Effects of fire on ground-dwelling spider (Araneae) assemblages in central Indiana forests

Marc Milne, Joseph Gonsiorowski, Nathan Tuft, Tyler Ploss, Janise Acosta, Lucas Frandsen and Casey Venable, Biology, University of Indianapolis, INDIANAPOLIS, IN, Brodrick Deno, Agronomy, Purdue University, West Lafayette, IN

Background/Question/Methods
Fire is a natural disturbance that occurs in many temperate and tropical ecosystems worldwide. As ubiquitous members of these ecosystems, spiders are often affected by fire and their response to this disturbance has been shown to be dependent on taxonomy, functional diversity, seasonality, and a variety of environmental factors. We examined the effect of fire on ground-dwelling spider assemblages in temperate forests in central Indiana over five years. Burns were conducted at all sites on staggered dates, allowing us to minimize seasonal differences among sites. Spiders were sampled five times a year over five years through pitfall trapping and leaf litter collecting and extraction using Berlese funnels. All collected spiders were sexed, had their maturity recorded, and identified to the deepest taxonomic level possible using various keys. Treatments were defined by years since burn (>1, 1-2, and 2-3) and unburned. Multiple one-way ANOVAs with Tukey post-hoc tests were used to compare the abundance of spiders, species richness of spider communities, and Shannon diversity of spider communities among these four treatments. Spiders were grouped into guilds and compared using one-way ANOVAs to determine the effect of burns on functional diversity. Taxonomic spider assemblages were compared among treatments using NMDS analyses and significant differences among groups were determined using the pairwise.adonis package in R.

Results/Conclusions
Overall spider abundance and diversity decreased while species richness remained unaffected. We also found that spider response depended heavily on the family and/or guild to which the spider belonged. Specifically, agelenids, gnaphosids, linyphiids, phrurolithids, and thomisids all showed significant decreases in abundance post-burn. The "Ambusher" and "Sheet web weaver" guilds also showed significant decreases in abundance post-burn. Finally, we found that while spider assemblages (male and female) were significantly different between burned and unburned time periods, it was the female-only spider assemblages that displayed high resilience to fire, becoming similar to unburned sections after 1 year. We suspect that altered habitat heterogeneity, the patchy nature of fire’s effect on leaf litter, and the high rate of re-colonization by spiders all played important roles in these observed patterns.