Marine biodiversity and ecosystem services: an elusive link

Abstract

Efforts to test the hypothesised positive link between ecosystem services and functions and biodiversity are increasing in order to forecast the consequences of the present erosion of biodiversity on ecosystem functions and to provide an additional basis for the conservation of biodiversity. These efforts have been, however, modest in marine ecosystems. An examination of seagrass communities, which are simple assemblages with a limited membership of about 50 species worldwide and <12 species in any one community, provides, however, strong evidence for the existence of such positive link between species richness and ecosystem functions. Ecosystem functions are, however, dependent on the particular membership of the community, rather that its number, for the functions are species-specific properties. Yet evidence, is provided, that an increasing species richness should be, on average, linked to an increase in the functional repertoire present in the community, will lead to a more efficient use of resources and a greater capacity to ensure the sustainability of ecosystem functions under disturbance or ecosystem change. Closer examination indicates that the functional variability of mixed-species seagrass assemblages is correlated to the variability in species size, whereas species of similar size tend to show similar functional capacities and, therefore, a greater degree of functional redundancy. In addition, the demonstration of positive interactions in seagrass communities, which are also dependent on the presence of engineering species in the community that facilitate the growth of other species, provides increasing grounds to expect an enhanced functional performance of mixed communities over that expected from a simple additive contribution of the community members. Multispecific communities also hold, within the functional repertoire they contain, many unrealised functional potentials that may prove instrumental to ensure the sustainability of ecosystem functions in the presence of disturbance or a changing environment. The arguments offered, illustrated for the comparatively simple seagrass communities, provide strong reasons to expect a strong — if difficult to test experimentally — positive relationship between species diversity and the functions of marine ecosystems and, thereby, the services they yield to humanity.

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.