The word habitat has two distinct usages. The true dictionary definition is the type of place where an animal normally lives or, more specifically, the collection of resources and conditions necessary for its occupancy. Following this definition, habitat is organism specific (e.g., deer habitat, grouse habitat). A second definition is a set of specific environmental features that, for terrestrial animals, is often equated to a plant community, vegetative association, or cover type (e.g., deer use different habitats or habitat types in summer and winter). Nonhabitat could mean either the converse of habitat in the first sense (a setting that an animal does not normally occupy) or the second (a specific vegetative type that the animal views as unsuitable); here, the two meanings of habitat converge (see also pages 392—396 in this volume).
Hall et al. (1997) argue that only the first definition of habitat is correct and that the second represents a confusing misuse of the term. They reviewed 50 articles dealing with wildlife—habitat relationships and, based on their definition, found that 82% discussed habitat vaguely or incorrectly. I suggest that given the prevalent use of habitat to mean habitat type, this alternative definition is legitimate and well understood in the wildlife literature. Moreover, this common usage of the term is consistent with the normally accepted meaning of habitat use: the extent to which different vegetative associations are used. Hall et al. (1997:175) define habitat use as "the way an animal uses . . . a collection of physical and biological components (i.e., resources) in a habitat" (emphasis mine), which seems difficult to measure.
Habitat selection and preference are also more easily understood in terms of differential use of habitat types. Selection and preference are often used interchangeably in the wildlife literature; however, they have subtly different meanings. I will adopt the distinction posed by Johnson (1980), who defined selection as the process of choosing resources and preference as the likelihood of a resource being chosen if offered on an equal basis with others. Peek (1986) suggested that innate preferences exist even for resources not actually available. Furthering this concept, Rosenzweig and Abramsky (1986) characterized preferred habitats as those that confer high fitness and would therefore support a high equilibrium density (in the absence of other confounding factors, such as competitors). Thus use results from selection, selection results from preference, and preference presumably results from resource-specific differential fitness. In controlled experiments, preferences can be assessed directly by offering equal portions of different resources and observing choices that are made
(Elston et al. 1996). In the wild, however, preferences must be inferred from patterns of observed use of environments with disparate, patchy, and often varying resources.
Generally, the purpose for determining preferences is to evaluate habitat quality or suitability, which I define as the ability of the habitat to sustain life and support population growth. Importance of a habitat is its quality relative to other habitats—its contribution to the sustenance of the population. Assessments of habitat quality and importance (i.e., habitat evaluation) are thus based on the presumption that preference, and hence selection, are linked to fitness (reproduction and survival) and that preference can be gleaned from patterns of observed use.
Use of habitat is generally considered to be selective if the animal makes choices rather than wandering haphazardly through its environment. Typically, the disproportionate use of a habitat compared to its availability is taken as prima facie evidence of selection. Although technically resource availability encompasses accessibility and procurability (Hall et al. 1997), these attributes are difficult to measure, so it seems reasonable to equate habitat availability with abundance (typically measured in terms of area), as is normally done in habitat selection studies. A habitat that is used more than its availability is considered to be selected for. Conversely, a habitat that is used less than its availability is often referred to as being selected against, or even avoided. This is poor terminology, however, in that it suggests that the animal preferred not to be in that habitat at all, but occasionally just ended up there. Use that is proportional to availability is generally taken to be indicative of lack of selection, which is also unfortunate terminology, as illustrated by the following examples.
Consider an animal living in an area with only two habitats and using each in proportion to its availability; from this we might assume that the animal was not exhibiting habitat selection. However, unless the animal was a very low life form, it certainly made choices as to when it visited each habitat and what it did when it got there; anytime it made a choice, and either stayed or moved, it selected one habitat over the other. Arguably, if one analyzed these movements on a short enough time scale, habitat use would be disproportionate to availability, enabling detection of habitat selection. As the time scale is shortened, though, the sheer physical constraint of moving between the two habitats (i.e., the distance between them) also affects their relative use.
On the flip side, imagine a dispersing animal attempting to traverse an area with no regard for habitat. If its route was frequently diverted by the presence of other, more dominant resident animals, living in their presumably preferred habitats, the disperser's movements would appear to reflect habitat selection
(i.e., selection for habitats not preferred by the residents). Indeed, one could reasonably assert that this represents true habitat selection as defined earlier, in that the disperser chose to avoid habitats with dominant conspecifics and thereby improved its chance of obtaining resources and not getting killed; however, one could also legitimately contend that the disperser was simply exhibiting avoidance of conspecifics, and used whatever cues, including markings, droppings, and possibly habitat characteristics, to do so.
These are not trivial complications, but rather examples of the intrinsic ambiguities associated with the application of these concepts. Terms such as selection and preference can be clearly defined, but not easily measured in the real world. Moreover, as I will show later, the link between selection, preference, and habitat-related fitness may be tenuous.
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