Factors Controlling Species Diversity: Overview and Synthesis

Abstract

Factors controlling species diversity are usually presented as a laundry list without organization-for instance, "The tropics have more species than the temperate zones because of greater stability and reduced seasonality and higher productivity and more diverse resources and ...."This paper instead proposes a fourfold grouping of factors, termed the QQID approach. Q = resource quality, consisting of the habitat and resource factors that determine the "number of niches."Q = resource and consumer quantity, consisting of factors determining the quantity of resources (i.e., area and productivity), and factors determining the number of consuming individuals (especially body size) that divide those resources. I = species interactions, which may boost or lower species diversity through effects on individual numbers or individual fitnesses. D = dynamic processes (e.g., extinction, immigration, and speciation), which affect species diversity in both equilibrial and nonequilibrial situations. I illustrate this approach by applying it to understand the famous gradients of species diversity over altitudinal and latitudinal gradients. A major problem for the future involves seeking generalizations as to which factors are likely to be most important under which circumstances. Another problem is to convert this empirical fourfold grouping into a natural hierarchy of factors, possibly based on a hierarchy of processes in space and time.