In fire-dependent jack pine forests of northern Lower Michigan, conservation and restoration efforts have been on-going for over 40 years as they are the breeding ground for the endangered Kirtland’s Warbler (KW). Current management practices, focused on increasing KW habitat (e.g. clear-cutting and an opposing-weave planting pattern), however, have been shown to simplify landscape structure. We suggest that without an understanding of how these forests respond to wildfire we have insufficient metrics to measure the success of habitat restoration efforts that emulate fire and natural post-fire succession. One such metric, or indicator of ecosystem function, could be spider community structure and diversity.
Spiders have been shown to be good bioindicators of ecosystem function because they are abundant, can be easily sampled, and are sensitive to environmental changes. Our objective was to quantify the changes in the ground spider community (diversity, abundance, and community composition) in response to stand development processes following wildfire.
To quantify the ground spider community we established pitfall traps in 12 former wildfire sites in northern Lower Michigan. Sites were categorized as either young (2-7 years post-fire) or mature (23-41 years post fire) and all were relatively undisturbed by human activity (e.g., no artificial jack pine regeneration).
Results/Conclusions
We observed significant differences in the ground spider community composition between the age classes (MRPP=0.001), and higher species diversity (Shannon Diversity Index) in mature (H’=2.32) versus young (H’=1.66) stands (p=0.0005). Non-metric multidimensional scaling (nMDS) suggests that these differences were primarily associated with the higher abundances of several Gnaphosidae and Liocranidae species in the mature versus higher abundances of several Lycosidae and Corinnidae species in the young jack pine stands. Specifically, indicator species analyses found eight species (e.g. Pardosa distinca, Castianeira spp.) were associated with the young stands while 17 species (e.g. Gnaphosa muscorum, Agroeca spp.) were associated with mature stands. Redundancy Analysis (RDA) revealed that stand age, canopy cover, jack pine density, ground cover, and understory plant community composition and density (below 0.5 m) are important drivers of ground spider abundance.
These data provide a more detailed understanding of the role of natural disturbances, and their legacies, play in shaping these important animal communities. Differences in spider communities suggest that heterogeneous stand structure associated with naturally regenerated jack pine stands, and absent from many KW managed stands, may provide a variety of microclimate areas suitable for maintaining a high diversity of ground spiders across stand ages.