The role of dispersal from natural habitat in determining spider abundance and diversity in California vineyards

Many generalist predators do not persist in crops year-round, and must colonize crops on a seasonal basis from natural vegetation. We monitored the spider community in the air, on the ground, and in the foliage of California vineyards and neighboring oak-woodlands, to assess whether dispersal from natural habitats influences the abundance and species diversity of spiders in agroecosystems. Early in the growing-season, low spider numbers in vineyard foliage indicated that few spiders overwintered in the vineyard. Aerial dispersal did not, however, occur primarily in the spring when generalist predators are likely to be important in pest suppression, but in mid-summer, while dispersal activity on the ground did not change over time. Aerial collection traps showed greater similarity to spider composition in vineyard foliage than ground collection traps, indicating that spiders in vineyards disperse more by air than over ground. Dwarf spiders (Linyphiidae) dominated spider composition on aerial traps but were not abundant on the vine, while other spider species showed an opposing pattern, suggesting that while spiders may have entered vineyards by air, other factors may have also contributed to spider composition in vineyards. These results are discussed with respect to control of vineyard pests and the establishment of spiders in vineyard agroecosystems.