Arthropods in urban habitat fragments in southern California: Area, age, and edge effects

The distribution of non-ant arthropods was examined in 40 urban habitat fra gments in coastal San Diego County, California, USA, to look for effects of fragmentation, proximity to developed edge, and the non-native Argentine a nt (Linepithema humile). Arthropods were sampled with pitfall traps and by vacuum sampling from California buckwheat shrubs (Eriogonum fasciculatum). Individual arthropods were identified to order and Recognizable Taxonomic U nit (RTU), or morphospecies. At the fragment scale we looked for correlatio ns in the point diversity and abundance of arthropods as a function of the age and area of the fragment being sampled. At the scale of the individual sample points we looked for correlations of abundance and diversity with va riables that describe the species composition of the shrub vegetation and d isturbance. As indicators of disturbance we used the cover of native woody and exotic non-woody vegetation, the distance to the nearest developed edge , and the abundance of Argentine ants. The following patterns were found: ( 1) In general, arthropods showed a fragmentation effect with point diversit y and abundance positively correlated with fragment area and negatively cor related with fragment age. (2) The pitfall samples were dominated by three primarily non-native orders, Isopoda (pillbugs), Dermaptera (earwigs), and Blattaria (roaches). Over 35% of all pitfall-captured arthropods belonged t o four species in these orders. Dermaptera and Blattaria increased in abund ance in smaller and older fragments, respectively. Isopod abundance, in con trast, was unrelated to fragment attributes. None of these groups appeared to be associated with edges, but were distributed throughout the fragments. (3) Point diversity and abundance in ground-active spiders appears to be e nhanced by fragmentation. (4) Total pitfall RTU richness and abundance, and abundance or richness in the Coleoptera (vacuum), Diptera, non-ant Hymenop tera, Hemiptera, Microcoryphia, and Acarina had significant partial negativ e correlations with Argentine ant abundance. The Diptera and Coleoptera had this negative partial relationship with the Argentine ants despite the fac t that both they and the ants were positively associated with edges. (5) In general, diversity in most orders was higher in sampling locations dominat ed by coastal sage scrub habitat than in those with appreciable cover of ch aparral shrub species. (6) There was a strong seasonal variation in abundan ce and diversity in most orders. Diversity and abundance were highest in sp ring, intermediate in winter, and lowest in the fall. (7) Although higher t rophic levels are often considered to be more sensitive to fragmentation, t wo groups of arthropod predators, spiders and carabid beetles, increased in abundance in older fragments. Abundance of these predators was positively correlated with the abundance of Argentine ants and the non-native Isopods, Dermaptera, and Blattaria.