Introduction

'Habitat' can be defined as a location in which a particular organism is able to conduct activities which contribute to survival and/or reproduction. This definition emphasizes the notion that the term habitat is organism-specific; that is, it focuses on the biotic and abiotic factors that affect the survival or reproduction of a particular type of organism, and on the areas that contain these factors. In addition, the term 'habitat' focuses on particular areas within a larger landscape. In contrast, the related term 'niche' considers the range of environmental conditions which allow the members of a species to live and reproduce, but does not specify the areas where these conditions exist.

The term habitat can refer to locations at several different spatial scales. At the largest spatial scale, habitat refers to large geographic areas in which the members of a population are able to survive and reproduce. At a medium spatial scale, habitat refers to areas in which a single member of a population is able to survive and reproduce. And finally, at a small spatial scale, habitat refers to areas in which a single member of a population can conduct particular activities (e.g., foraging, shelter, offspring production). Often, the term 'microhabitat' is used to refer to areas in which organisms conduct particular activities, while the term 'macrohabitat' refers to areas used by populations or species.

Implied in the concept of habitat at the intermediate (individual) level is that it contains all of the biotic and abiotic factors which affect survival and reproduction, throughout the life of that individual. However, it is frequently difficult (if not impossible) for humans to identify and measure all of the factors that affect survival and reproduction for taxa ofinterest. As a result, researchers interested in defining habitat for a particular species usually take 'shortcuts', trying to find factors that humans can readily measure which might be related to the factors which determine whether or not an organism is able to live and reproduce in a given area. Some examples of the shortcuts used by humans to describe habitat include using vegetation species or structural features to describe habitat for birds, using type of soil or incline to describe habitat for trees, or using salinity or distance from the sea to describe habitat for organisms that live in estuaries. In these and comparable cases, researchers assume that the factors they are using to describe habitat either have strong direct effects on survival and reproduction, or that they are strongly correlated with other biotic and abiotic factors that affect survival and reproduction.

Ifthese assumptions are valid, then surrogate measures of habitat can be reasonably successful. Thus, if a particular plant community provides a particular species of bird with nest-sites, food, water, protection from predators and inclement weather, and locations appropriate for social activities, then the space-use patterns of that species might map relatively closely onto the spatial distribution of that plant community. The assumption that some features of a habitat are more important than others is implicit in the term 'habitat-forming species', which refers to organisms that create habitat that can be used by other organisms. For instance, certain species of sea anemones provide habitat to anemonefish, which are able to live, grow, and reproduce within the protection of their tentacles. At a broader spatial scale, 'foundation species' create areas that are capable of supporting an entire community of other organisms. An obvious example are the corals which provide food, shelter from predators, protection from wave action, and other benefits to the large array of organisms that are only able to make a living on coral reefs.

However, while it is tempting to assume that one can determine habitat for one species by mapping the distributions of other organisms, this practice can sometimes lead one astray. Even if the members of a species only occur in association with another organism, this does not mean that every area that contains that organism will support members of that species. For instance, the presence or absence of avian brood parasites (e.g., cowbirds) in a landscape can have a major impact on the reproduction and sustainability of avian populations in that area, but vegetation maps alone are unlikely to reveal whether or not avian brood parasites are present in a particular region.

Given the considerable uncertainty that exists when humans attempt to describe habitats, there can be quite a gap between our notion of what constitutes a habitat and the constellation of biotic and abiotic factors which determine whether the members of a species would be able to sustain themselves in a given area. This uncertainty is one reason for use of the phrase 'suitable habitat', which ecologists often use to refer to areas which actually do contain the factors which determine whether an area can be used by the members of a population.

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