ECOTOXICOLOGY: A Comprehensive Treatment - Chapter 33 docx

Clements: “3357_c033” — 2007/11/9 — 12:39 — page 715 — #1 33 Patterns and Processes: The Relationship between Species Diversity and Ecosystem Function The economies of the Earth would grind to a halt without the services of ecological life-support systems (Costanza et al. 1997) The biodiversity–ecosystem function linkage appears to be another concept for which enthusiasm outweighs supportive evidence. (Schwartz et al. 2000) 33.1 INTRODUCTION Afundamental characteristic of most biological systems is their remarkable diversity. Our accounting of global diversity should not be restricted to the large number of species inhabiting the biosphere, estimated to be between 10 and 100 million (Wilson 1999), but should also include the genetic variation residing within each species as well as the functional diversity of processes for which they are responsible. The rapid loss of genetic, species, and functional diversity resulting from habitat destruction, exotic species, climate change, overharvesting, chemical stressors, and other sources of anthropogenic disturbance is a significant environmental concern with global consequences. Argu- ments for the protection of biological diversity have traditionally been based on moral or aesthetic perspectives. However, researchers and policymakers are becoming increasingly aware that species also provide ecological goods and services that are essential for human welfare. In this chapter, we describe theoretical and empirical evidence that supports the hypothesis that species diversity controls ecosystem function, describe the limitations in our understanding of this relationship, and discuss theimplications for ecotoxicology. Becauseof the controversysurrounding thesignificance of the diversity–ecosystem function relationship and its practical importance in managing ecosystems, the topic has received considerable attention in the ecological literature. The recent comprehensive review published by Hooper et al. (2005) is especially noteworthy because coauthors included both proponents and critics of the diversity–ecosystem function relationship. This review, which charac- terizes major points of agreement and uncertainty, represents a broad consensus within the scientific community (Table 33.1). Because reduced genetic, species, and functional diversity resulting from contaminants has important consequences for the services provided by ecosystems, we believe that the diversity–ecosystem function relationship has significant implications for ecotoxicology. We first review the observational and experimental studies that support the theoretical relationship between 715 © 2008 by Taylor & Francis Group, LLC Clements: “3357_c033” — 2007/11/9 — 12:39 — page 716 — #2 716 Ecotoxicology: A Comprehensive Treatment TABLE 33.1 Major Points of Agreement and Remaining Uncertainty within the Ecological Community Regarding the Relationship between Species Diversity and Ecosystem Function Points of certainty: 1. Species and functional diversities influence ecosystem function. 2. Anthropogenic alteration of ecosystem function and related services has been well documented. 3. The relationship between species diversity and ecosystem processes is context-dependent and varies among ecosystem properties and types. 4. The relative insensitivity of some ecosystem processes to loss of species or changes in composition results from the redundancy of some species, the fact that some species contribute relatively little to ecosystem function, and the dominant influence of abiotic environmental factors. 5. As spatial and temporal variability increases, more species are required to maintain the stability of ecosystem processes and services. Points of high confidence: 1. Although some combinations of species are complementary and can increase ecosystem function, environmental factors can influence the importance of complementarity. 2. The effects of exotic species are determined by community composition and are generally lower in communities with high species richness. 3. Variation in the sensitivity and susceptibility of species to anthropogenic stressors within a community can provide stability to ecosystem processes. Points of uncertainty and the need for future research: 1. Additional research is necessary to understand the mechanisms responsible for the relationship between species diversity, functional diversity, and ecosystem function. 2. Because most studies have focused on the relationship between diversity of primary producers and ecosystem function, effects of diversity across trophic levels are poorly understood. 3. Although there is broad theoretical support for the relationship between species diversity and stability, long-term field experiments are necessary to determine the importance of this relationship in natural ecosystems. 4. Because ecosystem function simultaneously influences and responds to biological diversity, understanding the feedback between these variables is critical. 5. Because the focus of diversity–ecosystem function research has been in terrestrial ecosystems and to a limited extent in freshwater ecosystems, little is known about this relationship in marine ecosystems. Source: From Table 1 in Hooper et al. 2005. species diversity and ecosystem processes. We then discuss the practical implications of this relation- ship and argue that, in addition to the direct effects of pollutants on energy flow and biogeochemical cycles describedin previouschapters, contaminant-induced changein diversitycan negatively impact ecosystem processes and services. We also review the evidence supporting the hypothesis that ecosystems with lower biological diversity have lower resistance and resilience to natural and anthro- pogenic stressors compared to species-rich ecosystems. Finally, we demonstrate that recent studies investigating the concept of ecological thresholds have important implications for understanding the diversity–ecosystem function relationship. We suggest that quantifying the level of perturbation (or species loss) where ecosystem function is significantly impaired will improve our ability to predict effects of anthropogenic perturbations. Many of the important ecosystem processes we have discussed in the preceding chapters, includ- ing primary productivity, nutrient dynamics, decomposition, and energy flow respond directly to anthropogenic perturbations. We also know that changes in abundance of keystone species and other ecologically important taxa as a result of physical and chemical stressors affect ecosystem function at local and global scales (Chapin et al. 1997). In this chapter, we consider the implications of reduced © 2008 by Taylor & Francis Group, LLC Clements: “3357_c033” — 2007/11/9 — 12:39 — page 717 — #3 Diversity–Ecosystem Function Relationship 717 species richness and diversity on ecosystem processes. The fundamental argument supporting this relationship is quite simple. From a probabilistic perspective, greater species richness increases the likelihood that functionally important species will be present in an ecosystem. Elimination of species in ecosystems with lower species diversity increases the likelihood that critical ecosystem processes will be affected. As described previously (Chapter 25), greater species diversity provides functional redundancy and increases the resistance and resilience of ecosystems to anthropogenic perturbations. For example, Frost et al. (1999) attributed the functional redundancy and increased resilience of acidified lakes to high zooplankton diversity. Although our understanding of the mechanistic linkages between community structure and eco- system function remains limited, it is becoming increasingly apparent that alterations in ecosystem processes as a result of species loss have important implications for ecosystem services provided to humans. Species loss within functionally related assemblages, such as pollinators and flowering plants, may impact ecosystem services at very large spatial scales (Biesmeijer et al. 2006). The unprecedented rate of species extinction occurring at a global scale requires that ecologists and ecotoxicologists develop a better appreciation of the relationship between patterns and processes. Despite the recent interest in this relationship within the basic ecological literature, surprisingly few studies have examined the consequences of contaminant-induced species loss on ecosystem function and services. In addition to measuring the direct effects of chemical stressors on ecosystems, we believe that it is important that ecotoxicologists recognize the indirect effects on ecosystem processes owing to loss of species. The goal of this chapter is to provide an ecotoxicological perspective on the critical relationship between community patterns and ecosystem processes. 33.2 SPECIES DIVERSITY AND ECOSYSTEM FUNCTION Although biologists have hypothesized about the relationship between species diversity and eco- system function for over a century, the topic remains controversial in the contemporary ecological literature (Grime 1997, Hooper and Vitousek 1997, Huston 1997). There is general agreement that high species diversity provides benefits to ecosystems beyond simple aesthetics. In a review of evid- ence supporting the diversity–ecosystem function relationship, Chapin et al. (1998) concluded that high species diversity maximizes resource acquisition across trophic levels, reduces the risk associ- ated with stochastic changes in environmental conditions, and protects communities from exposure to pathogens or exotic species. Because diversity’s relationship with ecosystem goods and services has important socioeconomic implications, this debate has also generated significant attention among policymakers. A key issue in the diversity–ecosystem function debate is that many of the ecosystem processes that have been linked directly to species diversity, such as the primary productivity of tropical rainforests, are clearly influenced by other environmental factors in addition to the number of species (Figure 33.1). Second, while most research has investigated the influence of diversity on ecosystem function, ecosystem processes such as primary productivity also regulate species diversity. The influence of ecosystem processes on species diversity may be very complex. For example, Chase and Leibold (2002) reported that the shape of the relationship between productivity and species diversity was scale dependent. At a local scale, the relationship was hump shaped, with diversity increasing up to a certain level of productivity and then declining at higher levels. In contrast, species diversity increased linearly with productivity at the regional scale. As a consequence of these complex, scale-dependent relationships, an understanding of potential feedbacks between diversity and ecosystem processes is critical. The relationships between species diversity, ecosystem function, and ecosystem services also must be interpreted within the context of changing global climate. The most contentious debates within the ecological community pertain to the mechanisms by which species diversity controls ecosystem function. It is possible that the positive relationship is simply a sampling artifact, by which greater species richness increases ecosystem function by increasing © 2008 by Taylor & Francis Group, LLC Clements: “3357_c033” — 2007/11/9 — 12:39 — page 718 — #4 718 Ecotoxicology: A Comprehensive Treatment Local chemical and physical stressors Species diversity and community structure Ecosystem processes Ecosystem services Human health Global atmospheric stressors FIGURE 33.1 The influence of anthropogenic stressors and species diversity on ecosystem processes and services. Because ecosystem processes also influence species diversity, an understanding of the potential feed- back is necessary for characterizing these relationships. All of these interactions must be interpreted within the context of global atmospheric stressors such as climate change, N deposition, and increased UV-B radiation. the likelihood that functionally important species are present. Alternatively, increased ecosystem function at higher species richness could be a result of positive interactions among species, described as complementarity or facilitation effects. Thus, while many researchers accept the existence of a relationship between species diversity and ecosystem function, the mechanisms responsible for this relationship remain a significant source of controversy. 33.2.1 EXPERIMENTAL SUPPORT FOR THE SPECIES DIVERSITY–ECOSYSTEM FUNCTION RELATIONSHIP Large-scale field experiments conducted in grasslands by Tilman and colleagues have contributed significantly toour understandingof therelationship betweendiversity andplant productivity(Tilman et al. 1997). By adding a known number of species (0–32) orfunctional groups (0–5) tolarge (169 m 2 ) grassland plots, these researchers foundthat both speciesdiversity and functional diversityinfluenced plant productivity (Figure 33.2). When results were analyzed based on functional composition, the relationships were stronger, suggesting that composition of the community was more important than the number of species. Similarly, Hooper and Vitousek (1997) reported that the composition of plant functional groups was more closely related to ecosystem processes than functional group richness. These results demonstrate that the different functional roles of species may be more important predictors of ecosystem integrity than the actual identity of those species. A large-scale experimental test of the relationship between grassland plant diversity and pro- ductivity was conducted at eight European field sites (Hector et al. 1999). Five levels of species richness were established at each site across a broad geographic region (Germany, Portugal, Switzer- land, Greece, Ireland, Sweden, and two sites in the United Kingdom). Productivity (measured as aboveground biomass) varied among locations, but the overall pattern at all sites was greater productivity with higher species richness. The mechanisms proposed to account for this pattern included positive mutualistic interactions among species and niche complementarity, whereby vari- ation among species resulted in more complete utilization of resources. Although distinguishing between these alternative explanations will not be simple, the results demonstrate that loss of species © 2008 by Taylor & Francis Group, LLC Clements: “3357_c033” — 2007/11/9 — 12:39 — page 719 — #5 Diversity–Ecosystem Function Relationship 719 * * * * * * * * * * * * * 010203035 0 50 100 150 200 Species diversity 012345 0 50 100 150 200 Functional diversity Plant biomass (g/m 2 ) FIGURE 33.2 The influence of species diversity and functional diversity on productivity (measured as above- ground biomass) in grassland plots. Species diversity and functional diversity were manipulated by adding a known number of species or functional groups to experimental plots. (Modified from Figure 1 in Tilman et al. (1997).) and the alteration in community composition will significantly alter ecosystem processes. Consistent with predictions of the drivers and passengers model described in Chapter 25 (Figure 25.2d), the loss of some functionally important species will have greater impacts on ecosystem function than the loss of other species. Taylor et al. (2006) reported that removal of a single detritivorous fish species from a species-rich tropical river had large effects on carbon flow and ecosystem metabolism. These results were contrary to the theoretical prediction that high species diversity at lower trophic levels provides insurance against changes in ecosystem function. If one of the key goals of basic ecology is to identify these functionally important species, we believe that one of the challenges in ecotoxicology is to predict the consequences of their local extinction owing to the presence of chemical stressors. 33.2.2 FUNCTIONAL REDUNDANCY AND SPECIES SATURATION IN ECOSYSTEMS The positiveinfluence of speciesrichness on ecosystem function reportedin many studieshas attained greater significance as conservation biologists have used this relationship to argue for species protec- tion. The accelerating loss of biodiversity has intensified efforts to clarify the diversity–productivity relationship and to identify mechanistic explanations. From a species conservation perspective, the shape of the relationship between richness and ecosystem processes may be at least as import- ant as the actual existence of this relationship. A linear relationship between ecosystem processes and richness implies that all species in a community are important and contribute to ecosystem function (Figure 33.3). However, if the relationship is curvilinear and ecosystem processes can be supported by a relatively small number of species, then ecosystems could potentially lose a signi- ficant number of species without affecting function. Schwartz et al. (2000) reviewed observational, experimental, and theoretical studies and found relatively little support for the linear dependence of ecosystem processes on species richness. These researchers recommended caution when using the © 2008 by Taylor & Francis Group, LLC Clements: “3357_c033” — 2007/11/9 — 12:39 — page 720 — #6 720 Ecotoxicology: A Comprehensive Treatment Species richness Ecosystem process A B C FIGURE 33.3 Three hypothetical relationships between species richness and ecosystem processes. TypeA is an example of where ecosystem processes are saturated at a relatively low number of species. This response also shows an abrupt threshold response when species richness is reduced below a certain critical number. Type B shows an intermediate relationship between species richness and ecosystem function. The linear relationship between species richness and ecosystem function depicted by Type C implies that all species contribute equally to ecosystem function. (Modified from Figure 1 in Schwartz et al. (2000).) diversity–ecosystem function relationship as an argument to support species conservation. Although a saturating response of ecosystem processes to increasing species richness is the most commonly observed pattern (Hooper et al. 2005), in a global analysis of marine ecosystems Worm et al. (2006) found no evidence of functional redundancy and reported a linear relationship between richness and ecosystem processes. 33.2.3 INCREASED STABILITY IN SPECIES-RICH ECOSYSTEMS In many respects, the relationship between species richness and ecosystem function is closely related to the diversity–stability relationship described in Chapter 25. Indeed, one of the responses frequently cited to support the existence of a positive diversity–ecosystem function relationship is greater sta- bility in species-rich ecosystems. This relationship is supported by mathematical models that predict that, if species abundances vary randomly or are negatively correlated, ecosystem processes will be more stable in diverse communities than in species-poor communities. This statistical averaging phe- nomenon, which has been termed the “portfolio” effect, provides a type of insurance for ecosystems where species have varying sensitivities to environmental conditions. Despite its broad theoretical support and intuitive appeal, there have been few long-term experiments testing the relationship between species diversity and ecosystem stability in nature. We agree with Hooper et al. (2005) that linking results of long-term experiments with theoretical and mathematical models will improve our understanding of the role that biological diversity plays in stabilizing ecosystem function. 33.2.4 C RITICISMS OF THE DIVERSITY–ECOSYSTEM FUNCTION RELATIONSHIP Critics of the diversity–ecosystem function relationship argue that ecosystem properties are not a direct consequence of species richness or diversity per se, but simply an outcome of the functional composition of dominant species. Experimental studies conducted in grasslands, greenhouses, and growth chambers that controlled for potential confounding variables have demonstrated a strong positive relationship between species diversity and plant productivity. However, large-scale com- parative field studies showed that this relationship was not consistent, suggesting that factors other than species diversity determined ecosystem processes (Chapin et al. 1997). Wardle et al. (1997) took advantage of natural variation in community composition of an island archipelago to examine © 2008 by Taylor & Francis Group, LLC Clements: “3357_c033” — 2007/11/9 — 12:39 — page 721 — #7 Diversity–Ecosystem Function Relationship 721 the relationship between island size and ecosystem processes. In contrast to predictions based on many studies, ecosystem processes were inversely related to species diversity. Another legitimate criticism of studies reporting a relationship between community structure and ecosystem processes is that biological diversity within a community is frequently reduced to a single number (e.g., species richness). However, there are other important community characteristics that are equally likely to respond to anthropogenic stressors and influence ecosystem processes. For example, in addition to reduced species richness and alterations in community composition, one of the mostconsistent responses tomany chemical stressors is increaseddominance of pollution-tolerant species. Dangles and Malmqvist (2004) reported that the relationship between species richness and leaf breakdownrates in 36European streams was determined bydominance of invertebrateshredders. Detrital processing rates were higher and showed an asymptotically increasing relationship with species richness in streams with high dominance, indicating considerable functional redundancy. In contrast, the relationship between species richness and detrital processing in streams with an even distribution of individuals was linear, indicating that all shredder species were important and contributed to ecosystem function. 33.2.5 MECHANISMS RESPONSIBLE FOR THE SPECIES DIVERSITY–ECOSYSTEM FUNCTION RELATIONSHIP Sacrificing those aspects of ecosystems that are difficult or impossible to reconstruct, such as diversity, simply because we are not yet certain about the extent and mechanisms by which they affect ecosystem properties, will restrict future management options even further. (Hooper et al. 2005) If we assume that different species in a community have different functional roles and that the functions performed by individual species are limited, it follows that alterations in community composition resulting from anthropogenic disturbance will affect ecosystem processes. However, identifying the specific mechanistic explanations for the diversity–ecosystem function relationship and characterizing its form (e.g., linear vs. curvilinear) have been challenging. Some ecologists suggest that this relationship is inconsistent among communities because the relative contributions of individual species to ecosystem function are context dependent and vary with environmental conditions (Cardinale et al. 2000). Others argue that the observed pattern is a sampling artifact resulting from inappropriate experimental designs and hidden treatment effects (Grime 1997, Huston 1997). An important research challenge will be to distinguish among these alternatives and to identify the specific mechanisms responsible for the diversity–ecosystem function relationship. Fox (2006) developed a framework to partition effects of species loss on ecosystem function. Effects were partitioned into those resulting from random loss of species, nonrandom loss of species, and changes in functioning of remaining species. Much of the debate about the relationship between biodiversity and ecosystem function centers on the hypothesis that ecosystem integrity is dependent on the number of species and that loss of species will affect critical ecosystem services. In addition, there is an obvious inconsistency between the hypothesis that all species in an ecosystem are important and the alternative that ecosystems with a large number of species have significant functional redundancy (Figure 33.3). Chapin et al. (1997) argue that this issue can be resolved by considering functional traits of species instead of simple measures of species richness and diversity. Species richness is predicted to influence ecosystem function in several fundamental ways. First, ecosystems with a large number of species have a greater probability of containing taxa with important functional roles. Second, ecosystems with more species will likely use available resources more efficiently, resulting in greater productivity. Finally, a large number of species provide functional redundancy in an ecosystem and a buffer against species loss owing to anthropogenic disturbance. Yachi and Loreau (1999) developed a stochastic dynamic model to test this “insurance hypothesis” and concluded that greater species © 2008 by Taylor & Francis Group, LLC Clements: “3357_c033” — 2007/11/9 — 12:39 — page 722 — #8 722 Ecotoxicology: A Comprehensive Treatment richness had both a buffering effect on temporal variance and a performance-enhancing effect on productivity. As described above, ecosystem processes are more likely to respond to the functional diversity of a community rather than the total number of species. Heemsbergen et al. (2004) measured the effects of species richness and functional dissimilarity (defined as the range of species traits that determine their functional role) on soil processes (leaf litter mass loss, nitrification, and respiration). Although the number of species had relatively little impacton soil processes, leaf litter decomposition and soil respiration significantly increased with functional dissimilarity. Finally, while the focus of the biodiversity–ecosystem function debate has been primarily on the role of species diversity, we should remember that genetic diversity within populations may also influence ecosystem processes. Crutsinger et al. (2006) reported that genotypic diversity in a population of plant species increased aboveground net primary productivity and species richness of arthropod herbivores and predators. This increase in consumer species richness was a result of both greater resource productivity and greater diversity of these resources. 33.3 THE RELATIONSHIP BETWEEN ECOSYSTEM FUNCTION AND ECOSYSTEM SERVICES Although a number of uncertainties remain, the importance of ecosystem services to human welfare requires that we adopt the prudent strategy of preserving biodiversity in order to safeguard ecosystem processes vital to society. (Naeem et al. 1999) The practical significance of understanding the relationship between community patterns and ecosys- tem processes is best illustrated by considering the services provided by ecosystems. We know that natural ecosystems supply irreplaceable benefits to society, and that many of these benefits are crit- ical for the health and survival of humanity. Some ecosystem services, such as removal of nutrients and other wastes, soil stabilization, pollination, and regulation of climate and atmospheric gasses, contribute directly to human welfare. The ecosystem service most familiar to ecotoxicologists is the biotic and abiotic attenuation of contaminants, often referred to as the assimilative capacity of an ecosystem. The purifying function of ecosystems has been widely reported in the literature (Havens and James 2005, Ng et al. 2006, Richardson and Qian 1999), but only recently have research- ers considered specific management practices that facilitate assimilative capacity (Vorenhout et al. 2000). Although researchers and policymakers have long recognized the qualitative importance of ecosystem services, collaboration between ecologists and economists has improved our ability to estimate their total economic value. Costanza et al. (1997) estimated that the global economic value of 17 ecosystem services across a range of aquatic and terrestrial biomes was US$ 16–54 trillion (average = US$ 33 trillion) per year, which was approximately 1.8 times the global gross domestic product (Table 33.2). These researchers also note that as ecosystems providing services become increasingly stressed, it is likely that their economic value will significantly increase. Disruption of ecosystem services is a result of alterations in ecosystem processes that are linked either directly or indirectly to physical, chemical and biological stressors (Figure 33.1). These linkages all occur within the context of global climate change, which operates at a much larger spa- tiotemporal scale. The dependence of ecosystem processes on community characteristics described in this chapter provides additional justification for the protection of biological diversity. Identifying quantitative relationships among community patterns, processes, and ecosystem services should be a research priority in ecotoxicology. Many ecologists now recognize that research conducted exclus- ively inundisturbed ecosystems providesan important but somewhat biasedperspective of ecosystem processes (Palmer et al. 2004). Although the inclusion of humans and associated anthropogenic © 2008 by Taylor & Francis Group, LLC Clements: “3357_c033” — 2007/11/9 — 12:39 — page 723 — #9 Diversity–Ecosystem Function Relationship 723 TABLE 33.2 Ecosystem Function, Services, and Annual Economic Global Value Ecosystem Function Ecosystem Services Examples Value (US$ 10 9 ) Regulation of atmospheric chemical composition Gas regulation CO 2 /O 2 balance; O 3 for UV-B protection 1,341 Regulation of global temperature Climate regulation Greenhouse gas regulation 684 Damping ecosystem response to environmental fluctuations Disturbance regulation Storm protection, flood control, and other response to environmental variability 1,779 Regulation of hydrological flows Water regulation Provision of water for agricultural and industrial processes 1,115 Storage and retention of water Water supply Provision of water by watersheds, reservoirs, and aquifers 1,692 Soil retention Erosion control Prevention of soil loss by wind and runoff 576 Soil formation processes Soil formation Geological weathering and accumulation of organic material 53 Nutrient storage, cycling and processing Nutrient cycling N fixation; N and P cycling 17,075 Retention of nutrients and immobilization of toxic chemicals Waste treatment Pollution control and detoxification 2,277 Movement of floral gametes Pollination Providing pollinators for plant reproduction 117 Trophodynamic regulation of populations Biological control Keystone predator control of prey species; herbivore control by top predators 417 Habitat for resident and transient populations Refugia Nurseries and habitat for migratory and commercially important species 124 Portion of GPP used as food Food production Production of fish, game, fruits, nuts, and crops 1,386 Portion of GPP used as raw materials Raw materials Production of lumber, fuel, or fodder 721 Sources of unique biological materials and products Genetic resources Medicine, products for materials science, and genes for resistance to plant pathogens 79 Opportunities for recreational activities Recreation Ecotourism, sport fishing, and other outdoor activities 815 Opportunities for noncommercial uses Cultural Aesthetic, artistic, educational, and spiritual value 3,015 Source: From Table 2 in Costanza et al. (1997). disturbances into the study of basic ecosystem processes is controversial, we believe this step is fundamental to understanding the complex relationship between ecosystems and the services they provide. Because there will likely be variation in the sensitivity among ecosystem processes to chemical stressors, quantifying stressor–response relationships should be a research priority. The lack of a consensus on which ecosystem services are critical and therefore should be protected also impedes our ability to make policy decisions based on the diversity–ecosystem function relationship (Schwartz et al. 2000). A critical step will be to prioritize the importance of ecosystem services and to determine which are irreplaceable and which can be maintained with technological advances (Palmer et al. 2004). © 2008 by Taylor & Francis Group, LLC Clements: “3357_c033” — 2007/11/9 — 12:39 — page 724 — #10 724 Ecotoxicology: A Comprehensive Treatment 33.4 FUTURE RESEARCH DIRECTIONS AND IMPLICATIONS OF THE DIVERSITY–ECOSYSTEM FUNCTION RELATIONSHIP FOR ECOTOXICOLOGY 33.4.1 E FFECTS OF RANDOM AND NONRANDOM SPECIES LOSS ON ECOSYSTEM PROCESSES Because most experimental investigations of the diversity–productivity relationship have focused on terrestrial primary producers, the widespread generality of these patterns in other ecosystems is uncertain. In addition, most studies investigating this relationship have assumed that elimination of species is a random process. However, the susceptibility of a species to anthropogenic disturbance in natural systems will be influenced by a wide range of life history features, including mobility, longevity, reproductive rates, and body size (Bunker et al. 2005, Raffaelli 2004, Solan et al. 2004). For example, specialized species are likely to be more sensitive to stressors than generalized species that rely on a broader range of resources. Our understanding of the diversity–ecosystem function relationship is also limited because species removals are generally restricted to a single trophic level. We can be confident that the relationship between species diversity and ecosystem processes is considerably more complex in natural systems with multiple trophic levels than what is predicted based on single-trophic models. The oft-cited metaphor that keystone species represent a critical supporting stone in an arch of subordinate species has recently been modified to account for the dynamic nature of food webs (de Ruiter et al. 2005). The potentially complex functional interactions among trophic groups require that ecologists adopt a multitrophic approach to predict ecosystem responses to changes in species diversity. For example, loss of species occupying higher trophic levels will likely have very different consequences for energy flow and other ecosystem processes compared to the loss of primary producers and herbivores. Furthermore, because species richness generally decreases at higher trophic levels and because species at higher trophic levels are often more susceptible to anthropogenic disturbances, an understanding of food web structure is necessary to predict the consequences of local species extinctions on ecosystem function (Petchey et al. 2004). In addition to the cascading effects through food webs, species occupying higher trophic levels may also have direct effects on ecosystem processes. Ngai and Srivastava (2006) reported that con- sumption of detritivores by damselfly predators reduced the export of N and increased N cycling. In systems regulated by top–down trophic interactions, we would expect that removal of species at higher trophic levels will have greater effects (Downing and Leibold 2002). However, the relation- ship between species richness and ecosystem processes is context dependent and will be influenced by many environmental factors (de Ruiter et al. 2005). For example, major changes in community composition of Tuesday Lake (Michigan, USA) resulting from removal of three planktivorous fish species and addition of one piscivorous fish species had remarkably little effect on trophic dynamics (Jonsson et al. 2005). Naeem et al. (2000) also reported that increased producer or decomposer diversity could not account for greater algal production observed in freshwater microcosms. Duffy et al. (2001) observed that species composition of grazers in marine seagrass beds strongly influ- enced productivity and was more important than species richness. These results indicate that studies focusing on a single trophic level may underestimate ecosystem effects of anthropogenic disturbance on biodiversity. Failure toconsider the consequencesof ordered versusrandom species lossesmay cause research- ers to underestimate the effects of species extinction on ecosystem function (Zavaleta and Hulvey 2004). Assuming that the loss of species from ecosystems will likely be nonrandom, understanding factors that influence the susceptibility of species to local extinction will improve our ability to predict ecosystem consequences. Solan et al. (2004) compared effects of species loss on ecosystem processes in marine sediments under random and nonrandom species extinction models. Removal of abundant, large, and highly mobile marine invertebrates had much greater effect on ecosystem © 2008 by Taylor & Francis Group, LLC [...]... For example, consider a nonlinear relationship between a response variable and a stressor variable If this function shows a dramatic change in slope at some point along a stressor gradient, then a threshold point has been defined However, this does not necessarily imply that the system has been shifted to an alternative stable state One important challenge in the evaluation of data used to estimate these... rapidly and without warning (Figure 33. 6) Catastrophic shifts to alternative stable states have been reported in a variety of ecosystems including lakes, coral reefs, deserts, and oceans (Scheffer et al 2001) Shifts to alternative stable states can be triggered by natural disturbance, such as fire or flooding, or anthropogenic factors such as climate change, nutrient accumulation, exotic species, and toxic... et al (2006) reconstructed historical baselines (300–1000 years bp) of species richness and ecosystem processes for 12 temperate estuaries and coastal ecosystems in Europe, North America, and Australia Patterns were remarkably consistent among these diverse geographic regions, showing gradual declines immediately after human settlement followed by accelerating degradation during the past 150–300 years... chemicals The loss of natural resistance caused by long-term exposure to a chronic stressor may increase the likelihood that an ecosystem will shift to an alternative stable state Although most ecosystems recover from natural disturbance through successional processes, human-induced disturbances are often unique and may move ecological Ecosystem response A B C S1 S2 Stressor level FIGURE 33. 6 Hypothetical... phytoplankton species and reduces primary productivity in marine ecosystems (Day and Neale 2002) Separating the relative importance of these direct and indirect effects will require that researchers move away from relatively small-scale, closed experimental systems to larger and more ecologically realistic outdoor systems 33. 4.5 BIODIVERSITY–ECOSYSTEM FUNCTION IN AQUATIC ECOSYSTEMS A key limitation... understanding of the consequences of reduced biological diversity at larger spatiotemporal scales is essential Schroter et al (2005) quantified the vulnerability of ecosystem services to climate and land use changes across broad geographic regions in Europe Significant species loss and increased vulnerability were observed in all regions, but effects were greatest in Mediterranean and mountainous areas Lotze... thresholds from background variation in a response variable It is also important to identify changes in slope of the function owing to external or internal factors that affect the response variable but act independently of the stressor Because of the uncertainties associated with quantifying a precise ecological threshold, some researchers have suggested that we identify a range of stressor values where... require that stressor levels be reduced below those that initially caused shifts to alternative stable states (Figure 33. 9) (Bellwood et al 2004) The need to reduce stressor levels below those that initially caused the shift to an alternative stable state clearly has important implications for the restoration and recovery of damaged ecosystems © 2008 by Taylor & Francis Group, LLC Clements: 335 7_c 033 —... shape of these relationships As explained above, linear and curvilinear increases in ecosystem processes as a function of species diversity have very different implications A linear relationship between ecosystem function and © 2008 by Taylor & Francis Group, LLC Clements: 335 7_c 033 — 2007/11/9 — 12:39 — page 727 — #13 Ecotoxicology: A Comprehensive Treatment 728 species diversity implies that all... negative (Wardle et al 2004) These linkages between above- and belowground communities and the processes they control also have important implications for distinguishing between direct and indirect effects of chemical stressors in terrestrial ecosystems We agree with Wardle et al (2004) that a combined aboveground–belowground approach may be necessary to understand the effects of global change and . other ecologically important taxa as a result of physical and chemical stressors affect ecosystem function at local and global scales (Chapin et al. 1997). In this chapter, we consider the implications. (Chapin et al. 1997). Wardle et al. (1997) took advantage of natural variation in community composition of an island archipelago to examine © 2008 by Taylor & Francis Group, LLC Clements: 335 7_c 033 . (Havens and James 2005, Ng et al. 2006, Richardson and Qian 1999), but only recently have research- ers considered specific management practices that facilitate assimilative capacity (Vorenhout et al. 2000).