Assessing detectability for monitoring of rare species: a case study of the cobblestone tiger beetle (Cicindela marginipennis Dejean)

Interpreting data on distribution or population trends may be difficult unless detection probability is accounted for. We wished to determine the detectability of the rare and patchily distributed cobblestone tiger beetle (Cicindela marginipennis) along the upper Genesee River in western New York for development of a monitoring strategy. We used occupancy surveys and distance sampling to examine two types of detectability. The first type was site-level detectability: the probability of detecting a single cobblestone tiger beetle on an occupied cobble bar, calculated using program PRESENCE. The second type was individual-level detectability: the probability of detecting an individual cobblestone tiger beetle in a population on a single cobble bar, calculated using program DISTANCE. Our occupancy surveys consisted of collecting presence and absence data on cobble bars along the Genesee River; these showed a relatively narrow range of site-level detection probabilities (0.60–0.68) for cobblestone tiger beetles in 2008 and 2009. Three visits were necessary to detect cobblestone tiger beetles on 90% of occupied cobble bars. Individual cobblestone tiger beetles were detectable one-half of the time (0.50) in our surveys. It is important for ecologists to distinguish between the two kinds of detectability, as monitoring implications could differ substantially depending on which is calculated. Our monitoring recommendations include (1) continuing occupancy surveys with at least three visits to each cobble bar; (2) conducting occupancy surveys between 10:00 and 17:00 on warm sunny days in mid-July and mid-August; and (3) conducting surveys at three- to five-year intervals depending on the study objective.