scholar] indicating that most paths of energy flow through food webs go through relatively few species. This is consistent with species removal experiments in BEF studies, which have suggested a small sub-group of species disproportionately influences productivity [9Hooper D.U. et al.Effects of biodiversity on ecosystem functioning: a consensus of current knowledge.Ecol. Monogr. 2005; 75: 3-35Crossref Scopus (5207) Google Scholar]. Weighted networks allow food-web metrics to include the strength of trophic interactions, and therefore provide an estimate of energy flow through the web. Relationships between energy fluxes and biodiversity have been proposed in the past, notably that systems with larger amounts of energy entering the food web should be able to support longer food chains and hence more biodiversity [6Lindeman R.L. The trophic-dynamic aspect of ecology.Ecology. 1942; 23: 399-418Crossref Google Scholar, 21Thompson R.M. Townsend C.R. Energy availability, spatial heterogeneity and ecosystem size predict food-web structure in streams.Oikos. 2005; 108: 137-148Crossref Scopus (133) Google Scholar], although there is the potential for interactions with ecosystem size and disturbance [22McHugh P.A. et al.Dual influences of ecosystem size and disturbance on food chain length in streams.Ecol. Lett. 2010; 13: 881-890Crossref PubMed Scopus (139) Google Scholar, 23Post D.M. et al.Ecosystem size determines food-chain length in lakes.Nature. 2000; 405: 1047-1049Crossref PubMed Scopus (540) Google Scholar]. The power of integrating ecosystem functions into food-web studies is clearly illustrated by the studies that have related food-chain length to basal energy supply. Importantly these are also readily applicable to management issues. For instance, food-web studies have shown that riparian vegetation influences energy inputs to streams, with consequences for fish populations [24Kawaguchi Y. et al.Terrestrial invertebrate inputs determine the local abundance of stream fishes in a forested stream.Ecology. 2003; 84: 701-708Crossref Scopus (126) Google Scholar]. A potential application is using basal productivity data and food-web models to determine the size of reserves needed to support top predators. There is considerable potential to use food webs as a tool for achieving greater understanding of the relationships between attributes of food webs and ecosystem function. As yet there have been relatively few studies of the effects of food-web attributes on various ecosystem functions in a multi-trophic level context. Studies of variability in food web attributes along productivity gradients have suggested that such relationships do exist [21Thompson R.M. Townsend C.R. Energy availability, spatial heterogeneity and ecosystem size predict food-web structure in streams.Oikos. 2005; 108: 137-148Crossref Scopus (133) Google Scholar], but there is a need for experimental studies to explore the underlying mechanisms. Most importantly, understanding the reciprocal nature of relationships between food web structure and ecosystem function is essential. This understanding will only come from manipulative studies of food webs where both species composition and ecosystem functions are measured and manipulated individually and in combination. New measures of structure of food webs also have potential to show correlations with function [18Bersier L.F. et al.Quantitative descriptors of food-web matrices.Ecology. 2002; 83: 2394-2407Crossref Scopus (375) Google Scholar]. A food-web motif is a recognisable regular pattern of connections between nodes. Using food-web motifs to decompose larger, more complex food webs into more basic building blocks [25Bascompte J. Melian C.J. Simple trophic modules for complex food webs.Ecol. Soc. 2005; 86: 2868-2873Crossref Scopus (119) Google Scholar, 26Stouffer D.B. et al.Evidence for the existence of a robust pattern of prey selection in food webs.Proc. R. Soc. B: Biol. Sci. 2007; 274: 1931-1940Crossref PubMed Scopus (135) Google Scholar] shows that motifs are shared by food webs from distinct habitat types
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