Density as a Misleading Indicator of Habitat Quality

Abstract

Current methods of evaluating wildlife habitat for management purposes can be arranged in a hierarchy of increasing generality. The most general level is evaluation of wildlife habitat for entire com- munities on the basis of inferences drawn from vegetational structure. At the base of the hierarchy the high resolution studies, upon which accuracy at the higher hierarchical levels depends, usually assume that habitat quality for a species is positively correlated with the density of the species. If habitat quality for a wildlife species is a measure of the importance of habitat type in maintaining a particular species, habitat quality should be defined in terms of the survival and production characteristics, as well as the density, of the species occupying that habitat. Situations in which habitat quality thus defined is not expected to be positively correlated with density are described, along with the species and environmental characteristics that are most likely to produce these situations. Examples drawn from the literature in which density and habitat quality are not positively correlated are described. The positive correlation of density with habitat quality in specific instances cannot be assumed without supporting demographic data. J. WILDL. MANAGE. 47(4):893-901 The foundation of any wildlife habitat management plan is the ability to assess habitat quality accurately. Without this key ingredient, the effort put into care- fully prepared objectives and elegant cat- egorizations of habitat types is largely wasted. Yet biologists often dwell on ob- jectives and categories while treating lightly the assumptions implicit in their assessments of habitat quality. For in- stance, they seldom question the assump- tion that the density of a species in a hab- itat is a direct measure of the quality of that habitat. Perhaps this is because any more accurate investigation of habitat quality to truly reflect the importance of that habitat in maintaining wildlife species populations must be intensive, often at the expense of the broader information base that could be achieved by simple surveys. Such surveys are a particularly common means of evaluating nongame wildlife habitat. The objectives of this paper are to pro- vide some examples of situations in which this correlation does not hold, and to make predictions regarding species and envi- ronmental types for which the density- habitat quality relationship is likely to be decoupled. In such cases, management policies based directly on species abun- dance may be misleading and these errors may be amplified when management ap- proaches are restricted to the higher levels of the hierarchy. This paper is dedicated to the late 0. C. Wallmo, who was always eager to dis- cuss ideas and whose refusal to be any- thing but completely honest in evaluating his own ideas, objectives, and research ideas, as well as those of others, set an example for us to follow.