Biocentric Stability

Often top—down controls by predators or pathogens have much greater effect on biomass and species composition of lower trophic levels than on the flow of energy or nutrients through the ecosystem, because declines in producer biomass are compensated by increased productivity and nutrient cycling rates by other trophic levels. Intensely grazed grassland systems such as the southern and south-eastern Serengeti, for example, have a low plant biomass but rapid cycling of carbon and nutrients due to treading and excretion by large mammals. Grazing prevents the accumulation of standing dead litter, which return nutrients to soil in plant-available forms (McNaughton 1985, 1988). Keystone predators or grazers thus alter the pathway of energy and nutrient flow, modifying the balance between herbivore-based and detritus-based food chains, but we know less about their effects on total energy and nutrients cycling through ecosystems.
Many species effects on ecosystems are indirect and not easily predicted. Species that themselves have small effects on ecosystem processes can have large indirect effects if they influence the abundance of species with large direct ecosystem effects, as described for trophic interactions. Thus a seed disperser or pollinator that has little direct effect on ecosystem processes may be essential for persistence of a canopy species with a greater direct ecosystem impact. Stream predatory invertebrates rates alter the behaviour of their prey, making them more vulnerable to fish predation, which leads to an increase in the weight gain of fish (Soluck and Richardson 1997).Thus all types of organisms—plants, animals, and microorganisms—must be considered in understanding the effects of biodiversity on ecosystem functioning. Although each of these examples is unique to a particular ecosystem, the ubiquitous nature of species interactions with strong ecosystem effects makes these interactions a general feature of ecosystem functioning (Chapin et al. 2000b). In many cases, changes in these interactions alter the traits that are expressed by species and therefore the effects of species on ecosystem processes. Consequently, simply knowing that a species is present or absent is insufficient to predict its impact on ecosystems. There is currently no clear theoretical framework to predict when these indirect effects are most important. Consequently, the introduction or loss of a species, such as a popular sport fish, often generates unanticipated surprises (Carpenter and Kitchell 1993).


Diversity Effects on Ecosystem Processes
Diversity within a functional type may enhance the efficiency of resource use and retention in ecosystems. Many species in a community appear functionally similar, for example, the nanoplankton in the ocean or the canopy trees in a tropical forest. What are the ecosystem consequences of changes in species diversity within a functional type? Evolutionary theory provides some clues. Ecologically similar species co-exist in a community in part because of niche partitioning. In other words, co-existing species differ slightly in their responses to environment, perhaps specializing to use different soil horizons, canopy heights, or times of season. They may also differ in the range of temperatures or water or nutrient availabilities that they exploit effectively (Tilman 1988). These subtle differences in environmental specialization might increase the efficiency of resource use by the community if some species use resources that would otherwise not be tapped by other species.
In experimental grassland communities, for example, plots that were planted with a larger number of species had greater plant cover and lower concentrations of inorganic soil nitrogen than did low-diversity plots (Tilman et al. 1996). The more diverse plots might use more resources because species have complementary patterns of resource use; in other words, species might differ in the types of resources, the location of their roots, or their timing of uptake. Alternatively, diverse plots might use resources more effectively because they are more likely to have a species that is highly effective in capturing resources or are more likely to include species with complementary patterns of resource use (Hooper et al., in press). In other cases, low-diversity ecosystems are quite efficient in using soil resources. Crop or forest monocultures, for example, are often just as productive as mixed cropping systems (Vandermeer 1995) and mixed-species forest stands (Rodin and Bazilevich 1967). Although there are many examples of a positive relations hip between species number and productivity or efficiency of resource use, this does not always occur. The effect of species richness
Diversity Effects on Ecosystem Processes frequently saturates at a much lower number of species (5 to 10) than characterize most natural communities.
Diversity of functionally similar species stabilizes ecosystem processes in the face of temporal variation in environment. In ecosystems in which functionally similar species differ in environmental response, this can buffer ecosystem processes from environment all fluctuations (McNaughton 1977, Chapin and Shaver 1985). Tropical tree species, for example, differ subtly in their growth response to nutrients. Conditions that favour some species will likely reduce the competitive advantage of other functionally similar species, thus stabilizing the total biomass or activity by the entire community. In other words, in compensation for the reduced growth by some species, other species grow more. For example, in one study, annual variation in weather caused at least a twofold variation production by each of the major vascular plant species in arctic tussock tundra. Years that were favourable for some species, however, reduced the productivity of others, so there was no significant difference in productivity at the ecosystem scale among the 5 years examined (Chapin and Shaver 1985). Directional changes in environment can also cause less change in total biomass than in the biomass of individual species for similar reasons; some species respond positively to the change in environment, whereas other species respond negatively. This stabilization of biomass and production by diversity has been observed in many studies (Cottingham et al. 2001), including grasslands, in response to the addition of water and nutrients (Lauenroth et al. 1978) and to grazing (McNaughton 1977);
in tundra, in response to changes in temperature, light, and nutrients (Chapin and Shaver
1985); and in lakes, in response to acidification (Frost et al. 1995). This stability of processes provided by diversity has societal relevance. Many traditional farmers plant diverse crops, not to maximize productivity in a given year but to decrease the chances of crop failure in a bad year (Altieri 1990).