Effects of food and oxygen availability on habitat selection by guppies in a laboratory environment

Using the ideal free distribution model proposed in 1970 by S. D. Fretwell and H. L. Lucas, we investigated how local food and oxygen availability influenced the distribution of a group of guppies (Poecilia reticulata, Teleostei, Poeciliidae) among five habitats in a large tank. Our study differed from previous investigations of ideal free distributions in the larger number of habitats available, the presence of resource-free zones between habitats, the visual isolation of the different habitats, the availability of resources over an entire day rather than a short feeding period, and the systematic variation of a potential cost component (low oxygen). In our system, the ideal free distribution model was a poor predictor of fish distributions. Although the proportion of fish in a habitat was positively correlated with food supply, there were too few fish where the food supply was high and too many fish where the food supply was low to equalize average feeding rates in all habitats. The proportion of fish in a habitat was not affected by oxygen concentration, whether food was limited or superabundant. Large, stable differences in density occurred between habitats receiving identical treatments. Time spent by the fish in sampling alternative habitats and the ease with which some individuals could restrict the access of others to the habitats may have contributed to the failure of the ideal free distribution model. The effectiveness with which guppies use the surface film for respiration under hypoxic conditions may explain the lack of a dissolved oxygen effect. Our study demonstrates the need to understand conditions that promote or hinder the appearance of ideal free distributions before they can be used to predict the relationship between animal density and resource availability in nature.