Long-term dynamics of the distribution of the invasive Argentine ant, Linepithema humile, and native ant taxa in northern California

Invasive species, where successful, can devastate native communities. We st udied the dynamics of the invasive Argentine ant, Linepithema humile, for 7 years in Jasper Ridge, a biological preserve in northern California. We mo nitored the distributions at the hectare scale of native ant taxa and L. hu mile in the spring and fall from 1993 to 1999. We also studied the invasion dynamics at a finer resolution by searching for ants in 1-m(2) plots. Our results are similar at both scales. The distributions of several native spe cies are not random with regard to L. humile; the distributions of several epigeic species with similar habitat affinities overlap much less frequentl y than expected with the distribution of L. humile. We found that season ha d a significant influence on the distributions of L. humile and several nat ive taxa. Over the 7-year period, L. humile has increased its range size in Jasper Ridge largely at the expense of native taxa, but there is seasonal and yearly variation in this rate of increase. Studies of invasions in prog ress which sample across seasons and years may help to predict the spread a nd effects of invasive species.