237 Chapter 11 Responses of a Tallgrass Prairie Spider (Araneae) Community to Various Burn Seasons and Its Importance to Tallgrass Prairie Management David J. Wade and Robert E. Roughley1 Department of Entomology, University of Manitoba Winnipeg, Manitoba, Canada R3T 2N2 Abstract. A four-year study was conducted in which spiders were used as bioindicators to determine if there was an optimal season to burn tallgrass prairie. The burn seasons examined were spring, summer, and fall. Overall, the species diversity of the spider community in the burned areas was not significantly different from the unburned areas for any burn season, although some significant differences occurred within the first year following the burns. Spider abundance decreased following each burn season treatment and the recovery period varied depending on the burn season. Burn season also affected the structure of the spider communities in that the summer and fall burn treatments became dominated by Pardosa distincta (Blackwall) rather than the initially dominant P. moesta Banks. The use of a mosaic of burn season treatments is recommended to maximize the diversity of spiders within the tallgrass prairie habitat. Résumé. Nous avons réalisé une étude de quatre ans afin de déterminer s’il existe une saison optimale pour les brûlis en prairies à herbes hautes en utilisant des araignées en guise de bioindicateurs. Les brûlis ont été effectués au printemps, en été ou en automne. Dans l’ensemble, la diversité des espèces d’araignées mesurée dans les zones brûlées n’était pas significativement différente de celle mesurée dans les zones non brûlées, sans égard à la saison choisie pour les brûlis, même si certaines différences significatives s’observaient au cours de la première année suivant les brûlis. L’abondance des araignées a diminué après chaque saison de brûlis, et la période de rétablissement a varié en fonction de la saison choisie. Le choix de la saison des brûlis a également influé sur la structure des communautés d’arachnides : dans les zones soumises à des brûlis d’été ou d’automne, le Pardosa distincta (Blackwall) est devenu dominant et a remplacé l’espèce qui dominait auparavant, le P. moesta Banks. On recommande de recourir à une mosaïque de traitements saisonniers afin de maximiser la diversité des araignées dans l’habitat de la prairie à herbes hautes. Introduction Approximately 300 years ago, tallgrass prairie covered 570,000 km2 of North America (Howe 1994), extending from southern Saskatchewan and Manitoba, south to Texas, and eastward to Iowa (Robertson et al. 1997). Only 1% of this habitat remains in North America and less than 1% in Manitoba (Morgan 1994; Robertson et al. 1997; Nature Conservancy of Canada 2000). The main tallgrass prairie sites in Manitoba are the Tall Grass Prairie Preserve (1,820 ha), the St. Charles Rifle Range (SCRR; 192 ha), and the Living Prairie Museum (12 ha). Tallgrass prairie is adapted to disturbance, and active management requires regular disturbances such as fire, grazing, or mowing (Howe 1994; Schwartz and Hermann 1997). Prior to European settlement, tallgrass prairie was regularly burned by lightning strikes in 1 Deceased. Wade, D. J. and R. E. Roughley. 2010. Responses of a Tallgrass Prairie Spider (Araneae) Community to Various Burn Seasons and Its Importance to Tallgrass Prairie Management. In Arthropods of Canadian Grasslands (Volume 1): Ecology and Interactions in Grassland Habitats. Edited by J. D. Shorthouse and K. D. Floate. Biological Survey of Canada. pp. 237-249. © 2010 Biological Survey of Canada. ISBN 978-0-9689321-4-8 doi:10.3752/9780968932148.ch11 238 D. J. Wade and R. E. Roughley late summer or early autumn and by Aboriginal people purposely igniting the habitat in the early spring or autumn (Warren et al. 1987; Collins 1990; Howe 1994; Schwartz and Hermann 1997). Fire is currently the most widely used management technique, with spring burns being most common (Schwartz and Hermann 1997; Collins and Steinauer 1998). Spring burns favour late season grasses such as big bluestem and other key tallgrass prairie grass species (Schwartz and Hermann 1997), although summer burns may maximize plant diversity (Howe 1994). Without regular disturbances, the plant community becomes dominated by few species that reduce species richness and productivity (Reed 1997; Schwartz and Hermann 1997; Collins and Steinauer 1998). However, the disturbance regime necessary to maintain high diversity and productivity has not yet been determined (Howe 1994; Collins and Steinauer 1998). The effects of fire on the spider (Araneae) fauna of prairie habitats are not well-known (see reviews by Warren et al. 1987; Reed 1997; Bell et al. 2001). Spiders are important predators and can survive the physical effects of fire by finding protection in places such as cracks in the soil (Warren et al. 1987). However, because of the upper lethal temperature of most spiders, biologists hypothesize that the majority of spiders are not able to survive a fire incident and that they recolonize following the burn (Bell et al. 2001). Spider survival after a fire also depends on the availability of prey, both its density and diversity, in the burned area (Warren et al. 1987). The pioneer spider fauna in grasslands following fire includes certain species of linyphiids, theridiids, and lycosids (Riechert and Reeder 1972; Bell et al. 2001). These species prefer bare ground, are more tolerant to microclimate changes, and are relatively less dependent on vegetation for web construction (especially lycosids) (Bell et al. 2001). These pioneering species often decline in numbers over time as the habitat recovers from the effects of fire and as old-growth species increase in abundance (Bellet al. 2001). In general, species diversity increases over time following fire in grasslands, but the short-term responses can vary (Bell et al. 2001). Bell et al. (2001) recommended that more studies be done to examine the effect of different burn regimes on spider communities. They also recommended that burning be conducted on large connected habitats with a rotation of regimes to conserve the highest species richness and range of stand ages. Maintaining refugia and keeping the inter-burn period long enough are also important so that the spider fauna can recover (Harper et al. 2000). Although no studies have been done of the impact of fire on spiders in tallgrass prairie in Manitoba, some have been conducted in similar tallgrass prairie habitats in Illinois (Rice 1932; Harper et al. 2000), Wisconsin (Riechert and Reeder 1972), and Kansas (Nagel 1973). Harper et al. (2000) found that spider abundance significantly decreased in the 10 weeks following a spring burn. Spider abundance was more negatively affected in the enclosure study sites (i.e., closed system), suggesting that recolonization from unburned areas is important for spiders. Rice (1932) found that spider numbers were lower in burned versus unburned subclimax tallgrass prairie following a spring burn. Riechert and Reeder (1972) saw the same trend on two separate prairie plots. On a subclimax prairie site, spider abundance recovered within a week and species composition favoured vagrant species. However, on a climax site, spider abundance did not recover after 45 days, albeit species composition was unaffected. These researchers also observed that spiders moved from burned areas to unburned areas. The long-term effect of burning on the spider community was inconclusive in their study. In the year following the burn, spider abundance on the climax prairie site had returned to pre-burn levels, whereas levels at the subclimax prairie site remained below pre-burn levels. Similarly, Nagel (1973) found that spider abundance was lower in the burned versus unburned prairie following a spring burn on a prairie in Response of spiders to fire in tallgrass prairie 239 Kansas. Johnson (1995) found that spider abundance and density was higher in annually burned Spartina pectinata Link wetlands, but species composition was similar. This increase in spider abundance was correlated with increases in insect prey. Howe (1994) predicted that fire season would affect the resultant tallgrass plant community from observations that species (1) differ in response to thatch removal and soil warming, (2) have different seeding phenologies and germination requirements, and (3) differ in patterns of rhizome recruitment. His prediction was validated for the tallgrass prairie plant community on the SCRR (Sveinson 2001). By extension, fire season also is predicted to affect spiders. Spider species have different phenologies and peak activity periods (Aitchison 1984) and differentially migrate into burned areas (Riechert and Reeder 1972; Harper et al. 2000). The objectives of this study were to determine (1) the effect of burn season on the spider community, (2) the optimal burn cycle interval, and (3) the optimal burn season for tallgrass prairie management. This research was part of a multidisciplinary study that also used the plant and ground beetle (Coleoptera: Carabidae) communities as bioindicators to evaluate the effectiveness of burn season as a tool for tallgrass prairie management. The results for these other bioindicators are discussed in Sveinson (2001), Roughley (2001), and Roughley et al. (see Chapter 10). A secondary component of the study was to determine the uniqueness of the tallgrass prairie spider and ground beetle fauna compared with the adjacent aspen forest fauna, the results of which are discussed in Roughley et al. (2006). Study Area The study was conducted at the SCRR, located just to the northwest of Winnipeg, Manitoba (49°54′35.1″ N, 97°20′47.3″ W). The 192 ha property is owned by the Canadian Department of National Defence and includes 47.9 ha of high-quality tallgrass prairie (Morgan 1994). The area has remained undisturbed for at least 100 years, but a portion of the study area may have been cultivated until 50 years ago. This area was coined the “go-back” prairie by Morgan (1994).