Effect of an invasive ground cover plant on the abundance and diversity of a forest floor spider assemblage

Ground dwelling spiders are important predators in the detrital food web, which plays important roles in nutrient cycling and energy flow in forest ecosystems. The cursorial spider assemblage in a Beech-Maple forest in southwestern Michigan at sites where and invasive plant, Vinca minor, has invaded was compared to a native site within the same forest and to the forest prior to invasion by the plant. Pitfall traps were used to sample cursorial spiders over the course of a summer. Vinca minor substantially altered the forest floor spider assemblage. The invasive plant reduced the total activity-abundance of spiders by nearly 49% and depressed species diversity and evenness; in contrast, species richness was not affected. We found that V. minor changed the guild and family structure with wolf spiders being common at sites where the plant had invaded. Vinca minor reduced the abundance of vagrant web building and crab spiders. Similarity indices revealed that the spider communities between the two sites were quite dissimilar (Bray-Curtis = 0.506; Jaccard’s = 0.520). Importantly, comparison to a study conducted in the same forest 28 years earlier showed that the cursorial spider assemblage in the forest prior to Vinca invasion was very different than it was after Vinca invaded but was similar to the current native site in species and guild composition. We conclude that invasion by Vinca has caused the striking changes we observed in community organization of this important group of forest floor predators. We suggest that changes in the physical structure of the litter/soil microhabitat with the invasion of V. minor are likely the cause of the substantial impacts of the plant on the spider assemblage.