ECOTOXICOLOGY: A Comprehensive Treatment - Chapter 33 docx

Because reduced genetic, species, and functional diversity resulting from contaminantshas important consequences for the services provided by ecosystems, we believe that thediversity–eco

33 Patterns and Processes:

A fundamental 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 geneticvariation residing within each species as well as the functional diversity of processes for which theyare responsible The rapid loss of genetic, species, and functional diversity resulting from habitatdestruction, exotic species, climate change, overharvesting, chemical stressors, and other sources ofanthropogenic disturbance is a significant environmental concern with global consequences Argu-ments for the protection of biological diversity have traditionally been based on moral or aestheticperspectives However, researchers and policymakers are becoming increasingly aware that speciesalso 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 diversitycontrols ecosystem function, describe the limitations in our understanding of this relationship, anddiscuss the implications for ecotoxicology Because of the controversy surrounding the significance ofthe diversity–ecosystem function relationship and its practical importance in managing ecosystems,the topic has received considerable attention in the ecological literature The recent comprehensivereview published by Hooper et al (2005) is especially noteworthy because coauthors included bothproponents 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 scientificcommunity (Table 33.1)

Because reduced genetic, species, and functional diversity resulting from contaminantshas important consequences for the services provided by ecosystems, we believe that thediversity–ecosystem function relationship has significant implications for ecotoxicology We firstreview the observational and experimental studies that support the theoretical relationship between

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TABLE 33.1

Major Points of Agreement and Remaining Uncertainty within the Ecological Community Regarding the Relationship between Species Diversity and EcosystemFunction

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 ship and argue that, in addition to the direct effects of pollutants on energy flow and biogeochemicalcycles described in previous chapters, contaminant-induced change in diversity can negatively impactecosystem processes and services We also review the evidence supporting the hypothesis thatecosystems 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 studiesinvestigating the concept of ecological thresholds have important implications for understanding thediversity–ecosystem function relationship We suggest that quantifying the level of perturbation (orspecies loss) where ecosystem function is significantly impaired will improve our ability to predicteffects of anthropogenic perturbations

relation-Many of the important ecosystem processes we have discussed in the preceding chapters, ing primary productivity, nutrient dynamics, decomposition, and energy flow respond directly toanthropogenic perturbations We also know that changes in abundance of keystone species and otherecologically important taxa as a result of physical and chemical stressors affect ecosystem function atlocal and global scales (Chapin et al 1997) In this chapter, we consider the implications of reduced

includ-species richness and diversity on ecosystem processes The fundamental argument supporting thisrelationship is quite simple From a probabilistic perspective, greater species richness increasesthe likelihood that functionally important species will be present in an ecosystem Elimination ofspecies in ecosystems with lower species diversity increases the likelihood that critical ecosystemprocesses will be affected As described previously (Chapter 25), greater species diversity providesfunctional redundancy and increases the resistance and resilience of ecosystems to anthropogenicperturbations For example, Frost et al (1999) attributed the functional redundancy and increasedresilience of acidified lakes to high zooplankton diversity.

Although our understanding of the mechanistic linkages between community structure and system function remains limited, it is becoming increasingly apparent that alterations in ecosystemprocesses as a result of species loss have important implications for ecosystem services provided

eco-to humans Species loss within functionally related assemblages, such as pollinaeco-tors and floweringplants, may impact ecosystem services at very large spatial scales (Biesmeijer et al 2006) Theunprecedented rate of species extinction occurring at a global scale requires that ecologists andecotoxicologists develop a better appreciation of the relationship between patterns and processes.Despite the recent interest in this relationship within the basic ecological literature, surprisingly fewstudies have examined the consequences of contaminant-induced species loss on ecosystem functionand services In addition to measuring the direct effects of chemical stressors on ecosystems, webelieve that it is important that ecotoxicologists recognize the indirect effects on ecosystem processesowing to loss of species The goal of this chapter is to provide an ecotoxicological perspective onthe 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 system function for over a century, the topic remains controversial in the contemporary ecologicalliterature (Grime 1997, Hooper and Vitousek 1997, Huston 1997) There is general agreement thathigh 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 thathigh species diversity maximizes resource acquisition across trophic levels, reduces the risk associ-ated with stochastic changes in environmental conditions, and protects communities from exposure

eco-to pathogens or exotic species Because diversity’s relationship with ecosystem goods and serviceshas important socioeconomic implications, this debate has also generated significant attention amongpolicymakers

A key issue in the diversity–ecosystem function debate is that many of the ecosystem processesthat have been linked directly to species diversity, such as the primary productivity of tropicalrainforests, 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 ecosystemfunction, ecosystem processes such as primary productivity also regulate species diversity Theinfluence of ecosystem processes on species diversity may be very complex For example, Chaseand Leibold (2002) reported that the shape of the relationship between productivity and speciesdiversity was scale dependent At a local scale, the relationship was hump shaped, with diversityincreasing 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 thesecomplex, scale-dependent relationships, an understanding of potential feedbacks between diversityand 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 Themost contentious debates within the ecological community pertain to the mechanisms by whichspecies 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

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 back is necessary for characterizing these relationships All of these interactions must be interpreted within the

feed-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 ecosystemfunction 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 arelationship between species diversity and ecosystem function, the mechanisms responsible for thisrelationship remain a significant source of controversy

DIVERSITY–ECOSYSTEMFUNCTIONRELATIONSHIP

Large-scale field experiments conducted in grasslands by Tilman and colleagues have contributedsignificantly to our understanding of the relationship between diversity and plant productivity (Tilman

et al 1997) By adding a known number of species (0–32) or functional groups (0–5) to large (169 m2)grassland plots, these researchers found that both species diversity and functional diversity influencedplant productivity (Figure 33.2) When results were analyzed based on functional composition, therelationships were stronger, suggesting that composition of the community was more important thanthe number of species Similarly, Hooper and Vitousek (1997) reported that the composition of plantfunctional 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 importantpredictors of ecosystem integrity than the actual identity of those species

A large-scale experimental test of the relationship between grassland plant diversity and ductivity was conducted at eight European field sites (Hector et al 1999) Five levels of speciesrichness 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

pro-as aboveground biompro-ass) varied among locations, but the overall pattern at all sites wpro-as greaterproductivity with higher species richness The mechanisms proposed to account for this patternincluded positive mutualistic interactions among species and niche complementarity, whereby vari-ation among species resulted in more complete utilization of resources Although distinguishingbetween these alternative explanations will not be simple, the results demonstrate that loss of species

Species diversity

0 50 100 150 200

Functional diversity

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 aknown 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 Consistentwith predictions of the drivers and passengers model described inChapter 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 aspecies-rich tropical river had large effects on carbon flow and ecosystem metabolism These resultswere contrary to the theoretical prediction that high species diversity at lower trophic levels providesinsurance against changes in ecosystem function If one of the key goals of basic ecology is toidentify 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

ECOSYSTEMS

The positive influence of species richness on ecosystem function reported in many studies has attainedgreater 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–productivityrelationship and to identify mechanistic explanations From a species conservation perspective, theshape 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 processesand richness implies that all species in a community are important and contribute to ecosystemfunction (Figure 33.3) However, if the relationship is curvilinear and ecosystem processes can besupported 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 ofecosystem processes on species richness These researchers recommended caution when using the

FIGURE 33.3 Three hypothetical relationships between species richness and ecosystem processes Type A is

an example of where ecosystem processes are saturated at a relatively low number of species This response alsoshows an abrupt threshold response when species richness is reduced below a certain critical number Type Bshows an intermediate relationship between species richness and ecosystem function The linear relationshipbetween 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 commonlyobserved 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 andecosystem processes

In many respects, the relationship between species richness and ecosystem function is closely related

to the diversity–stability relationship described inChapter 25 Indeed, one of the responses frequentlycited 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 predictthat, if species abundances vary randomly or are negatively correlated, ecosystem processes will bemore 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 ecosystemswhere species have varying sensitivities to environmental conditions Despite its broad theoreticalsupport and intuitive appeal, there have been few long-term experiments testing the relationshipbetween species diversity and ecosystem stability in nature We agree with Hooper et al (2005) thatlinking results of long-term experiments with theoretical and mathematical models will improve ourunderstanding of the role that biological diversity plays in stabilizing ecosystem function

RELATIONSHIP

Critics of the diversity–ecosystem function relationship argue that ecosystem properties are not adirect consequence of species richness or diversity per se, but simply an outcome of the functionalcomposition of dominant species Experimental studies conducted in grasslands, greenhouses, andgrowth chambers that controlled for potential confounding variables have demonstrated a strongpositive relationship between species diversity and plant productivity However, large-scale com-parative field studies showed that this relationship was not consistent, suggesting that factors otherthan 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

the relationship between island size and ecosystem processes In contrast to predictions based onmany studies, ecosystem processes were inversely related to species diversity.

Another legitimate criticism of studies reporting a relationship between community structureand ecosystem processes is that biological diversity within a community is frequently reduced to asingle number (e.g., species richness) However, there are other important community characteristicsthat are equally likely to respond to anthropogenic stressors and influence ecosystem processes Forexample, in addition to reduced species richness and alterations in community composition, one ofthe most consistent responses to many chemical stressors is increased dominance of pollution-tolerantspecies Dangles and Malmqvist (2004) reported that the relationship between species richness andleaf breakdown rates in 36 European streams was determined by dominance of invertebrate shredders.Detrital processing rates were higher and showed an asymptotically increasing relationship withspecies richness in streams with high dominance, indicating considerable functional redundancy

In contrast, the relationship between species richness and detrital processing in streams with aneven distribution of individuals was linear, indicating that all shredder species were important andcontributed to ecosystem function

DIVERSITY–ECOSYSTEMFUNCTIONRELATIONSHIP

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 ecosystemproperties, 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 thefunctions performed by individual species are limited, it follows that alterations in communitycomposition resulting from anthropogenic disturbance will affect ecosystem processes However,identifying the specific mechanistic explanations for the diversity–ecosystem function relationshipand characterizing its form (e.g., linear vs curvilinear) have been challenging Some ecologistssuggest that this relationship is inconsistent among communities because the relative contributions

of individual species to ecosystem function are context dependent and vary with environmentalconditions (Cardinale et al 2000) Others argue that the observed pattern is a sampling artifactresulting from inappropriate experimental designs and hidden treatment effects (Grime 1997, Huston1997) An important research challenge will be to distinguish among these alternatives and to identifythe specific mechanisms responsible for the diversity–ecosystem function relationship Fox (2006)developed a framework to partition effects of species loss on ecosystem function Effects werepartitioned 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 ofspecies will affect critical ecosystem services In addition, there is an obvious inconsistency betweenthe 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 simplemeasures of species richness and diversity Species richness is predicted to influence ecosystemfunction in several fundamental ways First, ecosystems with a large number of species have agreater probability of containing taxa with important functional roles Second, ecosystems withmore 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 bufferagainst species loss owing to anthropogenic disturbance Yachi and Loreau (1999) developed astochastic dynamic model to test this “insurance hypothesis” and concluded that greater species

richness had both a buffering effect on temporal variance and a performance-enhancing effect onproductivity.

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 theeffects of species richness and functional dissimilarity (defined as the range of species traits thatdetermine their functional role) on soil processes (leaf litter mass loss, nitrification, and respiration).Although the number of species had relatively little impact on soil processes, leaf litter decompositionand soil respiration significantly increased with functional dissimilarity Finally, while the focus ofthe biodiversity–ecosystem function debate has been primarily on the role of species diversity, weshould 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 increasedaboveground 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 andgreater 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 welfarerequires that we adopt the prudent strategy of preserving biodiversity in order to safeguard ecosystemprocesses vital to society

(Naeem et al 1999)

The practical significance of understanding the relationship between community patterns and tem processes is best illustrated by considering the services provided by ecosystems We know thatnatural 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 nutrientsand 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 thebiotic and abiotic attenuation of contaminants, often referred to as the assimilative capacity of anecosystem The purifying function of ecosystems has been widely reported in the literature (Havensand 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 ofecosystem services, collaboration between ecologists and economists has improved our ability toestimate their total economic value Costanza et al (1997) estimated that the global economic value

ecosys-of 17 ecosystem services across a range ecosys-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 domesticproduct (Table 33.2) These researchers also note that as ecosystems providing services becomeincreasingly 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 linkedeither directly or indirectly to physical, chemical and biological stressors (Figure 33.1) Theselinkages 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 Identifyingquantitative relationships among community patterns, processes, and ecosystem services should be

a research priority in ecotoxicology Many ecologists now recognize that research conducted ively in undisturbed ecosystems provides an important but somewhat biased perspective of ecosystemprocesses (Palmer et al 2004) Although the inclusion of humans and associated anthropogenic

exclus-TABLE 33.2

EcosystemFunction, Services, and Annual Economic Global Value

EcosystemFunction EcosystemServices Examples Value (US$ 10 9 )

Regulation of atmospheric chemical

composition

Gas regulation CO2/O2balance; O3for 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 isfundamental to understanding the complex relationship between ecosystems and the services theyprovide Because there will likely be variation in the sensitivity among ecosystem processes tochemical stressors, quantifying stressor–response relationships should be a research priority Thelack of a consensus on which ecosystem services are critical and therefore should be protected alsoimpedes 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 servicesand to determine which are irreplaceable and which can be maintained with technological advances(Palmer et al 2004)

33.4 FUTURE RESEARCH DIRECTIONS AND

IMPLICATIONS OF THE DIVERSITY–ECOSYSTEM

FUNCTION RELATIONSHIP FOR

ECOTOXICOLOGY

ECOSYSTEMPROCESSES

Because most experimental investigations of the diversity–productivity relationship have focused

on terrestrial primary producers, the widespread generality of these patterns in other ecosystems isuncertain In addition, most studies investigating this relationship have assumed that elimination ofspecies 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 speciesthat rely on a broader range of resources Our understanding of the diversity–ecosystem functionrelationship is also limited because species removals are generally restricted to a single trophiclevel 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 predictedbased on single-trophic models The oft-cited metaphor that keystone species represent a criticalsupporting stone in an arch of subordinate species has recently been modified to account for thedynamic nature of food webs (de Ruiter et al 2005) The potentially complex functional interactionsamong trophic groups require that ecologists adopt a multitrophic approach to predict ecosystemresponses to changes in species diversity For example, loss of species occupying higher trophiclevels will likely have very different consequences for energy flow and other ecosystem processescompared to the loss of primary producers and herbivores Furthermore, because species richnessgenerally decreases at higher trophic levels and because species at higher trophic levels are oftenmore 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 levelsmay 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 athigher 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 communitycomposition of Tuesday Lake (Michigan, USA) resulting from removal of three planktivorous fishspecies 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 decomposerdiversity 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 enced productivity and was more important than species richness These results indicate that studiesfocusing on a single trophic level may underestimate ecosystem effects of anthropogenic disturbance

influ-on biodiversity

Failure to consider the consequences of ordered versus random species losses may cause ers to underestimate the effects of species extinction on ecosystem function (Zavaleta and Hulvey2004) Assuming that the loss of species from ecosystems will likely be nonrandom, understandingfactors that influence the susceptibility of species to local extinction will improve our ability topredict ecosystem consequences Solan et al (2004) compared effects of species loss on ecosystemprocesses in marine sediments under random and nonrandom species extinction models Removal

research-of abundant, large, and highly mobile marine invertebrates had much greater effect on ecosystem