Douglas T. Bolger

EFFECTS OF FRAGMENTATION AND INVASION ON NATIVE ANT COMMUNITIES IN COASTAL SOUTHERN CALIFORNIA

We investigated the roles of habitat fragmentation and the invasion of an exotic species on the structure of ground-foraging ant communities in 40 scrub habitat fragments in coastal southern California. In particular, we asked: how do fragment age, fragment size, amount of urban edge, percentage of native vegetation, degree of isolation, and the relative abundance of an exotic species, the Argentine ant (Linepithema humile) affect native ants? Within these fragments, Argentine ants were more abundant near developed edges and in areas dominated by exotic vegetation. The number of native ground-foraging ant species at any point declined from an average of >7 to <2 species in the presence of the Argentine ant. Among fragments, a stepwise multiple regression revealed that the abundance of Argentine ants, the size of the fragment, and the number of years since it was isolated from larger continuous areas of scrub habitat best predict the number of remaining native ant species. The Argentine ant was found in every fragment surveyed as well as around the edges of larger unfragmented areas. Fragments had fewer native ant species than similar-sized plots within large unfragmented areas, and fragments with Argentine ant-free refugia had more native ant species than those without refugia. The relative vulnerability of native ants to habitat fragmentation and the subsequent presence of Argentine ants vary among species. The most sensitive species include army ants (Neivamyrmex spp.) and harvester ants (genera Messor and Pogonomyrmex), both of which are important to ecosystem-level processes. Our surveys suggest that the Argentine ant is widespread in fragmented coastal scrub habitats in southern California and strongly affects native ant communities.

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 fragments in coastal San Diego County, California, USA, to look for effects of fragmen- tation, proximity to developed edge, and the non-native Argentine ant (Linepithema humile). Arthropods were sampled with pitfall traps and by vacuum sampling from California buck- wheat shrubs (Eriogonumfasciculatum). 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 abun- dance, and abundance or richness in the Coleoptera (vacuum), Diptera, non-ant Hymenop- tera, Hemiptera, Microcoryphia, and Acarina had significant partial negative correlations with Argentine ant abundance. The Diptera and Coleoptera had this negative partial rela- tionship 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.

Avian assemblages along a gradient of urbanization in a highly fragmented landscape

Abstract Our goal was to evaluate how avian assemblages varied along a gradient of urbanization in the highly fragmented landscape of coastal southern California. We measured species richness and abundance of birds within continuous blocks of habitat, within urban habitat fragments that varied in landscape and local habitat variables, and within the urban matrix at different distances from the wildland interface. These comparisons allowed us to characterize patterns of avifaunal response to a gradient of urban fragmentation. At the fragment scale, we found that fragment area was a strong, positive predictor of the total number of breeding species detected per fragment; total bird abundance per point count also increased with fragment size. Tree cover was higher in small fragments, as was the abundance of birds that typically occupy wooded habitats. Comparisons between core, fragment, and urban transects revealed differing patterns of response of individual bird species to urbanization. In unfragmented habitat, we recorded a relatively high diversity of urbanization-sensitive birds. In urban transects, these species were rare, and a relatively few species of non-native and anthropophilic birds were common. These urbanization-enhanced birds were also recorded in previous urban gradient studies in northern California and Ohio. Bird communities along the urban gradient reached their highest richness and abundance in fragments. The marked difference in vegetation structure between urban and natural landscapes in this arid shrubland system likely contributed to this pattern; the presence of native shrubs and exotic trees in fragments enabled both shrub and arboreal nesters to co-occur. As is characteristic of biotic homogenization, urban fragmentation in coastal southern California may increase local diversity but decrease overall regional avifaunal diversity.

Occurrence Patterns of Bird Species in Habitat Fragments: Sampling, Extinction, and Nested Species Subsets

Comparison of the species-area relationship in unfragmented chaparral habitat with that in urban chaparral fragments confirmed that rapid population extinction of resident bird species has occurred in these fragments. A strong positive correlation between the relative persistence ability of a species and its density remains even after correcting for the sampling effect of area. We conclude that this pattern is due to differences between species in extinction vulnerability attributable to density; the more abundant species persist longer in fragments. This differential vulnerability to extinction produces a pattern in which the bird species present in species-poor fragments are nested subsets of those in species-rich fragments.

The effects of habitat fragmentation on chaparral plants and vertebrates

The effects of fragmentation in a scrub habitat in California on three taxa (plants, birds, and rodents) are concordant. Extinctions within the habitat remnants occur quickly and the sequence of species disappearances of birds and rodents is predictable based on population density in undisturbed habitat. Distance effects on species diversity are weak to non-existent, and habitat area effects are strong. Edge effects and cumulative habitat loss following isolation of the remnants are correlated with loss of species diversity. Recolonization in these taxa occurs rarely. Rodents appear to be extremely susceptible to extinction. Small, old patches retain a predictable subset of bird and rodent species, reinforcing the principle that larger reserves are generally superior

RESPONSE OF RODENTS TO HABITAT FRAGMENTATION IN COASTAL SOUTHERN CALIFORNIA

We employed an island biogeographic approach to determine whether small fragments of the shrub habitats coastal sage scrub and chaparral, isolated by urbanization, are capable of supporting viable populations of native rodent species. The distribution of native rodents in 25 urban habitat fragments was assessed by live-trapping. Over half of the fragments surveyed (13 of 25) did not support populations of native rodents. Fragments supported fewer species than equivalently sized plots in large expanses of unfragmented habitat, and older fragments (fragments that had been isolated for a longer period of time) supported fewer species. Both results implied that local extinctions occurred in the fragments following insularization. Stepwise multiple polychotomous logistic regression was used to determine which biogeographic variables were the best predictors of species number across fragments. The area of shrub habitat in each fragment was the most significant predictor of species diversity; age of a fragment was also significant and was negatively correlated with species number, but the isolation distance of a fragment had no relationship to species diversity. We found a negative relationship between extinction vulnerability of native rodent species and relative abundance: species that were more abundant in unfragmented habitat persisted in more habitat fragments. Random environmental and demographic fluctuations (island effects) and edge effects associated with fragmentation are proposed as causes of these local extinctions.

Variation in a sparrow's reproductive success with rainfall: food and predator-mediated processes

Abstract. From 1997 to 1999, we monitored the reproductive success of individual rufous-crowned sparrows (Aimophila ruficeps) in coastal sage scrub habitat of southern California, USA. Annual reproductive output of this ground-nesting species varied strongly with annual variation in rainfall, attributed to the El Niño-Southern Oscillation. Birds fledged 3.0 young per breeding pair in 1997, when rainfall was near the long-term mean, 5.1 offspring per pair in 1998, a wet El Niño year, and 0.8 fledglings per pair in 1999, a dry La Niña year. Variation in many components of reproductive output was consistent with the hypothesis that food availability was positively correlated with rainfall. However, the factor most responsible for the high reproductive output in 1998 was low early season nest predation which, combined with favorable nesting conditions, enabled more pairs to multiple-brood. Cool, rainy El Niño conditions may have altered the activity of snakes, the main predator of these nests, in the early season of 1998. Overall, more of the annual variation in fecundity was attributable to variation in within-season components of reproductive output (mean number of nests fledged per pair) than to within-nest components (mean brood size). Annual variation in rufous-crowned sparrow fecundity appears to be driven primarily by food resource-mediated processes in La Niña years and by predator-mediated processes in El Niño years.

INVASIONS AND COMPETITIVE DISPLACEMENT AMONG HOUSE GECKOS IN THE TROPICAL PACIFIC

Since the 1 930s the common house gecko, Hemidactylus frenatus, a sexual species, has been inadvertently introduced to many tropical Pacific islands. Using mark- recapture censuses and visual gecko searches we found that Lepidodactylus lugubris, an asexual gecko previously common on these islands, is nearly 800% more abundant on buildings in the urban/suburban environment on islands that lack H. frenatus than it is on islands where H. frenatus is present. On buildings in Hawaii and Fiji that have been surveyed in different years, the proportion of H. frenatus relative to L. lugubris has sig- nificantly increased over time. The degree of numerical dominance is also related to climate and habitat: L. lugubris is relatively more common on the more mesic, cooler, windward sides of large islands compared to the more arid sides of islands and in general H. frenatus does not penetrate forest habitats, where L. lugubris remains one of the most common geckos. Overall, buildings with external electric lights have more geckos than unlit buildings. Where the two species occur on the same buildings H. frenatus is closer, on average, to the prime feeding sites near lights that attract insects. In the absence of H. frenatus, L. lugubris is found closer to the lights. We hypothesize that the previously demonstrated agonistic dominance of H. frenatus over L. lugubris (Bolger and Case 1992) is likely to lead to competitive superiority when insect prey are concentrated into patches that are structurally simple (like flat building walls with lights), allowing easy detection of prey and intruders.

Avian reproductive failure in response to an extreme climatic event

Recently, climate change research has emphasized the potential increase in the frequency and severity of climatic extremes. We compared the reproductive effort and output among four species of passerine birds in coastal southern California, USA, a semi-arid region, during a normal precipitation year (2001) and the driest year in a 150-year climate record (2002). Both reproductive effort and output differed dramatically between years. Mean reproductive output among the four species was 2.37 fledglings/pair in 2001 and 88.4% of all pairs observed attempted at least one nest. The birds attempted a mean of 1.44 nests per pair and were successful in 47.7% of those attempts. In 2002, only 6.7% of the pairs even attempted a nest and only 1.8% were successful, for a total output of 0.07 fledglings per pair. The abundance of suitable arthropod prey items in the environment was also much lower in 2002, suggesting that low food availability was the proximal cause of the reproductive failure. The data for one of these species, the rufous-crowned sparrow (Aimophila ruficeps), were combined with reproductive and rainfall data from a previous 3-year study (1997–1999) in the same sites. The combined data sets suggest that the response of reproduction to rainfall variation is linear, and that the low end of the precipitation range brings the population near reproductive failure. Any change in climate that would increase the frequency of extreme dry conditions would likely endanger populations of these species.

Use of corridor-like landscape structures by bird and small mammal species

Abstract We investigated the use of corridor-like structures by birds and rodents in coastal San Diego County, California, USA, in areas characterized by dense residential development and small remnant habitat fragments. In 1992 and 1993 we compared the use of remnant strips of coastal sage scrub habitat and revegetated highway rights-of-way (ROW) with that of larger remnant patches of coastal sage scrub habitat to which these strips were connected. Using live trapping we compared rodent species composition, percent of females reproducing, and recapture frequency among the three site types. Rodent species richness did not differ significantly among the three site types. One species, Peromyscus eremicus was significantly more abundant in ROW sites, while several other species, Rheithrodontomys californicus , Chaetodipus californicus and C. fallax showed a similar, though non-significant trend. Peromyscus californicus was most abundant in remnant strips while Neotoma fuscipes used ROW sites less than remnant strips or patches. No significant differences were detected among the three site types in recapture frequency or proportion of females in reproductive condition. Common bird species were a priori divided into two categories based on previous research in this region: fragmentation-sensitive and fragmentation-tolerant. Species richness in remnant strips of habitat was similar to that in remnant patches for both categories of species. Species richness of the fragmentation-sensitive species was significantly lower in ROW sites than in remnant strips or patches by a factor of 4–10. Species richness in fragmentation-tolerant species was significantly lower in ROW sites in 1993 by less than a factor of two. The greater reduction in fragmentation-sensitive species suggests they have more stringent corridor requirements than do fragmentation-tolerant species. The species richness per site for fragmentation-sensitive bird species was significantly correlated with the percent cover of native shrubs in 1993 in patch and remnant strip sites. Fragmentation-tolerant bird species showed no consistent trend with shrub cover. Rodent species richness trended higher with increasing shrub cover in five of six site-type/year combinations, but none of the correlations were significant. Within the range of width and length examined there was no correlation between bird or rodent species richness and length or width of these corridor-like structures. These results indicate that remnant strips of habitat and revegetated highway ROW have potential to serve as habitat linkages for native rodents and fragmentation-tolerant bird species, but only remnant habitat strips appear suitable for fragmentation-sensitive bird species.