Leaf-cutting ant nests near roads increase fitness of exotic plant species in natural protected areas.

FARJI-BRENER, A. AND L. GHERMANDI

Proceedings Royal Society

Año: 2007

Summary   1.      Roadsides in natural protected areas often act as a source of exotic plants, but the mechanisms that promote this pattern of invasion are not well understood. We demonstrated that nests of the leaf-cutting ant, Acromyrmex lobicornis, near roadsides promote the abundance and fitness of two exotic plant species, Carduus nutans and Onopordum acanthium, in a national park of northern Patagonia, Argentina; and documented the associated mechanism. 2.      Field sampling involved the measurement of 40,000 seeds, 10,000 inflorescences, 300 plants and 50 ant nests. Exotic plants in refuse dumps of ant nests were 50-600 % more abundant; seedlings showed 100-1000% more foliar area and root and leaf biomass, and adult plants showed 100-300% more seeds than non-nest soil plants. Refuse dumps showed higher nutrient content than non-nest soils; measures of foliar nutrient content and isotopic signature of 15N strongly suggest that plants can reach and use these nutrients. These results support the hypotheses of resource availability as a mechanism of invasions. 3.      Synthesis and applications. Refuse dumps of leaf-cutting ants concentrate and supply limiting nutrients that favour exotic plants. Our extrapolations suggest that exotics produce 3,311,000 more seeds /ha in areas with ant nests compared to areas without. We propose several management strategies to prevent this effect: reduce roadside areas, minimize soil-surface disturbances adjacent to roads, and control the introduction of alien seeds into leaf-cutting ant nests away from roads.  We suggest focusing removal efforts in ant nest sites, since exotic plants are denser and represent a higher source of propagules in refuse dumps.