HABITAT SELECTION BY 2 SPECIES OF NECTARIVORE - HABITAT QUALITY ISOLINES

I present a model predicting the distribution and abundance of two species of competing nectarivore exploiting nectar in two types of flower. The model uses a submodel of resource renewal and depletion, and the principle that individuals attempt to maximize individual fitness, to construct habitat isolines. The habitat isoline for a population of nectarivores is a line in an abundance phase-plane along which all individuals of the population, regardless of the habitat they are currently exploiting, have equal expected fitness. At equilibrium the habitat isoline determines the distribution of a population between two habitats given a fixed abundance of competitors. The habitat isolines for populations of two species exploiting two habitats can be used to predict the equilibrium distribution of both species. I illustrate the model using data about bumble bees foraging for nectar in flowers (Inouye 1978). An asset of the model is its ability to make a priori predictions about flower exploitation patterns using information about resource dynamics and the foraging abilities of the nectarivores. Two warnings regarding the empirical detection of exploitation competition between species emerge from the model analysis. Depending on the range over which density manipulations are carried out in exploitation competition systems, competition may or may not be detected, while interspecific competition may or may not appear to be more intense than intraspecific competition. Although the model is directed at nectarivores competing for nectar in flowers, the results have wider implications for all systems where species compete exploitatively for resources.