Sampling through time

Abstract Many ecological research and resource monitoring programs must deliver good estimates of both current resource status and long-term trend. The simple two-occasion context frames the trade-offs in design of surveys to achieve these objectives. If the objective is to estimate change in status (trend), then most precise estimation is achieved by full retention of a random sample selected at time 1. If the objective is to estimate average status, then most precise estimation is achieved by selecting independent random samples. If a survey has both objectives, then a compromise design, involving partial retention and partial replacement of the initial sample, is optimal (i.e., will have intermediate performance for status and trend). Sampling designs for long-term monitoring (and before/after assessment monitoring) have two distinct components: a membership design which specifies selection of groups of units to be designated as sample panels, and a revisit design that specifies when these panels of units should be visited (revisited). For example, some randomly selected panels might be visited in years one to three, then dropped out of rotation for three years and then revisited in years–seven to nine, and so on. One panel might be revisited every year, and other panels might be visited only a single time. Design-based estimates of measures of status and trend are derived for some simple membership and revisit designs. The theory of dual frame sampling is applied to estimation of the number of active bald eagle nests on a wildlife refuge.