Besides simply asking whether or not a habitat can support an organism, we can further categorize suitable habitats based on their 'quality', where the quality of a given habitat can be defined as the fitness an individual can expect if it lives in that habitat, after controlling for any positive or negative effects of conspecifics on fitness. In practice, habitat quality can be estimated by measuring fitness components of organisms living in different types of habitats at the same population density, or by controlling statistically for positive and negative relationships between population density and fitness.
Habitat quality measured at the spatial scales that are relevant to individuals has important implications for habitat quality at the larger spatial scales that are relevant to populations and species. For instance, if habitat type A supports higher individual survivorship and reproduction than habitat type B at any given population density, then habitat A will contribute more new biomass and more new recruits to the population than habitat type B per unit area. Hence, accurately estimating habitat quality is a major concern both for basic ecologists interested in the effects of habitat features on population dynamics, and for applied ecologists interested in identifying the locations with the largest potential impact on endangered or pest species.
As should be apparent from the previous section, population density should not be used as a surrogate for habitat quality. Although organisms sometimes accumulate in high-quality habitats, there are plenty of situations in which this does not occur. Indeed, the best habitats from the perspective of individual survival and reproduction may be areas which currently contain moderate to low population densities, or even areas which currently do not contain any members of the species. Every decade or so, ecologists write influential articles explaining the various reasons why population density should not be equated with habitat quality. Unfortunately, however, this practice is still fairly common, perhaps because it is easier for humans to estimate the number of organisms living at a given location than to measure the survivorship and reproductive success of those organisms.
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