Ted J. Case

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.

An Experimental Demonstration of Exploitation Competition in an Ongoing Invasion

A native asexual gecko, Lepidodactylus lugubris, declines numerically when the sexual gecko Hemidactylus frenatus invades urban/suburban habitats throughout the Pacific. Previous studies showed that the competitive displacement occurs rapidly and is facilitated by clumped insect resources. Five lines of evidence suggest that the mechanism of displacement is primarily due to differences in the ability of each species to exploit insect resources. (1) These species show nearly complete diet overlap. (2) Insects are a limiting resource for both geckos as evidenced by positive demographic effects with in- creased insect abundance. (3) Hemidactylus frenatus depletes insect resources to lower levels than L. lugubris, which results in reduced rates of resource acquisition in L. lugubris. (4) This reduced resource acquisition translates into significant reductions in the body condition, fecundity, and survivorship of L. lugubris individuals. (5) Evidence for inter- ference (and other) mechanisms does not account for these negative demographic effects on L. lugubris. Interspecific competition is stronger than intraspecific competition for L. lugubris, with increasing L. lugubris density having negligible effect on H. frenatus, mirroring the asym- metry of the large-scale displacement. The superior harvesting ability of H. frenatus is most pronounced when insects are clumped spatially and temporally, and is attributable to a variety of species-specific traits such as their larger body size, faster running speed, and reduced intraspecific interference while foraging. We conclude that clumped resources can increase interspecific exploitation competition, and this mechanism may contribute to spe- cies turnover when human environmental alterations redistribute resources.

Behavioral and genetic differentiation between native and introduced populations of the Argentine ant

In this paper, we examine the hypothesis that reduced intraspecific aggression underlies the competitive prowess of Argentine ants in their introduced range. Specifically, we test three predictions of this hypothesis by comparing the genetic diversity, behavior, and ecology of Argentine ants in their native range to introduced populations. Differences between native and introduced populations of Argentine ants were consistent with our predictions. Introduced populations of the Argentine ant appear to have experienced a population bottleneck at the time of introduction, as evidenced by much reduced variation in polymorphic microsatellite DNA markers. Intraspecific aggression was rare in introduced populations but was common in native populations. Finally, in contrast to the Argentine ants ecological dominance throughout its introduced range, it did not appear dominant in the native ant assemblages studied in Argentina. Together these results identify a possible mechanism for the widespread success of the Argentine ant in its introduced range.

Global patterns in the establishment and distribution of exotic birds

Abstract I use three separate data bases to examine recipient community and site factors that might be influencing the establishment, persistence, and distribution of avian exotics. All in all, about half the variance between islands/regions in their numbers of successfully and unsuccessfully introduced species can be accounted for by recipient site-specific variables; the most important correlate of success is the number of native species extinctions over about the last 3000 years, which reflects the degree of human activity and habitat destruction and deterioration through intrusions of exotic predators, herbivores, and parasites. Consequently, the number of exotic species gained is close to the number of species lost through extinction. Even after controlling for avian extinctions, island area correlates positively with introduced species number. Invasion success does not decline significantly with the richness of the native avifauna (after controlling for the effects of extinctions and island area) nor the variety of potential mammalian predators. The relative proportion of extinct native species across islands/regions is negatively correlated with area and positively correlated with introduced species number and the number of endemic species. A strong correlation exists between the number of successes and the number of failures, attesting to the role of persistent acclimatization societies in increasing species numbers despite high failure rates. The relative success to failure rate increases with the number of extinct native species. The correlation between introductions and native extinctions seems to arise because native birds are usually more common, if not restricted, to native habitats while introduced birds are primary occupants of disturbed and open habitats. As more of an islands area is converted to urban, agricultural and disturbed habitats or altered through the introduction of herbivores and exotic predators, most natives lose good living space while most introduced birds, that frequent open and disturbed areas and have evolved in predator-rich areas, gain habitat. I find little support for the notion that rich avifaunas in themselves repel the establishment of avian invaders at the level of whole islands or archipelagoes. However, interactions between established exotics and natives may be influencing habitat distributions of species in both sets within islands. In both man-made habitats and native forest habitats, exotic species number and the relative abundance of exotic birds is negatively related to the number of native species. After accounting for this local variation, exotic species number is positively related to exotic species number for the entire island/region. In local surveys the relative abundance of exotic birds compared to native birds is affected by habitat (non-native habitats have more exotics) and also by the numbers of species of exotics and natives on the island. The relative importance of biotic interactions like competition, apparent competition through differential disease transmission or susceptibility, and predation in shaping the abundance and habitat affinities of exotics and native species can be difficult to unravel when regional affects are so important.

ROLE OF ABIOTIC FACTORS IN GOVERNING SUSCEPTIBILITY TO INVASION: A TEST WITH ARGENTINE ANTS

Hypotheses concerning community-level vulnerability to invasion often emphasize biotic interactions but fail to consider fine-scale variation in the physical environment. In this study, the interplay between interspecific competition and abiotic factors is examined with respect to whether scrub habitats in southern California become invaded by the Argentine ant (Linepithema humile). Argentine ants penetrate further into and attain higher abundances in mesic scrub fragments than they do in xeric scrub fragments. Probably as a result, native ant richness is lower in small (<30 ha), mesic fragments than in either small, xeric fragments or in small plots in unfragmented areas. Compared to six species of native ants, Argentine ants ranked lowest in their ability to tolerate high temperatures in the laboratory with 100% of field-collected workers dying after 60 min of exposure to temperatures ≥46°C. Field data corroborate these results; mean temperatures at which colonies of Argentine ants attained maximum abundance at baits (34.0°C) and abandoned baits (41.6°C) were both lower than for the native ant Dorymyrmex insanus. Laboratory studies further revealed that low levels of soil moisture depress mean worker survival in experimental colonies of Argentine ants. In a factorial laboratory experiment that varied both physical conditions and interspecific competition, Argentine ants exhibited greater worker activity and survival under warm, moist conditions than under hot, dry conditions, whereas the presence of a competitor, Forelius mccooki, had no significant effect. Experimental colonies of F. mccooki, in contrast, were more active under hot, dry conditions than under warm, moist conditions and exhibited reduced activity in the presence of L. humile irrespective of physical conditions. Taken together, these experimental data demonstrate how the abiotic environment impinges on both colony-level activity and colony growth in the Argentine ant and provide a general explanation for the patterns observed at the community level. A key consequence of the condition-specific nature of the competitive asymmetry between Argentine ants and native ants is that community-level vulnerability to invasion appears to depend primarily on the suitability of the physical environment from the perspective of L. humile.

Higher Order Interactions in Ecological Communities: What Are They and How Can They be Detected?

The detection and significance of higher order interactions (HOIs) between species has been a matter of debate and experimentation in community ecology for several decades. HOIs are considered potentially significant because their presence is assumed to mean that the dynamic behavior of a full community of species is unpredictable based on observations of interactions between subsets of the species within the community. Despite such attention, the causal mechanisms that produce HOIs have been inadequately discussed. We discuss three different usages of the term HOIs and provide insight as to why HOIs might be found within a given community. HOIs may be detected for three reasons: inappropriate assumptions made concerning species interactions that influence statistical tests, unmeasured parameters and variables, and interaction modifications (i.e., a functional change in the interaction of two species caused by a third species. This confusion concerning the defining attributes of HOIs has made their detection problematic. While the statistical tests being used in the ecological experiments to detect HOIs are described in detail in most papers, the dynamic models underlying these tests are often not made explicit. Additionally, we demonstrate the equivalency of three different statistical tests: the Case and Bender (1981) test, analysis of variance, and a multiplicative test (Wootton 1994). However, the choice of a response variable (i.e., population densities, population growth rates, per—capita growth rates, etc.) and different data transformations applied to these response variables alter the underlying dynamics model that is being tested. The result is that the statistical test applied does not always perform the intended comparison but instead tests a different and sometimes unjustified or even inappropriate dynamic model. Finally, we review the relationship between indirect effects and HOIs. Whereas some researchers have lumped HOIs and indirect effects, we argue that the two represent completely unique and separate phenomena. Additionally, indirect effects can complicate detection of HOIs, and we review several methods by which to separate the two processes.

The role of introduced species in shaping the distribution and abundance of island reptiles

SummarySpecies interactions, as revealed by historical introductions of predators and competitors, affect population densities and sometimes result in extinctions of island reptiles. Mongoose introductions to Pacific islands have diminished the abundance of diurnal lizards and in some cases have led to extinctions. Through these population level effects, biogeographic patterns are produced, such as the reciprocal co-occurrence pattern seen with the tuatara and its predator, the Polynesian rat, and with the tropical gecko competitorsHemidactylus frenatus andLepidodactylus lugubris in urban habitats in the Pacific. Although competition has led to changes in abundance and has caused habitat displacement and reduced colonization success, extinctions of established reptile populations usually occur only as a result of predation.These introductions, along with many manipulative experiments, demonstrate that present day competition and predation are potent forces shaping community structure and geographic distributions. The human introduction of species to islands can be viewed as an acceleration of the natural processes of range expansion and colonization. The immediate biotic consequences of these natural processes should be of the same intensity as those of the human introductions. Coevolution may subsequently act to ameliorate these interactions and reduce the dynamical response of one species to the other. The role played by coevolution in mediating interactions between competitors and predator and prey is highlighted by the susceptibility of predator-naive endemic species to introduced predators and the invalidity of species-poor communities.

Models of character displacement and the theoretical robustness of taxon cycles

The appropriateness of the techniques used in modeling character displacement has been the focus of vigorous debate. In this paper, the three competing methods (the coevolutionarily stable community (CSC), the evolutionarily stable strategy (ESS), and quantitative genetic recursion (QGR)) are compared in models using a common ecological setting. Specific predictions of the CSC model have been used to understand features of character displacement among Cnemidophorous lizards on islands off Mexico, Anolis lizards in the Lesser Antilles and Galápagos finches. Nonetheless, the validity of the approach has been repeatedly questioned. Conceptually the three formalisms vary in the degree to which within species variability is allowed in the models. The predictions of the CSC are found not to be robust to even small violation of its fundamental assumption of absolute species monomorphy. We show by simulation and analytical observations that the CSC is not valid under frequency dependent selection, and that the ESS is the limiting case of QGR as intraspecific phenotypic variation goes to zero. Thus the ESS and the QGR models agree closely when the between‐phenotype component (BPC) of the niche width is small. However, as the BPC increases, quantitative discrepancies between ESS and QGR predictions increase, although model behavior remains qualitatively similar. A fourth approach, termed “Quantitative Genetic Optimization” (QGO) analysis, is suggested, combining advantages of both the ESS and QGR. Although all approaches support the possibility of taxon cycles, the cycle patterns predicted are qualitatively different and strongly model dependent.

Relationships among native and introduced populations of the Argentine ant (Linepithema humile) and the source of introduced populations.

The Argentine ant (Linepithema humile) is a damaging invasive species that has become established in many Mediterranean‐type ecosystems worldwide. To identify likely sources of introduced populations we examined the relationships among native Linepithema populations from Argentina and Brazil and introduced populations of L. humile using mitochondrial cytochrome b sequence data and nuclear microsatellite allele frequencies. The mitochondrial phylogeny revealed that the populations in Brazil were only distantly related to both the introduced populations and the native populations in Argentina, and confirmed that populations in Brazil, previously identified as L. humile, are likely a different species. The microsatellite‐based analysis provided resolution among native and introduced populations of L. humile that could not be resolved using the mitochondrial sequences. In the native range, colonies that were geographically close to one another tended to be genetically similar, whereas more distant colonies were genetically different. Most samples from the introduced range were genetically similar, although some exceptions were noted. Most introduced populations were similar to native populations from the southern Rio Parana and were particularly similar to a population from Rosario, Argentina. These findings implicate populations from the southern Rio Parana as the most likely source of introduced populations. Moreover, these data suggest that current efforts to identify natural enemies of the Argentine ant for biological control should focus on native populations in the southern Rio Parana watershed.