Biodiversity and ecosystem functioning in the Anthropocene
BEF research has shown clearly, as have many other lines of ecological and oceanographic research, that there are many interacting mechanisms by which species richness, identity, and composition can affect ecosystem-level properties and processes. But BEF is moving—or attempting to move—from a basic field of science to a more applied field designed to address specific questions. Applying insights from the primarily academic tradition of BEF to understand how complex, real-world ecosystems will respond to particular environmental stressors is a daunting challenge and has only begun. As the previous section showed, pursuing this goal will require alternative designs and approaches to test the projected scenarios of extinction, depletion, and invasion that are of interest in applied ecology and management. At the same time, we must realize that the insights emerging from designs aimed at specific inferences will necessarily be less general, and will apply primarily to particular sites, species, and sets of conditions. Thus, there is an inherent tradeoff: more realistic diversity gradients may tell us a great deal about likely effects of diversity change in a particular system, but relatively little about the function of changing diversity per se.
We are nevertheless optimistic that research linking biodiversity to ecosystem functioning can valuably inform our approach to dealing with real-world challenges of global change (see also Duffy 2009; Palumbi et al. 2009 ). Although important details will inevitably vary across systems and taxa, previous research appears to be converging on several fairly robust generalizations, outlined above, regarding both how diversity is changing and how it is likely to affect marine ecosystem structure and processes. Notably, the research summarized in section 12.2 indicates that decline in density and diversity is typically most pronounced among higher consumers, whereas local diversity tends to be increasing at the level of detritivores and suspension feeders. These patterns clearly have implications for the functioning of marine food-webs and ecosystems. Below we consider a few of several possible hypotheses for how these changes may affect ecosystem functioning based on our current knowledge of BEF relationships outlined in section 12.3.
- 1. Intense harvesting shifts marine communities toward smaller average body sizes and lower average trophic levels, altering top-down control. Disproportionate impacts on large-bodied, slow-growing, and predatory species will tend, on average, to reduce food- chain length and shift communities toward dominance by small-bodied, fast-maturing, and ecologically generalized omnivores, detritivores, and suspension-feeders. The altered top-down control expected as a result of this trophic skew likely will be central to understanding effects of global change on marine ecosystems. However, the specific consequences of top predator decline will depend on the effective length of food chains (Wootton and Power 1993; Stibor et al. 2004) and various other factors (Bruno and Cardinale 2008, see also Figure 12.2), and thus may either increase or decrease top-down control on particular species or assemblages.
- 2. Invasions by suspensionfeeders may increase local to regional scale primary consumer richness, potentially altering standing stocks of phytoplankton. The introduction of exotic suspension-feeders has sometimes dramatically reduced phytoplankton biomass in both estuarine (Alpine and Cloern 1992) and lake ecosystems (Nicholls and Hopkins 1993). Although an experiment that increased species richness of marine suspension-feeders did not affect community filtration rate in the short term, and there was no clear differences in filtration capacity between invaders and natives, differences in seasonal phenology of novel versus resident species may increase the consistency of filtration over longer periods (Byrnes and Stachowicz 2009a). The potential implications of changing suspension feeder diversity are broad since they can alter the stock of food at the base of the web. Whether changing diversity of suspension-feeders might make them more resistant to consumers, as meta-analyses suggest for prey generally (section 12.2 Figure 12.4), remains to be tested.
- 3. Invasions will increase richness of decomposers and detritivores, leading to faster processing of detrital matter and nutrient cycling. The growing diversity of species at low trophic levels in many marine systems (Byrnes et al. 2007), coupled with a general trend toward greater decomposition rate as detritivore richness increases (Srivastava et al.
- 2009), suggests that rates of organic matter turnover and nutrient recycling might increase in detritus-based marine ecosystems. These processes have important rippling affects through ecosystems, and could have profound consequences for functioning.
- 4. Reduced diversity of primary producers, as a result of habitat degradation and/or dominance by exotic invaders, will reduce habitat and food quality for herbivores, decreasing trophic transfer and fish productivity. Benthic habitats worldwide are being altered or degraded by dredging and trawling (Thrush and Dayton 2002), loss of macrophyte vegetation due to eutrophication (Duarte 2002), and invasions of aggressive habitat-forming species such as the alga Caulerpa sp and ascidian Didemnum (Ruiz et al. 1999; Airoldi and Beck 2007; Byrnes et al. 2007). In many cases, these processes convert diverse benthic communities to near monocultures or depauperate assemblages of basal species. Robust associations between habitat diversity and associated animal diversity, and between prey—including algae—diversity and consumer fitness (Lefchek et al. submitted) suggest that these alterations are likely to reduce the diversity and productivity of animals in affected habitats, including both invertebrate primary consumers and the fishes that feed on them, with consequences for fisheries.
Finally, although we have striven to be as concrete and specific as possible in our predictions, it must be emphasized that human economies, ecosystems, and the multivariate stressors that affect them are all inherently complex and their interactions are often non-linear. Thus we can be certain of ecological surprises (Doak et al. 2008). The evolving distributions of species mediated by climate change and human commerce are creating 'novel' or 'emerging' ecosystems populated by species with little or no shared evolutionary history and without close analogues in natural communities (Hobbs et al. 2006; Williams and Jackson 2007). To the extent that our current understanding of BEF—and ecology generally—is based on existing communities, the prospect of these novel ecosystems emphasizes the need for testing the robustness of our conclusions and models under new conditions. Positive surprises are also possible. For example, human population growth is declining more rapidly than predicted in several parts of the world. Marine protected areas and other measures have successfully changed the trajectory of ecosystem decline in several systems (Halpern and Warner 2002; Edgar et al. 2009), and it is conceivable that more widespread implementation of sustainable fishing practices might turn the tide of marine degradation on a larger scale (Worm et al. 2009). Nevertheless, from a practical, management-oriented standpoint, the certainty of future uncertainty argues for a precautionary approach to managing our interaction with the biosphere. In this context, the practical lesson from research linking biodiversity to ecosystem functioning may be a very simple and general one: maintaining viable populations of as many species, genes, and landscape elements as possible may be our best insurance against ecosystem failure in the face of change.