subject: Understanding Diversity-Stability relationship of an ecosystem [print this page] Understanding Diversity-Stability relationship of an ecosystem
The belief worldwide by many biologists and scientist is that the more diverse the ecosystem is, the more it is stable. Many conclusions have been too general and hence there have been gaps in most research on ecosystem diversity. But to fully understand the relationship, there is need to note that there are different types of diversity i.e. diversity can be within a particular ecosystem or between ecosystems. Robert May (1973) was the first to describe the relationship between diversity and stability in what are called local stability analysis. With these he proves that an increase in number of species and interaction strength all lead to a decrease in the stability of a community. Otherwise there are many empirical tests and mathematical models that try to prove that stability decreases with increasing complexity.
Diversity and stability of an ecosystem depends on how species interact with each other and how the environment will affect each species. Different properties of an ecosystem lead to different relationships in diversity and stability.
Diverse ecosystem means that it can absorb stress and other pressures before it collapses.
The number of species and the number of individuals in each species are the major factors determining the stability of an ecology. For example, moving towards the poles i.e. North Pole and south pole, the communities tend to be simpler in terms of species i.e. there are more individuals in each species but the number of species become fewer. And in appearance these communities seem instable or less stable. And because of the greater number of individuals in each species, any change present is conspicuous. In equatorial populations, the species are many and though the number individuals are few, any change occurring becomes inconspicuous.
An ecosystem with abundant species hence lower population per species and will equilibrate fluctuations in that the predators will switch from the less abundant species to the more abundant species as prey. In this way, the predator will allow the population of the less common species to increase and that of the more common to decrease.
Changes in population affects economic activities especially agriculture whereby an increase in population leads to encroachment of forests and this affects the stability of the ecology. Research shows that an increase in diversity causes community stability to increase but population stability to decrease.
A greater plant diversity will lead to greater primary productivity. This is because of the chance that a more productive species will be present in a higher diversity. Thus there will be complete utilization of limited resources in a higher diversity and thus will increase resource retention and more productivity. The lower levels of limited resources at a higher diversity will decrease the susceptibility of an ecosystem to invasion. Hence the community will be stable.
The benefit of a more diverse community is that they are able to tap resources due to their difference of acquiring resources at each trophic level. It also reduces the risk of changes in the ecosystem due to other variations e.g. in the environment, invasion by the pathogens or by predators (Chapin et al.)
Other tests have been done by J.S.McNaughton (1977, 516) who argues that true diversity-stability relationship depends on empirical tests. He and his colleagues conducted a study at Serengeti-Mara ecosystem in the stretch of Tanzania and Kenya on the effect that African Buffaloes have on grazing on the grasslands. His conclusion was that in the more diverse stand, species diversity decreased than in the less diverse stands. Other researchers have contradicted Mc Naughton e.g. Pimm and Anthony King (1983),who devised food web models and they examined 3 types of complexities i.e. species diversity, species richness and connectance.Hence they concluded that when stability is determined by species ,then it decreases with increasing complexity.
It was Kristin Shrader-Frechette and Earl McCoy (1993) who concluded that the terms stability and diversity are ambiguous, inconsistent, and imprecise.
There are different types of stability and increasing diversity can decrease or increase different types of stability independently.
So to accept or reject the fact the more diverse an ecology/landscape is the more stable it is, one must consider all other factors present in an ecosystem.
