Environmental Problems, Their Causes, and Sustainability 1

advantages over other species by occupying as much ofits fundamental niche as possible.This change in the way we view what is happeningin nature explains why a growing number of ecologistsprefer terms such as biotic change instead ofsuccession—which implies an ordered and expected sequenceof changes. Many ecologists have also replacedthe term climax community with terms such as maturecommunity or a mosaic of vegetation patches at variousstages of succession.8-6 ECOLOGICAL STABILITY,COMPLEXITY, AND SUSTAINABILITYWhat Is Stability? Long-Term Sustainabilitythrough Continuous ChangeLiving systems maintain some degree of stability orsustainability through constant change in response tochanging environmental conditions.All systems from a cell to the biosphere are constantlychanging in response to changing environmental conditions.Continents move, the climate changes, anddisturbances and succession change the compositionof communities. Even without human actions, ecologicalcommunities have always faced change.So how do living organisms, communities, ecosystems,and the biosphere face changes and survive? Allliving systems, from single-celled organisms to thebiosphere, contain complex networks of negative andpositive feedback loops that interact to provide somedegree of stability or sustainability over each system’sexpected life span.This stability is maintained only by constantchange in response to changing environmental conditions.For example, in a mature tropical rain forest,some trees die and others take their places. However,unless the forest is cut, burned, or otherwise destroyed,you would still recognize it as a tropical rainforest 50 or 100 years from now.It is useful to distinguish among three aspects ofstability or sustainability in living systems. One is inertia,or persistence: the ability of a living system toresist being disturbed or altered. A second is constancy:the ability of a living system such as a populationto keep its numbers within the limits imposed byavailable resources. A third factor is resilience: theability of a living system to repair damage after an externaldisturbance that is not too drastic.Does Ecological Complexity IncreaseEcological Stability? Mixed ResultsHaving many different species can provide someecological stability or sustainability for communities,but we do not know whether this applies to allcommunities nor the minimum number of speciesneeded for such stability.In ecological terms, complexity refers to the number ofspecies in a community (species richness) at eachtrophic level and the number of trophic levels ina community. It is one measure of a community’sbiodiversity.In the 1960s, most ecologists believed the greaterthe species diversity and the accompanying web offeeding and biotic interactions in an ecosystem, thegreater its stability. According to this hypothesis, acomplex and biodiverse community with a diversityof species and feeding paths has more ways to respondto most environmental stresses because it does nothave “all its eggs in one basket.” This is a useful hypothesis,but recent research has found exceptions tothis intuitively appealing idea.Because no community can function withoutsome producers and decomposers, there is a minimumthreshold of species diversity below which communitiesand ecosystems cannot function. Beyondthis, it is difficult to know whether simple communitiesare less stable than complex and biodiverse onesor to identify the threshold of species diversity neededto maintain community stability. Recent research byecologist David Tilman and others suggests that communitieswith more species tend to have a higher netprimary productivity and can be more resilient thansimpler ones.Many studies support the idea that some level ofbiodiversity provides insurance against catastrophe.But how much biodiversity is needed in various communitiesremains uncertain. For example, some recentresearch suggests that the average annual netprimary productivity of an ecosystem reaches a peakwith 10–40 producer species. Many ecosystems containmore producer species than this, but it is difficultto distinguish among those that are essential andthose that are not. We need much more of this type ofresearch.Part of the problem is that ecologists disagree onhow to define stability. Does an ecosystem need bothhigh inertia and high resilience to be considered stable?Evidence suggests that some ecosystems have oneof these properties but not the other. For example,tropical rain forests have high species diversity andhigh inertia; that is, they are resistant to significant alterationor destruction.However, once a large tract of tropical forest is severelydegraded, the community’s resilience sometimesis so low that the forest may not be restored. Nutrients(which are stored primarily in the vegetation,not in the soil), and other factors needed for recoverymay no longer be present. Such a large-scale loss offorest cover may also change the local or regional climateso that forests can no longer be supported.160 CHAPTER 8 Community Ecology