PS 30-9
Friendly neighborhood spiders: Factors driving abundance and biodiversity in urban gardens
Urbanization is a major threat to arthropod biodiversity and abundance because of reduction and loss of suitable natural habitat. But green spaces and small-scale agroecological areas may provide habitat and resources for arthropods within a densely developed city. Arthropod diversity and abundance can, in turn, benefit these green spaces by pollinating, cycling nutrients, and providing other ecosystem services. Spiders generally consume a variety of prey items and can have a high fecundity, and thus are important predators that maintain food web balance, and control herbivore and pest density. We studied spider abundance and diversity in urban community gardens, in Santa Cruz, San Jose, and Monterey counties, in central California. We sampled for spiders with pitfall traps, and sampled vegetation and other site attributes for 5 months in 19 garden sites. We identified spiders to family. We classified the land surrounding each of the gardens sites into categories (natural, developed, agriculture, etc.) with GIS and the National Land Cover Database (NLCD). We correlated spider abundance and family richness with vegetation variables, other site attributes, and surrounding land cover with conditional inference trees, in order to examine the relative importance of local and landscape factors for predicting urban spider abundance and richness.
Results/Conclusions
We collected a total of 2904 spider individuals, spanning across 21 families. The most abundant family was Lycosidae (68% of individuals), and the second most abundant family was Gnaphosidae (15%). Females represented 52% of individuals captured, and 68% of individuals were sexually mature. Spider abundance differed with surrounding land use; we found more individuals in gardens that had a higher percentage of surrounding agriculture, and fewer individuals in sites with a high percentage of surrounding urban area. Statistically, abundance was positively correlated with percent of agricultural land within 1km, and also positively correlated with the presence of trees, shrubs, and ornamental flowers. Furthermore, spider family richness was positively correlated with the number of crop species within 1m. Interestingly, spiders did not respond to changes in the amount of natural land surrounding gardens. In short, spider abundance and family richness respond negatively to developed land use, likely due to the lack of vegetation presence and diversity in these habitats, but were positively associated with another relatively disturbed habitats (agriculture). Thus, spiders may respond positively to some level of habitat disturbance, and may be most abundant in disturbed areas with high crop, tree, and shrub diversity and abundance.