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.

Effects of forest fragmentation on the beetle assemblage at the relict forest of Fray Jorge, Chile

“…In agreement with theory, the small fragment has a low abundance and richness because of its small area and likely also because of strong edge-effects, due to its proportionally higher perimeter (e.g., Murcia 1995; but see Bolger et al 2000). This is consistent with the ANOVA results, where position (inside vs outside fragments) affected the number of individuals and species and its effect varied with the size of the fragment, such that differences in abundance and species richness between inside and outside fragments decreased as fragments become smaller (Figs.…”

“…Area-effects on arthropod abundance and richness have been well established in both experimental and field studies (e.g., Kareiva 1990;Collinge 2000; but see Golden and Crist 2000). Nonetheless, seasonal effects (Bolger et al 2000) have not been fully analyzed with regard to their interaction with area-or edge-effects in fragmented landscapes. In our study, season significantly interacted with the effects of area and position.…”

Carbohydrate supply limits invasion of natural communities by Argentine ants

“…The duration of sugar availability may have been insuYcient in relation to known rates of spread (15-270 m/year;Holway et al 2002a). Alternatively, their spread in response to sugar may have been positively mediated by edge eVects linked to the urban boundary Bolger et al 2000;Bolger 2007;Holway 2005) or Argentine ants may depend on edge disturbances limiting them to edge eVect status themselves (Bolger 2007). While this may seem to lessen the capacity for Argentine ants to invade natural habitat even with an available carbohydrate source, ongoing fragmentation of forest from human land-use (Wilcove et al 1986) increases the potential exposure of such habitat to Argentine ants Bolger 2007).…”

Edge effects and intraguild predation in native and introduced centipedes: evidence from the field and from laboratory microcosms

“…Not all introduced species cause declines in their native counterparts. For example, Bolger et al (2000) found that native spiders and carabid beetles increased in abundance with increasing fragment age, despite increases in exotic species in these habitat fragments. Our results were similar in that native centipedes did not appear to decline with decreasing fragment size.…”