Niche Width: Biogeographic Patterns Among Anolis Lizard Populations

One aspect of the niche width of a population refers to the variety of resources used by the entire population. This aspect may be measured by the variance of the population's resource-utilization function. Two components make up a population's niche width. The within-phenotype component measures the spread of resources used by the average individual in the population. The between-phenotype component measures the variety of individual specializations in the population. The sum of these two components yields the total niche width. Calculation of these measures is illustrated with data from Anolis lizards. If the population's niche width is mostly composed of the within-phenotype component, it is virtually a monomorphic population of generalists, whereas if mostly the between-phenotype component, it is virtually a population polymorphic with pure specialists. Anolis populations tend more to be monomorphic with generalists than polymorphic with specialists. Recent ecological theory entails four predictions supported by the data on Anolis populations. (1 and 2) A population incurs a smaller total niche width and becomes less polymorphic with specialists either as environmental productivity decreases or the number of competing species increases. (3) If environmental productivity decreases and the number of competing species also decreases, there is little change in the total niche width, but the population becomes less polymorphic with specialists. (4) Evolutionary compression or release of jaw-length variation occurs more slowly than displacement of average jaw length.