An ecological comparison of spiders from urban and natural habitats in California

Abstract

A comparative study of spider communities in urban, native, and semi-native habitats was conducted in 1980 in northern California. Selected biotic and abiotic factors thought to be responsible for observed differences in spider-community composition among habitats were investigated. Three backyards in the city of Berkeley comprised the urban study site. Plants found at this site included: Sequoia sempervirens, Juniperus sabina ‘Tamariscifolia,’ Hedera helix, Camellia japonica, and various annual and perennial flowers. Spider-community composition at the urban habitat consisted primarily of web-building spiders above ground level (aerial). Ground-surface spiders were fewer in number and species due to manipulation of the environment by homeowners. A relatively undisturbed coastal chaparral habitat, 7.7 km east of Berkeley (in Contra Costa County), was chosen to represent the native site. A limited number of plant species and structural diversity was characteristic of this habitat. Baccharis pilularis, Lupinus albifrons, Lupinus bicolor, and various grasses (Avena fatua, Bromus mollis, etc.) dominated the plant community at this study site, Briones Reservior. Total numbers and numbers of species of aerial spiders were low at this habitat. However, ground-surface spiders were more numerous, probably due to an abundance of prey. A semi-native habitat (Tilden Park) located between the urban and native habitats shared plant species with both these habitats. Plant species included both ornamental plants (Juniperus sabina, Pyracantha sp., and Prunus sp.) and native plants (Quercus agrifolia and Baccharis pilularis). The climate was most similar to the native habitat, although more humid. The aerial and ground-surface spider communities resembled the urban and native habitats, respectively. Plant architecture in conjunction with local climate influenced aerial spider community composition. A greater diversity of plant species provided a heterogeneous habitat for aerial spiders at both urban and semi-native habitats. The cooler and more humid climates at these sites reduced exposure to potentially limiting factors such as desiccation. Low diversity of plant architecture and a hot and dry climate restricted aerial spider numbers and species at the native site. Greater numbers of ground-surface spiders at the native and semi-native habitats were probably due to an abundance of prey, while owner manipulation of urban gardens negatively influenced ground-surface spider numbers and species at the urban habitat.