Journal of Experimental Marine Biology and Ecology
Marine biodiversity and ecosystem services: an elusive link
Introduction
Present concern about the preservation of biological diversity is partially based on the belief that loss of biodiversity would result in the loss of ecosystem functions and the many services they provide to society (see Constanza et al., 1997). These services encompass a number of functions that derive from the interactions of the organisms with their environment, both physical and chemical, as well as their own value as food or raw materials (Table 1). Except for the values as food or raw materials, which are clearly species-specific, most functions are shared among many of the organisms present in any one community. Moreover, both the contribution to ecosystem functions and the viability as harvestable food or raw material of any one species seem to be very much dependent on the development of a significant abundance, for rare species can hardly have a significant impact on their environment or be reliable sources of food or raw materials for human populations. Hence, the link between biodiversity and ecosystem functions and services is not straightforward. Yet the test of these links is essential to demonstrate a significant ecosystem role for biological diversity (Tilman, 1997), which would provide reasons other than ethical for the preservation of biodiversity as a whole, rather than that of target species alone. The demonstration of this link has, therefore, inspired research efforts of many scientists (see Tilman, 1997), and ranks highest in the agendas of international research programmes on biological diversity (e.g. Diversitas; http://www.icsu.org/diversitas/).
The investigation of the link between biodiversity and ecosystem functions typically involves the test of the null hypothesis that key ecosystem functions are independent of the number of species present in the community. Hence, the original question is in fact narrowed down to the test for the existence of functionally redundant species in natural communities. A species can be considered to be functionally redundant when the community contains functionally-analogous species, so that its disappearance from the community entails no measurable loss of functionality. Explicit experimental tests of this concept have been conducted in terrestrial ecosystems (e.g. Naeem et al., 1994, Tilman and Downing, 1994, Tilman et al., 1996) with some success. The experimental designs used included confounding factors, largely derived from the fact that the synthetic communities used were not composed of randomly drawn species. These confounding factors weakened the power of the experiments, so that the conclusive test of the link between species diversity and ecosystem functions and services remains an elusive goal.
Efforts to demonstrate such links in the marine environment have been far fewer, even though there is a widespread belief that they are strong (Table 1). The reasons for this are unclear, and may derive from a more limited concern about the consequences of species loss, due to the comparatively low probability of species losses in marine compared to terrestrial ecosystems, together with greater difficulties in conducting ecosystem-level experiments. Yet, the elucidation of the link between changes in species diversity and ecosystem functions and services is very relevant in marine ecosystems where an emerging biodiversity crisis is becoming apparent. The marine biodiversity crisis is most evident in the coastal zone, where losses of mangroves, coral reefs and seagrass beds are widespread (Fortes, 1988, Short and Wyllie-Echeverria, 1996), but it also extends to the pelagic, where the populations of large pelagic fish have been decimated (Pauly et al., 1998).
My goal here is to examine the relevance of the hypothesised link between species diversity and ecosystem functions and services for marine ecosystems. I shall base this examination on the particular type of ecosystem, seagrass meadows, with which I am most familiar. Seagrass beds are particularly well suited subjects for this task because (1) they are angiosperms — similar to the species used in some of the experimental tests of the link between species diversity and the functions of land ecosystems (e.g. Tilman and Downing, 1994, Tilman et al., 1996); (2) they encompass a limited set of species, with only about 50 closely-related species worldwide and no more than 12 species coexisting in any one community (Duarte, 2000); and (3) they are experiencing worldwide losses, with a substantial loss in cover (Short and Wyllie-Echeverria, 1996), providing direct empirical evidence of the changes in ecosystem functions and services upon biodiversity loss and rendering the elucidation of this link particularly important. The reader will, however, note that the arguments formulated are not derived from exclusive consideration of seagrasses, and can be, with minor adjustments, transferred to most other marine communities. Hence, while using seagrass communities as an example, I intend to provide quasiuniversal arguments in support of the hypothesised link between species diversity and the functional performance of marine communities.
Section snippets
Seagrass biodiversity
Seagrasses comprise a group of about 50 species of closely-related angiosperms, all of them belonging to two families of monocots, and comprising 12 genera (Duarte, 2000). Seagrasses are, therefore, amongst the poorest taxa, in terms of species membership, present in the sea. The paucity of species in the seagrass flora is reflected in a low species diversity in the communities they form, with about half of the meadows described in the literature being monospecific (Duarte, 2000). Two-thirds of
Functions of seagrass meadows: ecosystem services
The ecosystem-relevant functions seagrasses perform are consequences of their physiological functions, such as primary production and nutrient recycling, by which they provide food to consumers and trap carbon and nutrients, but are also dependent on their structural role (Table 1). Seagrasses form dense communities with a high biomass (mean biomass 460 g dw m−2, Duarte and Chiscano, 1999), which offer habitat and refuge for a wide diversity of organisms. Hundreds of species are found living
Species loss in seagrass meadows: evidence for ecosystem consequences
Seagrass loss has been recorded frequently throughout the world (Short and Wyllie-Echeverria, 1996), so there is ample evidence of what its effects on the ecosystem are. Seagrass loss often occurs as catastrophic events, rather than gradual processes, for the initiation of seagrass loss triggers a number of negative effects (e.g. sediment reduction, sediment resuspension) which further accelerates their loss (Duarte, 1995). Hence, seagrass loss — similar to seagrass recovery — is often a
Similar functions, different roles: are seagrass species equivalent?
While seagrasses share a common physiological basis and architectural design, their roles in the ecosystem differ considerably. The maintenance of seagrass populations depends largely on clonal growth, which in turn depends on the speed of horizontal extension of the species (Duarte, 1991a, Marbà and Duarte, 1998). The rate of seagrass species differs by a factor of 50 across species, from slow-growing (<10 cm year−1) species, such as Posidonia oceanica, to fast-growing (>5 m year−1) species,
Positive interactions in seagrass communities
The preceding discussion of the role of species diversity in providing functional diversity to the community assumes that the effect of high species diversity is additive. This is not necessarily the case, and positive interactions within the community could, if present, enhance the functional performance of the communities beyond that encountered by adding the functions of meadows of the individual species alone. Indeed, positive interactions in plant communities have received far less
Disturbance and the role of biodiversity in the maintenance of ecosystem functions
The perception of the functional role of species richness in the community depends on whether the communities examined are in approximate steady-state or experience intense dynamics. Seagrass communities are subject to frequent disturbance, whether anthropogenic (e.g. shoreline construction, eutrophication, mechanical damage) or natural (e.g. sand wave motion, storms and hurricanes/typhons, overgrazing). These disturbances may, if sufficiently strong, lead to partial or total loss of the
Species diversity and the resilience of the functions of marine ecosystems
The discussion of the functioning of mixed seagrass meadows leads to the conclusion that the link between species richness and ecosystem functions and services is not a direct one. The reason is that the functional performance of the community is a property of the species present therein and not of their number. A few species, such as, for instance, Trichodesmium species, the main pelagic nitrogen fixer in the ocean, play key roles in marine environments which cannot be assumed by other species
Acknowledgments
This is a contribution to the PREDICT project, funded by the INCO-DC programme of the European Commission (contract ERB3514PL972504). I thank Richard Warwick, editor in chief of J. Exp. Mar. Biol. Ecol., for the invitation to write this essay, which has proven a stimulating task. [RW]
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