Demography of some non-native isopods (Crustacea, Isopoda, Oniscidea) in a Mid-Atlantic forest, USA

Introduced species dominate the terrestrial isopod fauna in most inland habitats of North America, including urban landscapes. These non-native species are often very abundant and thus potentially play a significant role in detritus processing. We monitored isopod assemblages in an urban forest for a year to examine the relationship between surface activity and abiotic environmental factors, and to analyze reproductive characteristics that might contribute to their successful establishment. Using pitfall trap samples we recorded five species, two of which, Trachelipus rathkii and Cylisticus convexus, were highly abundant. We determined size, sex and reproductive state of each individual. Surface activity of both species reflected variability in abiotic stress factors for isopods, such as soil moisture and soil temperature. Early spring the main trigger was soil temperature while later in the season increasing temperature and decreasing soil moisture jointly affected population dynamics. Activity significantly correlated with soil moisture. The temporal pattern of sex ratios supported the secondary sex ratio hypothesis. Males dominated the samples on the onset of the mating season in search of females. The pattern was reversed as females searched for suitable microsites for their offspring. Size independent fecundity decreased as conditions became more stressful late in the season.