What Is Habitat Suitability and how Should it be Measured?

Recent theory and empirical evidence suggest that current methods of avian habitat evaluation are ineffective. Time scales over which habitats are evaluated are too short, variables used in evaluation are often inadequate, and lack of biologically based concepts of habitat quality (HQ) and habitat suitability (HS) weaken evaluation. Here we: (1) compare concepts of habitat quality and suitability, (2) review methods used in determining HQ and HS, (3) evaluate the potential efficacy of those methods in predicting persistence of avian populations, and (4) suggest improvements on those approaches. We define HS as the ability of a habitat to support a viable population over an ecological time-scale. Population viability is defined in terms of the finite rate of population increase (λ=er) where λwill be ≥ 1 for viable populations. Although in practice HQ often represents management goals that are based on human perceptions of habitat value, we suggest that HQ be defined as the expected value of λ or E (λ) .Finally, we suggest that habitats be evaluated in terms of HS before using HQ. Habitats are normally evaluated by relating changes in habitat features to changes in species density, richness, and diversity. These variables do not necessarily relate to species persistence in an area. We suggest evaluation procedures adopt the approach of directly assessing the potential of a habitat to support viable populations.