Chapter 8 - Basic Presence/Absence Situation

Abstract

The occupancy status of sampling units will often change over meaningful timescales, which is of interest in many areas of ecology, including metapopulation studies, studies of distributional change, and monitoring programs. In this chapter we turn our attention to the problem of estimating changes in occupancy from data collected over multiple seasons, where occupancy is defined in terms of two possible states, species presence or absence. We consider two general approaches for modeling changes in occupancy over time: (1) a model where underlying dynamics are implied but not explicitly accounted for (effectively combining several single-season models); and (2) explicitly modeling potential changes in the occupancy state of a unit over time with colonization and local extinction probabilities. Both types of model can be used to obtain time-specific estimates of occupancy and estimates of rate of change, or ‘trend’, in occupancy, which is the focus of many species monitoring programs. However, the explicit dynamics modeling approach can yield greater understanding about the underlying vital rates, colonization and local extinction, that govern occupancy dynamics (i.e., why is there a trend), which may be useful when investigators seek to influence the rate of change in occupancy. Predictor variables or covariates can be incorporated into the models to investigate effects of environmental variables and management actions on these rate parameters. We believe that this chapter may be the most important in the book, as studies that extend over multiple seasons provide the strongest inferences about processes that influence occupancy dynamics and determine observed occupancy patterns.