Document: ROB-3-68-22

Influence of prescribed fire on spider and carabid beetle communities in southwestern Oregon.

PECK, R.W.* and C.G.NIWA

USDA Forest Service, Corvallis, Oregon USA ¹

Abstract:
Prescribed fire is an important management tool on many forests in eastern and southwestern Oregon, but its influence on arthropod communities is poorly known. The objective of this study was to determine if prescribed fire effects spider and carabid beetle abundance and whether the magnitude of any differences detected decreased with increasing time since fire. Within mixed conifer stands (dominated by Douglas-fir, ponderosa pine and white fir), nine understory, fuels-reduction burns, ranging from <1 year to 15 years old, were compared to adjacent unburned sites. Pitfall traps were used to compare arthropod abundance over a 5-month period. In total, 3441 spiders within 24 families and 126 species; and 15,028 carabid beetles from 16 species, were identified from the samples. Seven spider families and 5 species of carabid beetles were abundant enough to be compared statistically. Significant differences between burned and unburned sites were found for all taxa analyzed, although the nature of the response differed among taxa. For spiders, Antrodiaetidae, Cybaeidae, Thomisidae and Linyphiidae were more abundant in unburned sites while Lycosidae, Gnaphosidae and Dictynidae were more abundant in burned sites. All carabid beetle species analyzed were more abundant in unburned sites. They were Omus cazieri, Pterostichus herculaneus, P. setosus, Scaphinotus rugiceps and Zacotus mathewsii. No difference in Simpson's diversity index was found for either group. Overall, the relationship between abundance and time since burning was weak, with only marginal significance found for Dictynidae and Gnaphosidae (p=0.0475 and 0.0523, respectively). We suggest that changes in foraging substrate, prey availability or microclimatic conditions after fire influenced the abundance of these organisms. Differences in fire intensities among years may have masked patterns in arthropod abundance associated with time since burning.

Keywords: prescribed fire, spiders, carabid beetles, Oregon

This abstract is being presented at: 10:30 AM in session:
Poster Session #12: Disturbance Ecology.