What can habitat preference models tell us? Tests using a virtual trout population

Habitat selection (preference) models are widely used to manage fish and wildlife. Their use assumes that (1) habitat with high animal densities (highly selected habitat) is high quality habitat, and low densities indicate low quality habitat; and (2) animal populations respond positively to the availability of highly selected habitat. These assumptions are increasingly questioned but very difficult to test. We evaluated these assumptions in an individual-based model (IBM) of stream trout that reproduces many natural complexities and habitat selection behaviors. Trout in the IBM select habitat to maximize their potential fitness, a function of growth potential (including food competition) and mortality risks. We know each habitat cell's intrinsic habitat quality, the fitness potential a trout in the cell would experience in the absence of competition. There was no strong relation between fitness potential and the density of fish in the IBM; cells where fitness potential was high but density low were common for all age classes, and fitness potential was not proportional to density. This result was consistent at high and low abundance and high and low overall habitat quality. We developed a statistical model of trout density observed in the IBM as a function of the four habitat variables that vary among cells. We then tested the ability of modeled mean density to predict population response to habitat changes resulting from stream flow modification. Modeled density partially explained population response to flow, but only at flows near the flow at which the density model was developed, and not for groups (e.g., juveniles) experiencing strong competition for habitat. Modeled density predicted population response opposite that observed for age-0 trout and incorrectly predicted response of all age classes to major changes in flow. These results make sense if habitat selection is understood as an emergent property of (1) the mechanisms by which habitat affects fitness, (2) habitat availability, (3) population abundance and size structure, and (4) how, individuals compete with each other. We identified eight reasons why animal density may not reflect habitat quality and several inherent limitations of habitat selection modeling.