Impact of a catastrophic natural disturbance on fluctuating asymmetry (FA) in a wolf spider

Abstract

Animals subject to environmental stress often exhibit higher levels of developmental instability, frequently measured as Fluctuating Asymmetry (FA), small, random deviations from perfect left-right symmetry in bilaterally symmetrical animals. We used FA measurement as a means of determining the impact of a catastrophic ecosystem disturbance (a major Class F3 tornado) on populations of Schizocosa ocreata, a common forest-dwelling wolf spider. As FA in sexually selected ornamental traits in males has been shown in some (but not all) species studied to be a sensitive indicator of environmental stress, we measured FA of foreleg tufts in male S. ocreata. Spiders from the first post-disturbance generation were collected by pitfall trapping in the spring of 2000, and preserved specimens were photographed and measured using digital imaging. Spiders from disturbed and undisturbed areas within the forest did not differ significantly in body size parameters, although body condition varied significantly between sites. Signed (R—L) FA of male tuft area was normally distributed with a mean of zero, indicating “ideal” or “true” FA, a reflection of Developmental Instability (DI). Although tuft area was not significantly different between sites, tuft area FA in male S. ocreata was significantly higher in the disturbed site, suggesting that ecosystem-level stress may produce higher levels of developmental instability in arthropod populations. Results of our study indicate that FA in this secondary sexual character (male leg tufts) is a more sensitive indicator of putative environmental stress than a non-sexual trait (femur length). To our knowledge, this is the first study of FA in invertebrates associated with environmental stress from a catastrophic natural disturbance.