We're at that point in the course where we're switching gears. We've spent the last six weeks focusing on the biology of small populations: an overview of the U.S. Endangered Species Act and how it is used to manage and protect species that are recognized as threatened or endangered by the federal government, understanding the deterministic and stochastic threats to persistence, integrating our understanding of those threats into analyses of population viability, and applying that understanding to several well-studied examples of threatened populations - the black-footed ferret, the northern spotted owl, the bay checkerspot butterfly, and the black sea turtle. For the next five weeks we're going to be talking about conservation biology as it applies to more than one species at a time. We'll be talking, for example, about the cumulative effects of species loss on ecological communities and about principles of conservation reserve design.
It is commonly believed that the more diverse an ecosystem is, the more stable it will be. I should point out right now that even though I'll be raising some substantial doubts about simple formulations of this belief and how useful it is for conservation purposes, it's not an unreasonable notion. It was, after all, first formalized by none other than Robert MacArthur in a very influential paper in Ecology [10]. Before we can talk about this relationship, however, we need to decide just what diversity really is.
Whittaker [16] distinguished three types of diversity.
For me an analogy from population genetics helps clarify the point. Rosenberg et al. [12] studied microsatellite variation in human beings at several hundred loci. They were able to distinguish five major geographical groupings of populations: Africa, Eurasia, East Asia, America, and Oceania. Of the total diversity in human populations, roughly 10% of the diversity is a result of differences among the different geographical groupings. Diversity within each geographical grouping corresponds to alpha diversity, the total diversity within human beings corresponds to gamma diversity, and the proportion of diversity due to differences among geographical populations (10%) corresponds to beta diversity.
Most analyses for conservation purposes have focused only on species diversity - attempting to identify regions with a large number of species. You all probably realize, however, that there are several aspects to diversity:
We also won't discuss formal definitions of ecological diversity,1 which are primarily definitions of alpha diversity although they can be generalized to allow partitioning of gamma diversity into its alpha and beta components.2These definitions treat all species as equivalent, ignoring aspects of ecological and evolutionary distinctiveness. Until relatively recently, many experimental evaluations of the diversity-stability hypothesis did the same.