Habitat Selection, Dispersal and Detectability of Cobblestone Tiger Beetles (Cicindela marginipennis Dejean) along the Genesee River, New York

Abstract

The objectives of my two year study were to (1) understand the dispersal dynamics of the adult cobblestone tiger beetles (Cicindela marginipennis); (2) identify environmental variables associated with suitable habitat; (3) model habitat selection; (4) describe important features of their natural history; and (5) determine their detectability in the riparian habitat along the Genesee River, NY. Data on cobblestone tiger beetle habitat selection and populations established a baseline for monitoring environmental change and population status of this species of management concern in riverine and riparian habitats in western New York. Cobblestone tiger beetles dispersed distances that far exceeded the maximum distance between surveyed cobble bars, and they sometimes moved between cobble bars. Cobblestone tiger beetles were more likely to occur in habitat patches with greater interior area and elevational relief. Occupied cobble bars also had few boulders and shrubs. I found cobblestone tiger beetles throughout occupied cobble bars and not restricted to the upstream end of cobble islands or sandy beaches as cited in most cobblestone tiger beetle literature. My surveys examined two levels of detection probability - individual-level (the probability of detecting an individual cobblestone tiger beetle in a population on a single cobble bar) and site-level (the probability of detecting a single cobblestone tiger beetle on an occupied cobble bar). My results for individual-level detectability show that there was a lower probability of seeing an individual cobblestone tiger beetle than detecting the co-occurring and more common bronzed tiger beetle (C. repanda). The best-fit model for cobblestone tiger beetles had no covariates. Although cobblestone tiger beetle detection probabilities were the same for both models (no covariate and with ground temperature), the results for site-level detectability showed similar detection probabilities for cobblestone tiger beetles in 2008 and 2009, even though the number of sites surveyed and the number of visits per cobble bar differed between years. In addition, an evaluation of a smaller subset of cobble bars surveyed during both years and with the same level of effort showed that the site level detectability and occupancy continued to be consistent with the individual year results. Based on results from my study, I recommend ( 1 ) continuing occupancy surveys with at least three visits to each cobble bar as long as the site-level detection probability is greater than 0.5, in order to detect cobblestone tiger beetles on at least 90% of occupied cobble bars; (2) conducting occupancy surveys when cobblestone tiger beetles are the most active – in mid-July and mid-August; (3) conducting surveys between 10:00 and 17:00 on warm sunny days when ambient and ground temperature are at their highest, preferably when ambient temperatures are above 1 8 . 8 C; and (4) conducting surveys at three- to five-year intervals depending on the study objective - shorter times for better understanding of metapopulation dynamics or longer intervals for simply determining continued occupancy.