Kathleen G. Human

Bio: Kathleen G. Human is an academic researcher from Stanford University. The author has contributed to research in topics: Linepithema & Argentine ant. The author has an hindex of 5, co-authored 5 publications receiving 907 citations.

Exploitation and interference competition between the invasive Argentine ant, Linepithema humile, and native ant species.

Abstract: Interactions between the invasive Argentine ant, Linepithema humile, and native ant species were studied in a 450-ha biological reserve in northern California. Along the edges of the invasion, the presence of Argentine ants significantly reduced the foraging success of native ant species, and vice versa. Argentine ants were consistently better than native ants at exploiting food sources: Argentine ants found and recruited to bait more consistently and in higher numbers than native ant species, and they foraged for longer periods throughout the day. Native ants and Argentine ants frequently fought when they recruited to the same bait, and native ant species were displaced from bait during 60% of these encounters. In introduction experiments, Argentine ants interfered with the foraging of native ant species, and prevented the establishment of new colonies of native ant species by preying upon winged native ant queens. The Argentine ants' range within the preserve expanded by 12 ha between May 1993 and May 1994, and 13 between September 1993 and September 1994, with a corresponding reduction of the range of native ant species. Although some native ants persist locally at the edges of the invasion of Argentine ants, most eventually disappear from invaded areas. Both interference and exploitation competition appear to be important in the displacement of native ant species from areas invaded by Argentine ants.

Effects of Argentine Ants on Invertebrate Biodiversity in Northern California

Abstract: Although most biological invasions usually fail (Elton 1958; Simberloff 1981; Lodge 1993), some invasive species can change the composition and function of communities and ecosystems through competition, predation, and habitat alteration (Elton 1958; Simberloff 1981; Mooney & Drake 1986; Drake et al. 1989; Vitousek 1990). The effects of invasive ant species on native ant fauna have been described in many systems (Foster 1908; Crowell 1968; Haskins & Haskins 1965, 1988; Erickson 1971; Tremper 1976; Haines & Haines 1978; Clark et al. 1982; Lubin 1984; Medeiros et al. 1986; Ward 1987; Porter et al. 1988; De Kock 1990; Porter & Savignano 1990; Cole et al. 1992; Holway 1995). Most of these studies have reported striking declines in the abundance and species richness of native ants in areas invaded by exotic ant species, although particular groups of ants occasionally coexist with the invaders (Haskins & Haskins 1965, 1988; Ward 1987). The effects of invasive ants on non-ant invertebrates have been less extensively studied than effects on other ant species. Invertebrate displacement by invasive species could have cascading effects on ecosystems because many invertebrates play important roles in ecosystem processes. Moreover, understanding how broad classes of organisms are associated with an invasive species can provide insight into the mechanisms of invasion and displacement. We compare the composition of invertebrate communities in areas invaded and uninvaded by the Argentine ant (Lipepithema humile). The Argentine ant is native to South America, and it is an increasingly common invasive species world-wide (Ward 1987; Holway 1995; Human & Gordon 1996).

Behavioral interactions of the invasive Argentine ant with native ant species

Abstract: The Argentine ant, Linepithema humile, has invaded many areas of the world, displacing native ants. Its behavior may contribute to its competitive success. Staged and natural encounters were observed at food resources in the field, between Argentine ants and eight ant species native to northern California. There was no relation between the frequency of aggression by any ant species and the outcome of encounters, though Argentine ants were more likely than ants of native species to behave aggressively. When an ant of one species initiated an encounter of any kind with an ant of another species, the ant that did not initiate was likely to retreat. This was true of all species studied. Most encounters between ants were initiated by Argentine ants. Thus the native species tended to retreat more frequently than Argentine ants. Interactions between Argentine ants and native species at food resources, causing ants of native species to retreat, may help Argentine ants to displace native species from invaded areas.

Effects of abiotic factors on the distribution and activity of the invasive Argentine ant (Hymenoptera: Formicidae)

Abstract: The Argentine ant, Linepithema humile (Mayr), has spread worldwide, often decimating native ant populations and other arthropod species in invaded areas. It is not known what abiotic and biotic conditions limit its distribution. We investigated the distribution of the Argentine ant in the Jasper Ridge Biological Preserve in northern California, a nature preserve that has been partially invaded by these ants. Canonical correlation analysis showed that the Argentine ant is most likely to occur near the edges of the preserve, which are next to disturbed areas and in low-elevation areas. Native ant species are associated with higher-elevation areas farther from the preserve edge. Distance to surface water and insolation were less important in predicting Argentine ant distribution. This suggests that dispersal from disturbed areas strongly determines the spread of the invasion. We examined how the daily activity patterns of Argentine ants and several native ant species depend on soil temperature, air temperature, and relative humidity. There was considerable overlap in activity times, and there were no values of any of the abiotic factors measured in which native ant species were more likely to be active than Argentine ants. Because the Argentine ant is usually active at the same times as any native species, most native species in invaded areas are likely to encounter the invaders frequently. Diet overlap was high with most native ant species except for seed-eating species. Overlap in activity times may intensify both exploitative and interference competition between the Argentine and native ant species.

Effects of harvester ants on plant species distribution and abundance in a serpentine grassland

Abstract: Seed harvesting ants can have important effects on the composition and structure of plant communities. We investigated two effects of Messor andrei, the black seed-harvesting ant, on a serpentine grassland plant community in northern California. First, to determine if selective seed predation by ants affects plant community composition, we excluded harvester ants from 1-mediameter circular plots of grassland. Abundances of all species on these plots and on control plots were measured before and after exclosure. Second, to determine if M. andrei nest mounds affect plant community composition, we compared plant species abundances on and off nest mounds. M. andrei deposit large amounts of organic matter on their nest mounds over a foraging season, so mounds may alter the edaphic environment. The exclusion of seed-harvesting activity did not cause changes in the plant community. Nest mounds had a strong effect on plant communities: there were many more grasses and fewer forbs on ant mounds, although at least one forb, Lepidium nitidum, produced twice as many seeds when it grew on nest mounds. We found that nest mounds formed islands of higher-temperature soil in the serpentine grassland.

The evolutionary impact of invasive species.

Abstract: Since the Age of Exploration began, there has been a drastic breaching of biogeographic barriers that previously had isolated the continental biotas for millions of years. We explore the nature of these recent biotic exchanges and their consequences on evolutionary processes. The direct evidence of evolutionary consequences of the biotic rearrangements is of variable quality, but the results of trajectories are becoming clear as the number of studies increases. There are examples of invasive species altering the evolutionary pathway of native species by competitive exclusion, niche displacement, hybridization, introgression, predation, and ultimately extinction. Invaders themselves evolve in response to their interactions with natives, as well as in response to the new abiotic environment. Flexibility in behavior, and mutualistic interactions, can aid in the success of invaders in their new environment.

Does global change increase the success of biological invaders

Abstract: Biological invasions are gaining attention as a major threat to biodiversity and an important element of global change. Recent research indicates that other components of global change, such as increases in nitrogen deposition and atmospheric CO2 concentration, favor groups of species that share certain physiological or life history traits. New evidence suggests that many invasive species share traits that will allow them to capitalize on the various elements of global change. Increases in the prevalence of some of these biological invaders would alter basic ecosystem properties in ways that feed back to affect many components of global change.

The Causes and Consequences of Ant Invasions

Abstract: Invasions by non-native ants are an ecologically destructive phenomenon affecting both continental and island ecosystems throughout the world. Invasive ants often become highly abundant in their introduced range and can outnumber native ants. These numerical disparities underlie the competitive asymmetry between invasive ants and native ants and result from a complex interplay of behavioral, ecological, and genetic factors. Reductions in the diversity and abundance of native ants resulting from ant invasions give rise to a variety of direct and indirect effects on non-ant taxa. Invasive ants compete with and prey upon a diversity of other organisms, including some vertebrates, and may enter into or disrupt mutualistic interactions with numerous plants and other insects. Experimental studies and research focused on the native range ecology of invasive ants will be especially valuable contributions to this field of study.

Reduced genetic variation and the success of an invasive species

Abstract: Despite the severe ecological and economic damage caused by introduced species, factors that allow invaders to become successful often remain elusive. Of invasive taxa, ants are among the most widespread and harmful. Highly invasive ants are often unicolonial, forming supercolonies in which workers and queens mix freely among physically separate nests. By reducing costs associated with territoriality, unicolonial species can attain high worker densities, allowing them to achieve interspecific dominance. Here we examine the behavior and population genetics of the invasive Argentine ant (Linepithema humile) in its native and introduced ranges, and we provide a mechanism to explain its success as an invader. Using microsatellite markers, we show that a population bottleneck has reduced the genetic diversity of introduced populations. This loss is associated with reduced intraspecific aggression among spatially separate nests, and leads to the formation of interspecifically dominant supercolonies. In contrast, native populations are more genetically variable and exhibit pronounced intraspecific aggression. Although reductions in genetic diversity are generally considered detrimental, these findings provide an example of how a genetic bottleneck can lead to widespread ecological success. In addition, these results provide insights into the origin and evolution of unicoloniality, which is often considered a challenge to kin selection theory.

Ecological effects of invasive alien insects

Abstract: A literature survey identified 403 primary research publications that investigated the ecological effects of invasive alien insects and/or the mechanisms underlying these effects. The majority of these studies were published in the last 8 years and nearly two-thirds were carried out in North America. These publications concerned 72 invasive insect species, of which two ant species, Solenopsis invicta and Linepithema humile, accounted for 18% and 14% of the studies, respectively. Most publications investigated effects on native biodiversity at population or community level. Genetic effects and, to a lesser extent, effects on ecosystem services and processes were rarely explored. We review the effects caused by different insect invaders according to: their ecosystem roles, i.e. herbivores, predators, parasites, parasitoids and pollinators; the level of biological organisation at which they occur; and the direct and indirect mechanisms underlying these effects. The best documented effects occur in invasive ants, Eurasian forest herbivores invasive in North America, and honeybees. Impacts may occur through simple trophic interactions such as herbivory, predation or parasitism. Alien species may also affect native species and communities through more complex mechanisms such as competition for resources, disease transmission, apparent competition, or pollination disruption, among others. Finally, some invasive insects, particularly forest herbivores and ants, are known to affect ecosystem processes through cascading effects. We identify biases and gaps in our knowledge of ecological effects of invasive insects and suggest further opportunities for research.