Arthropods in Urban Habitat Fragments in Southern California: Area, Age, and Edge Effects

Abstract

The distribution of non‐ant arthropods was examined in 40 urban habitat fragments in coastal San Diego County, California, USA, to look for effects of fragmentation, proximity to developed edge, and the non‐native Argentine ant (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 Unit (RTU), or morphospecies. At the fragment scale we looked for correlations 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 variables that describe the species composition of the shrub vegetation and disturbance. 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 diversity and abundance positively correlated with fragment area and negatively correlated 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 to four species in these orders. Dermaptera and Blattaria increased in abundance in smaller and older fragments, respectively. Isopod abundance, in contrast, 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 enhanced by fragmentation. (4) Total pitfall RTU richness and abundance, and abundance or richness in the Coleoptera (vacuum), Diptera, non‐ant Hymenoptera, Hemiptera, Microcoryphia, and Acarina had significant partial negative correlations with Argentine ant abundance. The Diptera and Coleoptera had this negative partial relationship with the Argentine ants despite the fact that both they and the ants were positively associated with edges. (5) In general, diversity in most orders was higher in sampling locations dominated by coastal sage scrub habitat than in those with appreciable cover of chaparral shrub species. (6) There was a strong seasonal variation in abundance and diversity in most orders. Diversity and abundance were highest in spring, intermediate in winter, and lowest in the fall. (7) Although higher trophic levels are often considered to be more sensitive to fragmentation, two 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.

Local and landscape drivers of arthropod abundance, richness, and trophic composition in urban habitats

“…Not surprisingly, different arthropod groups were more abundant in different habitats. Isopods accounted for nearly two-thirds of all arthropods encountered and were as common in other urban arthropod studies (Bolger et al 2000). Isopods were most abundant in gardens and forests, likely due to high amounts of organic inputs (e.g.…”

“…Spiders, generally, are more abundant in more disturbed habitats, such as the lots and gardens examined here that experience more mowing and soil manipulation during the summer season (e.g. Samu et al 1999;Bolger et al 2000;Pinkus-Rendón et al 2006). Species richness of ants, beetles, and spiders differed with habitat type.…”

“…Bolger et al (2000) examined the impacts of habitat fragmentation, fragment vegetation type (native vs. non-native woody vegetation), and fragment age on non-ant arthropods, and again, different arthropod groups responded to different factors. Abundance and composition of multiple arthropod groups vary with changes in fragment size, age, and quality (Bolger et al 2000;Gibb and Hochuli 2002). But to our knowledge, no study has examined local and landscape impacts of urbanization on multiple arthropod groups.…”

“…Few actually quantify differences in ecosystem services, and only recently have some observed change in trophic structure and identified the drivers of those changes. For example, urbanization and associated fragmentation may affect the relative abundance of different trophic groups and result in changes in predator composition and trophic structure (Bolger et al 2000;Gibb and Hochuli 2002). Likewise, the abundance of canopy herbivores, predators, and parasitoids may change depending on the location within sampled fragments (edge vs. interior) (Christie et al 2010).…”

Composição e diversidae da araneofauna (Araneae) de serapilheira em três florestas urbanas na cidade de São Paulo, São Paulo, Brasil

“…Além disso, visitas noturnas á área, mostraram grande número de formigueiros e de formigas em atividade de forrageamento. Alguns estudos mostram que formigas podem influenciar diretamente na diversidade ou abundância das aranhas de determinadas áreas (Vieira & Höfer, 1994;1998;Colef et al, 1992;Bolger et al, 2000).…”