ESTIMATING SPECIES RICHNESS - THE IMPORTANCE OF HETEROGENEITY IN SPECIES DETECTABILITY

Estimating species richness (i.e., the actual number of species presen t in a given area) is a basic objective of many field studies carried out in community ecology and is also of crucial concern when dealing w ith the conservation and management of biodiversity. In most studies, the total number of species recorded in an area at a given time is tak en as a measure of species richness. Here we use a capture-recapture a pproach to species richness estimation with North American Breeding Bi rd Survey (BBS) data in order to estimate species detectability and th us gain insight about its importance. In particular competing models m aking different assumptions about species detectability are available. We carried out analyses on all survey routes of four states, Arizona, Maryland, North Dakota, and Wisconsin, in two years, 1970 and 1990. T hese states were chosen to provide contrasting habitats, bird species composition, and survey quality. We investigated the effect of state, year, and observer ability on the proportions of different models sele cted, and on estimates of detectability and species richness. Our resu lts indicate that model M-h, which assumes heterogeneous detection pro bability among species, is frequently appropriate for estimating speci es richness from BBS data. Species detectability varied among states a nd was higher for the more skilled observers. These results emphasize the need to take into account potential heterogeneities in detectabili ty among species in studies of factors affecting species richness.