Asymmetric competition, body size, and foraging tactics: testing the ideal free distribution in two competing fish species

Question: Do habitat preferences of two competing fish species correspond to the two-species ideal free distribution (IFD)? Prediction: Consumer distributions across habitats will match the distribution of resource supply rates. Organisms: A mixture of white cloud mountain minnow Tanichthys albonubes and giant danio Danio aequipinnatus. The choice of allopatric species avoids confounding the factors of common evolutionary history and can reveal competition patterns that follow biological invasions. Methods: Record habitat preferences of the two competing fish species in an aquarium with two feeding sites. Develop the two-species IFD model and compare the data to its predictions. Results: In the presence of conspecifics alone, individuals of each species conform to Parker's matching rule, distributing themselves according to the ratio of the food supply rates. When both species are present, the minnow distribution still follows the matching rule but the danio selects the feeding site with the lower food supply rate disproportionately more often than when it is alone. Hence, while intraspecific competition does not affect feeding site preferences in either of the species, interspecific competition between minnows and danios is asymmetric; the minnow is the dominant competitor. We trace the asymmetry to species-specific foraging tactics. Minnows stay much closer to a feeding site and reach food items twice as quickly as danios. Our model, based solely on exploitative competition, does not fit these observations. Instead, interference between species appears to drive the asymmetric competition and make Parker's matching rule generalized to a two-species environment inadequate.