- Up to 99.9% of native North American grasslands have been degraded since European settlement, primarily due to agricultural conversion. Today, grasslands are a top priority for restoration as they provide essential habitat for many rare and endangered species; however, the majority of studies in grasslands have focused on vegetation or vertebrate responses to restoration while largely neglecting invertebrates. Grassland invertebrates are highly diverse and provide important ecosystem services such as pollination, nutrient cycling, food for vertebrates, and pest control. This dissertation seeks to understand the structure of spider and native bee communities within arid bunchgrass prairies and determine how grassland restoration and wildfire impact these beneficial invertebrates.
In Chapter 2, I focus on spider communities in a low-elevation, arid Pacific Northwest bunchgrass prairie and compare degraded, native, and restored sites to examine how spider communities and habitat respond to arid grassland restoration. Spider communities responded strongly to invasive annual grass, litter, and biological soil crust cover. Native sites differed from those in restored and degraded sites by community composition and abundance, with fewer spiders found in native sites than degraded and restored sites. However, native and restored sites had more species than degraded sites. I also examine how responses varied with the age of the restoration project. Chronosequence data showed trends for lower abundance, higher species richness, and changing community composition as restoration projects mature.
Chapter 3 describes the unique bee community found within the same bunchgrass prairie, identifies environmental variables associated with variation in bee abundance, richness, diversity, and community structure, and assesses the effect of grassland restoration on bee communities. I identify temporal trends within the bee and floral resource community that span over several seasons and years. As with the spider community, the bee community composition at native sites differed from both the degraded and restored communities, which did not differ from each other. However, there was no statistically significant difference in bee abundance, richness, and diversity among degraded, restored, and native sites. Bee abundance was most closely associated with litter cover, bee richness was associated with maximum vegetation height and floral abundance, and bee diversity was associated with floral abundance.
Chapter 4 examines spider community response to restoration at the regional scale. I compared degraded and restored communities at three separate grassland locations in eastern Oregon to determine what impact landscape context has on restoration and identify the environmental variables that underlie spider community patterns at this larger scale. Spider communities did not respond similarly to restoration among locations, indicating that landscape context may play a larger role in responses than restoration treatments. Regionally, spider abundance responded largely to changes in invasive grass and litter cover, while richness and diversity responded to changes in maximum vegetation height and forb cover.
Wildfire frequency has increased across the western United States, yet it is unclear how these fires affect beneficial invertebrates in arid grasslands. In Chapter 5, I examine bee, spider, and vegetative communities one year before and one year after wildfire. Both native bee and spider community composition were significantly altered one year after the fire. The fire did not affect bee or spider abundance, or spider diversity or richness but significantly increased native bee diversity and richness. Habitat variables such as invasive annual grass and biological soil crust declined significantly, while forb abundance increased after the burn.
Taken together, the findings show that invertebrate responses to grassland restoration and wildfire in inland Pacific Northwest grasslands are complex. Spider communities appear to largely respond to changes in vegetative structure (grass cover and height) and litter, while bee communities appear to be sensitive to changes in potential nesting sites (grass and litter cover) and floral resources (floral abundance). Depending on the degree that these environmental factors are influenced by future grassland restoration or wildfire, managers may expect to see strong effects on spider and bee communities.