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4 + ward ' s science Biodiversity (continued) or satiating effect of species number on ecosystem processes is frequently observed [panel (b) of Fig. 3], suggesting that eco- system function may be relatively unaffected by initial losses of species, but may become severely impaired after some critical number of species is lost. Ecosystem stability A second purported benefit of biodiversity is that more diverse ecosystems may be more stable or more predictable through time when compared to species-poor ecosystems. Stability can be defined at the community level as fewer invasions and fewer extinctions. This means that a more stable community will con- tain a more stable composition of species. However, stability also can be defined at the population level as reduced fluctua- tions in population size, meaning that a more stable population will contain a more constant number of individuals. The idea that biodiversity confers stability on ecosystems has a long and controversial history. Early ecologists used several lines of reasoning to argue that diverse ecosystems are more stable than those with fewer species. First, attempts to cre- ate simple, low-diversity ecosystems in the laboratory tended to fail, with most or all of the species declining to extinction. Second, unpredictable or strongly cyclical population dynam- ics are often observed in animals that live at high latitudes, and high-latitude ecosystems generally include relatively few spe- cies. Finally, islands, which generally have fewer species than mainlands, tend to be more easily invaded by introduced spe- cies. In the mid-1950s, an additional argument was proposed in favor of a positive relationship between biodiversity and ecosystem stability. With more species in an ecosystem, there are more paths through which energy and nutrients can flow; therefore, in diverse ecosystems, each species should be less affected by changes in the abundance of other species, leading to higher overall stability. Thus, the general consensus among early ecologists was that more diverse ecosystems should be more stable. In the early 1970s, mathematical models of many species interacting simultaneously were used to explore the relation- ship between biodiversity and population stability. The major outcome was that higher species diversity led to less stable population sizes of individual species. However, the apparent conflict between these modeling results and the intuitions of earlier ecologists remained unresolved for many years. In-depth studies in which the number of species has been manipulated experimentally have helped to resolve this long- standing controversy. For example, manipulations of plant diversity were used to examine not only the productivity of a grassland ecosystem, but also the stability of ecosystem productivity over time. Although this and other studies have shown that the abundance of individual species fluctuates more dramatically in high-diversity ecosystems, the total abun- dance or productivity of all species combined is actually more stable. High biodiversity decreases the stability of each species' population, lending support to mathematical modeling results, whereas the positive relationship between biodiversity and the stability of overall ecosystem productivity supports the propos- als of the earlier ecologists. Although the relationship between biodiversity and ecosystem stability is fairly clear, the mechanisms generating this pat- tern are not. In particular, diverse groups of species may be more stable because complementary species compensate for changes in one another's abundance. Alternatively, variation in aggregate measures (such as total productivity) may increase with richness as a result of averaging of random fluctuations in the growth of each species. The strength of this averaging effect depends on correlations among the species' fluctuations, but a positive relationship between biodiversity and the stabil- ity of aggregate measures of ecosystem function should usually be expected, due simply to averaging. In all likelihood, biodiversity is related to both the overall rates and the stability of ecosystem functions. However, document- ing a relationship between biodiversity and some measure of ecosystem function or stability does not reveal its underlying cause. Current ecological research continues to explore the mechanisms by which species diversity and functional diversity contribute to ecosystem function. Fig. 3: Ecosystem function (a) as a positive, linear function of biodiversity and (b) as a nonlinear, satiating function of biodiversity. (Copyright © McGraw Hill)