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FORUM FORUM FORUM FORUM is intended for new ideas or new ways of interpreting existing information. It provides a chance for suggesting hypotheses and for challenging current thinking on ecological issues. A lighter prose, designed to attract readers, will be permitted. Formal research reports, albeit short, will not be accepted, and all contributions should be concise with a relatively short list of references. A summary is not required. Functional redundancy in ecology and conser7ation Jordan S. Rosenfeld, Fisheries Research Section, Pro7ince of British Columbia, 2204 Main Mall, Uni7. of British Columbia, Vancou7er, BC, Canada V6T 1Z4 ( [email protected]). Multiple studies have shown that biodiversity loss can impair ecosystem processes, providing a sound basis for the general application of a precautionary approach to managing biodiversity. However, mechanistic details of species loss effects and the generality of impacts across ecosystem types are poorly understood. The functional niche is a useful conceptual tool for understanding redundancy, where the functional niche is defined as the area occupied by a species in an n-dimensional functional space. Experiments to assess redundancy based on a single functional attribute are biased towards finding redundancy, because species are more likely to have non-overlapping functional niches in a multi-dimensional functional space. The effect of species loss in any particular ecosystem will depend on i) the range of function and diversity of species within a functional group, ii) the relative partitioning of variance in functional space between and within functional groups, and iii) the potential for functional compensation (degree of functional niche overlap) of the species within a functional group. Future research on functional impairment with species loss should focus on identifying which species, functional groups, and ecosystems are most vulnerable to functional impairment from species loss, so that these can be prioritized for management activities directed at maintaining ecosystem function. This will require a better understanding of how the organization of diversity into discrete functional groups differs between different communities and ecosystems. The accelerating loss of biodiversity has become a primary concern of society and ecologists. The negative impacts of species loss are based on both the intrinsic value of individual taxa, and the potential effects of biodiversity loss on ecosystem function, where function is defined in terms of energy transformation and matter cycling (Lawton 1994, Ghilarov 2000). The intrinsic worth of species, while a critical (and arguably paramount) rationale for protecting biodiversity, is a matter of opinion that is largely value-based and not a scientific issue. In contrast, the impact of species loss on ecosystem function is clearly a scientific issue that lends itself to hypothesis testing and experimentation. Consequently, the effects of species loss on ecosystem function has been an area of active research (Tilman et al. 1996, 1997a, b, Naeem and Li 1997, Hector et al. 1999). 156 The concept of functional redundancy is at the core of theory relating changes in ecosystem function to species loss. Functional redundancy is based on the observation that some species perform similar roles in communities and ecosystems, and may therefore be substitutable with little impact on ecosystem processes (Lawton and Brown 1993). Although functional redundancy is a relatively recent construct in ecology, it’s roots extend back to the concept of ecological guilds (Root 1967), whereby species are grouped together based on similarities in what they do within communities. In terms of practical conservation issues, the concept of functional redundancy is a double-edged sword. It is an important tool for justifying and prioritizing species protection, since functional differentiation argues for the preservation of individual species (Walker 1995) and biodiversity in general (Lawton 1994) to maintain ecosystem processes. However, at its heart is the assumption that some species perform similar roles in ecosystems, and redundant species can therefore be lost with minimal impact on ecosystem processes. While the logic underlying this assumption is somewhat flawed because even similar species will always differ (by definition) to some unknown degree along some functional axes, it remains reasonable to expect that some species are more similar than others in terms of what they do in communities and ecosystems. It is also worth noting that this interpretation of redundant species as ‘‘expendable’’ was not part of the original construct of ecological redundancy (Walker 1992), where redundant species were considered necessary to ensure ecosystem resilience to perturbation (Walker 1995). The objective of this note is to consider the relationship of functional redundancy to other concepts in ecology, to introduce the concept of the functional niche as a heuristic tool for understanding functional redundancy, and to review problems and limitations in OIKOS 98:1 (2002) both the evaluation of functional redundancy and the search for generalities in effects of species loss on ecosystem function. Functionalism in ecology The concept of functional redundancy – the degree to which organisms have evolved to do similar things – is closely related to several fundamental issues in ecology and evolution: organization of species into ecological guilds and trophic levels, the Huchinsonian (or ecological) niche, competition, and limiting similarity. Ecologists have long recognized that taxa can be organized into relatively discrete groupings based on similarities in what they do. Trophic levels, guilds of stream fishes (Winemiller and Pianka 1990), and functional feeding groups of aquatic invertebrates (Cummins 1973) are all based on commonality of function within identified groups. The usefulness of these classifications depends largely on the variation in function within each group relative to the variation between groups (similarity within groups vs dispersion across functional space; see Fig. 1). Several authors have questioned the utility of discrete functional classifications that oversimplify complex interactions (e.g. Polis and Strong 1996), and it is clear that ecosystems differ in the degree to which species are organized into discrete groups (Hairston and Hairston 1997). In terms of understanding functional redundancy, ecologists need to understand when such classifications are appropriate, why communities and ecosystems differ in the degree to which taxa are organized into discrete functional groups, and how this affects the functional impacts of species loss. Ecologists have also recognized that similarity between species will affect the strength of interspecific competition, and a large body of theory was developed to assess the level of similarity that permits species to co-exist (Abrams 1983). Limiting similarity is based on the assumption that competition is most intense when organisms occupy the same spatial and temporal habitats and use the same resources. Hutchinson (1957) Fig. 1. Hypothetical distribution of stream invertebrate functional groups (grazers , predators , and detritivores ) in a three-dimensional functional space, for well-differentiated functional groups (A) and for functional groups that are less discrete (B). OIKOS 98:1 (2002) proposed the niche as a conceptual tool for quantifying the similarity between species, with the habitat space occupied by an organism defined by an n-dimensional hypervolume where different axes are environmental factors such as temperature or resources. The Hutchinsonian niche separates species based primarily on the habitats and resources they use and their environmental tolerances. It is a useful construct for understanding how species overlap in space and compete for similar resources, but it is less useful for understanding the impact that species have on ecological processes. Functional effects of a species – the influence of a taxon on particular rates and processes – are output variables or consequences of organism traits and resource use. Consequently, the functional effects of a species on ecological processes are only indirectly represented by the Hutchinsonian niche. Below I explore the concept of the functional niche as a tool for understanding and quantifying functional redundancy in organisms. The functional niche A functional niche can be defined that is analogous to the Hutchinsonian niche, except that the axes represent functional attributes or process rates (e.g. oxygen generation, predation rate, etc.; Fig. 1) rather than the environmental or resource axes defining the ecological niche. The functional niche therefore represents an n-dimensional hypervolume in functional space, where axes are functions or processes (e.g. oxygen generation, decomposition rate, etc.) associated with different functional attributes. Thus the Hutchinsonian niche defines where and under what circumstances a species will exist, while the functional niche defines the ecological effect that a species will have in any given habitat. Just as the Hutchinsonian niche represents a useful heuristic tool for understanding and modeling the consequences of species similarity for competitive interactions, the functional niche may serve as a useful conceptual framework for understanding functional redundancy. Axes used to define the Hutchinsonian niche are key resource or environmental factors that influence the survival, growth, and reproduction of a species. Their selection is usually based on empirical observation or experiments. Selection of axes for defining the functional niche is more subjective, and is based on identification of key functions or processes that are thought to be important for ecosystem functioning (Walker 1992, 1995). This approach should be viewed as flexible rather than arbitrary, since it permits both an exhaustive n-dimensional definition of the functional niche of an organism, or a simplified one that reflects the often utilitarian objective of assessing ecosystem function. A flexible application of the functional niche concept permits a focus on processes that are identified as impor157 tant either as diagnostics or because they directly influence critical processes of value to society or ecosystems, but it also requires that researchers explicitly state the functions (i.e. axes) being studied. The functional niche as applied to individuals expresses the effects of a species in functional space on a per capita or per biomass basis (e.g. oxygen generation per individual, or per g). While per capita functional effects are useful for assessing functional equivalency of individuals from different species or for identifying potential keystones (species with unusually high per capita effects; Power et al. 1996), the functional impact of a species in a community or ecosystem will be a function of both per capita effects and abundance (dominance). The aggregate functional effects of a species within a trophic level or community will be the per capita rate multiplied by organism abundance. It is important to distinguish between functional equivalency in terms of per capita impacts, and redundancy in population-level functional effects, because taxa that are apparently functionally equivalent at the individual level on selected functions may differ in their equilibrium population sizes, and therefore in aggregate functional effects within a community or ecosystem. Thus the functional niche of an organism can be defined in terms of three types of factors: i) ecological processes, such as per capita (or biomass) oxygen generation, decomposition rate, predation rate, etc., which will be related to intrinsic morphological and physiological attributes of an organism, ii) demographic attributes of a species (e.g. population r and K), and iii) en6ironmental factors such as temperature that will influence where an organism can perform its function. Species that are functionally equivalent on a per capita basis may differ in demographic attributes, so that they may not be able to compensate for one another at the population level, as previously discussed. However, even when species can compensate functionally at the population level, this does not ensure functional redundancy in all circumstances, i.e. apparently redundant species may have different environmental optima, so that their functional niches do not overlap when environmental axes are included (Fig. 2). Replacement of functionally equivalent species along environmental gradients (Frost et al. 1995) represents functional complementarity rather than functional redundancy. To be truly functionally redundant, taxa must overlap in both their population-level functional effects and environmental tolerances, which can be adopted as an effective criteria for assessing functional redundancy. Although defining a functional niche in terms of processes, demographic attributes, and environmental tolerances may seem needlessly complex, identifying the relative contribution of these three variables provides a basis for understanding how species are likely to be functionally differentiated or redundant. It is also worth noting that the definition of functional redun158 Fig. 2. Hypothetical detritivore species that are apparently redundant become well-differentiated in functional space when additional demographic or environmental axes are included. dancy above differs somewhat from that of Walker (1992, 1995), who considered species in a functional group to be redundant even if they were differentiated in their environmental tolerances. However, Walker (1995) makes is clear that his usage of redundant is not equivalent to ‘‘dispensable’’, which has nevertheless become the more common interpretation (Lawton and Brown 1993). Evaluation of functional redundancy Having established that species loss has the potential to impact ecosystem function (Schläpfer et al. 1999, Mooney et al. 1995a), future research on functional impacts of species loss needs to focus on both the generality of effects and their mechanistic basis. The functional niche may serve as a useful conceptual tool for understanding how differences in the structural organization and diversity of ecosystems (e.g. grasslands, woodlands, lakes, streams, deserts, etc.; Schläpfer and Schmid 1999, Mooney et al. 1995b, 1996) influences the effects of species loss. In particular, the width and overlap of individual niches in functional space (corresponding to generalist and specialist species; Fig. 3) will influence the effects of species loss in different ecosystems, since the ability of a species to expand beyond it’s realized niche (or equivalently, the presence of competition between two species) is one measure of the potential for functional compensation (Lawton and Brown 1993). The degree to which particular communities or ecosystem types are likely to have functionally redundant (or differentiated) species will depend not only on total species richness, but also on the abundance of generalist or specialist species within each functional group, and the degree of saturation of functional space (Fig. 4); however, it is presently unOIKOS 98:1 (2002) Fig. 3. Implied functional redundancy of temporal specialist species with narrow niches (Sps. 2 –4) relative to a generalist detritivore with a broad temporal niche (Sp. 1). Species three and four are temporally redundant with species one while species two is not. clear whether these properties differ systematically between ecosystems, communities, trophic levels, or functional groups. The width and overlap of entire functional groups (Fig. 1) may also differ systematically between ecosystems as a consequence of differences in energy flow and adaptive constraints. For example, the dispersion in functional space of primary producers in forests may be very broad (herbs to understory shrubs to canopy dominants), whereas in lakes it may be very narrow (i.e. phytoplankton). These differences in the breadth of functional groups are implicated in the greater frequency of trophic cascades in aquatic systems (Power 1992, Strong 1992). There may also be strong interactions between diversity within functional groups, the breadth of function within a functional group, and the functional effects of species loss. For example, one simple prediction would be that the expected variance (e.g. predictability) in ecosystem function following species loss should be much greater for functional groups that are broad rather than narrow, particularly in low diversity systems (Fig. 5A and B), assuming no compensation by remaining species. Alternatively, if the Fig. 5. Hypothesized change in variance of detrital processing rate with increasing species diversity for random assemblages of detritivores for narrow (A) and broad (B) detritivore functional groups (corresponding to Fig. 1A and B, respectively, assuming no functional compensation for species loss). If remaining species can fully compensate for species loss in low diversity assemblages, then total population size within a functional group should have lower variance in narrow (C) rather than broad (D) functional groups. remaining species can numerically compensate (sensu Ruesink and Srivastava 2001) to maintain function in low-diversity assemblages, then the expected variance in total population size within a functional group (all species combined) should be much larger for broadly dispersed functional groups (Fig. 5C and D). This discussion highlights the need to consider the functional traits of individual species within the broader context of their functional assemblage. Researchers have generally used two approaches for evaluating functional redundancy. The first is to define the effects of individual taxa in functional space using measured Fig. 4. Three contrasting scenarios for species overlap in functional space for hypothetical detritivorous (shredding) aquatic invertebrates. Species in a functional group may have narrow niches with no overlap (A), implying no functional redundancy and a large proportion of vacant functional space. Species may have broad functional niches with no overlap (functional complementarity; B), again implying little functional redundancy, but with minimal vacant functional space. Or species may broadly overlap in their functional niches (C), implying a substantial potential for functional redundancy. OIKOS 98:1 (2002) 159 functional traits as described above (Jonsson and Malmqvist 2000). However, absolute differences between species are not very informative without the context of the assemblage they are part of. For instance, the functional significance of loosing a species will be much greater if a functional group has 2 vs 20 species, and if the range of function within the group is large. It is also difficult to predict the emergent properties of assemblages based on the traits of individual taxa, because of the multiplicity of complex interactions between species that are difficult to predict (e.g. facilitation or overyielding). For these reasons, researchers often evaluate the effects of biodiversity loss by comparing the function of entire assemblages that differ in species diversity (Tilman et al. 1996, 1997a), rather than by comparing functional attributes of individual taxa. However, understanding the functional attributes of individual species is a necessary first step towards mechanistically linking the properties of species to the properties of ecosystems. Regardless of the approach favoured to assess functional redundancy, there are serious limitations to the inferences that can be drawn from biodiversity loss experiments. These are considered in the section below. Limitations to the assessment of functional redundancy Because there is a need to apply the results of biodiversity gradient experiments to inform policy and management decisions, their limitations must be carefully considered. In particular, several implicit assumptions tend to make the experiments systematically biased towards falsely concluding that functional redundancy exists. First, the detection of functional redundancy depends in part on the metrics chosen to represent ecosystem function. Primary production is usually the measure of ecosystem function in diversity experiments with artificial plant assemblages. While primary production is a legitimate measure of ecosystem function, it is only one of many functional attributes of an ecosystem. Sec- ondary production of herbivores, for instance, is another important measure of ecosystem function. Plant assemblages that differ in the proportion of edible and inedible plants will support different productivities of herbivores. However, they may have similar levels of primary production, leading to the erroneous conclusion of functional redundancy when primary production is used as the sole metric of ecosystem function (Rosenfeld 2002). In general, experiments are more likely to conclude that species are functionally redundant as the number of different measures of ecosystem function (e.g. primary production, secondary production, resistance to disturbance, etc.) decreases. Since most biodiversity loss experiments only consider one measure of ecosystem function, they tend to be biased towards demonstrating functional redundancy. Second, indices of ecosystem function are aggregate measures of processes at the ecosystem scale (by definition). This reflects an interest in the effects of biodiversity loss on ecosystem services that are provided at a large spatial scale (Costanza et al. 1997). However, loss of biodiversity will also affect processes at subsidiary scales (Table 1). Loss of single species will have functional impacts that can be measured in terms of changes in energy flow within communities; changes in ecological processes can also be measured at the guild, functional group, trophic, and finally ecosystem levels. Functional impairment at all of these scales has the potential for significant impacts on humans and biota, and the absence of functional change at the ecosystem scale may mask changes in function at subsidiary scales (Frost et al. 1995) that are of significant consequence. For instance, overfishing of desirable predatory species in marine food chains may result in species substitution with little change in total fish biomass (Fogarty and Murawski 1998). However, these changes in species composition may result in significant alterations in energy flow within the ecosystem, as well as changes in services provided to humans (e.g. viability of the fishing industry). Focusing on ecological function at the ecosystem scale indirectly discounts the potential for significant functional impairment at smaller scales that may not result in ecosystem-level impacts. Table 1. Hierarchy of species loss and functional effects within ecosystems. All effects may occur at spatial scales ranging from local ecosystems (e.g. loss of a population from a single lake) to regional or global effects (e.g. global extinction) with corresponding functional impacts over increasingly large areas. Level of biodiversity loss Functional effect within an ecosystem Human value affected Single non-keystone species Multiple non-keystone species Dominant species Weak direct/indirect effects on other species Intrinsic value of individual species Change in community structure Potential loss of health and economic opportunities (e.g. pharmaceuticals) Change in availability of commercially important species Provision of ecosystem services (e.g. nutrient cycling, primary production, etc.) Keystone species or functional group 160 Large change in community and food-web structure Large change in community structure and functions at the ecosystem scale OIKOS 98:1 (2002) Third, biodiversity gradient experiments use random species assemblages, which is equivalent to assuming that species extinction is a random process (Huston et al. 2000). This is likely a poor analogue for natural systems, where human activities selectively impact particular species, functional groups, and trophic levels. We need to better understand how non-random patterns of species extinctions may alter predictions of functional impairment. Selective loss of particular species (e.g. top predators; Pauly et al. 1998) may well cause greater functional change than expected from the random pattern of extinction implicit in biodiversity gradient experiments. Fourth, biodiversity experiments are usually performed over short time and space scales where the environment is relatively invariant (Wellnitz and Poff 2001). Species that are apparently functionally redundant over small scales may be functionally differentiated when the environment changes over larger space and time scales. A good example is the changing functional roles of species over long time scales as biological communities change with the widespread invasion of exotics. This is illustrated by the herbivorous aquatic weevil Euhrychiopsis lecontei (Dietz), which switched from being a minor grazer of native aquatic plants in eastern North America to a keystone herbivore controlling populations of the exotic Eurasian watermilfoil (Myriophyllum spicatum L.), over 30 years after its introduction (Creed 2000). Given the widespread certainty of future exotic invasions, the functional roles of many species will likely change in unpredictable ways. Similarly, rare and apparently redundant species of zooplankton may become functionally significant when environmental conditions change (Finlay et al. 1997), as observed following lake acidification (Frost et al. 1995). Again, this argues strongly for a general precautionary approach to conserving biodiversity. Finally, it is necessary to put the impact of species loss on ecosystem function in the correct context. While changes in biodiversity may affect ecosystem function, it is clear that direct conversion of ecosystems will have more far-reaching and severe effects on global ecosystem function (Lawton and Brown 1993, Tilman et al. 2001) than will species loss. For example, conversion of tropical or temperate forest to rangeland or conversion of rivers to reservoirs will alter function far more than loss of diversity within a forest or stream ecosystem. Conclusions Given the rapid rate at which humans are altering communities, a predictive understanding of how species loss affects ecosystem function seems imperative. However, the factors identified above cause manipulative experiments to underestimate the true functional conseOIKOS 98:1 (2002) quences of species loss. Until functional redundancy experiments specifically address these shortcomings, they should be viewed as simplistic and interpreted with caution. This conclusion may appear biased and driven by a conservation ideology that is not open to objective falsification of hypotheses (i.e. that species are functionally redundant). However, few experiments to date have adequately tested for functional redundancy over the broad suite of functional traits and environmental conditions necessary to convincingly demonstrate it. While there is strong evidence for functional complementarity (e.g. compensation by functionally equivalent species along environmental gradients; Frost et al. 1995), there is poor evidence for true functional redundancy. Even once functional redundancy has been convincingly demonstrated for selected species, it is not feasible to assess the degree of functional redundancy for all species in all environments and communities. Inability to convincingly predict functional redundancy is further compounded by unpredictable changes in the potential functional roles of species following environmental change, such as the widespread invasion of exotics in natural communities (Creed 2000). All of these factors argue strongly for the precautionary approach as an overarching strategy for managing biodiversity. If one accepts a precautionary approach as the fundamental principle for managing species loss, then why bother studying the mechanisms and processes underlying functional redundancy? While a broad precautionary approach should serve as the foundation for species management, the reality is that development and other human impacts will continue, and some understanding of the functional relationships between species will help prioritize conservation decisions and provide a basis for anticipating the impact of different management scenarios on ecosystem function (Walker 1995). 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