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.

Invasive species pose serious threats to community structure and ecosystem function worldwide. The impacts of invasive species can be more pervasive than simple reduction of species numbers. By using 7 years of data in a biological preserve in northern California, we documented the disassembly of native ant communities during an invasion by the Argentine ant. In sites without the Argentine ant, native ant communities exhibit significant species segregation, consistent with competitive dynamics. In sites with the Argentine ant, native ant communities appear random or weakly aggregated in species co-occurrence. Comparisons of the same sites before and after invasion indicate that the shift from a structured to a random community is rapid and occurs within a year of invasion. Our results show that invasive species not only reduce biodiversity but rapidly disassemble communities and, as a result, alter community organization among the species that persist.community organization ͉ null model ͉ Linepithema humile ͉ invasive ants B y displacing native species or reducing their abundance, biological invaders have severely disrupted the organization and function of many native communities and ecosystems (1, 2). The loss of biodiversity in the face of invasive species reflects the breakdown of community assembly rules (3) that structure intact assemblages. However, the disruption of community assembly rules has not been documented during the course of a biological invasion. Studies of invasions as natural experiments (4) are usually snapshot analyses that often lack preinvasion data on community structure. In this study, we used 7 years of data from monitoring an invasion in progress to assess the impact of a widespread invasive species, the Argentine ant (Linepithema humile), on the organization and disassembly of native ant communities in northern California. We used null model analysis (5) to ask three related, but distinct, questions with these data: (i) Is there evidence of nonrandom co-occurrence of native species within intact or invaded regions? (ii) Do co-occurrence patterns differ between invaded and intact regions? (iii) Do co-occurrence patterns in intact regions change once L. humile invades? MethodsThe Argentine ant is native to South America and has been introduced to Mediterranean and subtropical climates worldwide. As of 2001, the Argentine ant occupied 335 counties in 21 states in the United States and six continents in the world (6). Perhaps because the Argentine ant locates resources efficiently and interferes with native species in its introduced range (7,8), native ant (9-13) and arthropod (9, 14) species richness is drastically reduced in the presence of Argentine ants.Survey Methods. Since 1993, we have monitored the distribution of the ground-foraging native ant fauna and the invasive Argentine ant at Jasper Ridge Biological Preserve, a 481-hectare preserve in northern California. By using aerial photographs, we divided the preserve into 133 1-hectare quadrats. Each May and September since 1993, a...

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This study examines how the spatial distribution of nests is related to the behavioral interactions of conspecific neighbors in a population of the seed-eating ant, Pogonomyrmex barbatus. Colonies live for 15-20 yr, reaching reproductive age and a stable size at =5 yr. Spatial distributions were measured for 6 yr (1988)(1989)(1990)(1991)(1992)(1993) in a population of =250 colonies of known age. The probability that a 1-yr-old colony occurs in a given location is related to the distance to, and ages of, its five nearest conspecific neighbors.One-year-old colonies are most likely to occur near small, 2-and 3-yr-old colonies.Neighboring colonies encounter each other when foragers of both colonies search the same area on the same day. The probability of an encounter between two colonies decreases with the distance between their nests. For colonies of all ages, encounters are most likely with their larger neighbors, ~5 yr old. Encounters are more likely if there was an encounter the previous day, and this effect can overwhelm the effect of distance. Two-yr-old colonies are more likely than colonies of other ages to lose ground in t;epeated encounters with a particular neighbor.Though encounters are frequent, their costs are low: few of the foragers on a trail that meets a neighbor's actually encounter an ant of the neighboring colony, and interaction with a neighboring colony does not increase the typical duration of a foraging trip. If foragers of two colonies do meet, most fights are brief, with few resulting in injury or death. These results suggest that the cost of conspecific neighbors searching the same ground for seeds may be greater than the cost of behavioral interaction itself. Exploitative competition may have more important effects than interference competition on founding colony survival and thus on the spatial distribution of nests.

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