Spider (Araneae) Diversity at Red Banks Alvar State Natural Area, Wisconsin

Abstract

Wisconsin alvar is an understudied ecological landscape. With minimal published literature and research on the subject, not much is known about critically endangered habitat type. Red Banks Alvar State Natural Area (SNA) is one of Wisconsin’s best examples of an alvar. The goal of this study was to investigate how the rare and unique alvar plant community of Red Banks Alvar SNA may be influencing the spider community composition and diversity.

The spider community was sampled over the growing season (May-October) of 2021 through three main methods: five sets of pitfall trapping, two Rapid Assessment Protocols, and auxiliary sampling. The plant community was sampled once through a modified wetland delineation protocol during the month of August to collect information on what plant species are present at Red Banks Alvar SNA, as well as cover and height data.

4,094 spiders were captured and identified. 173 spider species were identified across 23 families and 101 genera. Of the adults identified to species, 21 are Wisconsin state records including 2 species range extensions. Vegetative sampling identified 65 plant species across 34 families. Spider diversity was measured using the Shannon-Weiner index. Each of the five focal study sites showed high spider diversity, including Powerline Trail, a site heavily invaded by common buckthorn (Rhamnus cathartica) and aggressive native red cedar (Juniperus virginiana). Several spider species were identified as habitat indicators, including Pardosa saxatilis and Neoantistea magna, through an indicator species analysis.

A non-metric-multidimensional scaling plot was produced to determine if sub-habitat types of the alvar (alvar grassland, alvar woodland, and degraded alvar) contain distinct spider and plant community assemblages. Distinct spider and plant communities were identified in each sub-habitat type. Spider diversity was then compared to plant diversity and average plant height diversity to understand how plant community diversity and structure influences spider community diversity and structure. Spider diversity did not show a strong relationship with plant diversity, but there is evidence to support the hypothesis that diversity in plant structure influences spider community composition and diversity.