Professor National Autonomous University of Mexico, Mexico
Seed predation by ants may affect plants negatively by diminishing their numbers, but positive effects on plants by dispersing their seeds. We assessed whether –and how– ants affect plants and diversity in a semiarid grassland in Mexico. We expected species preferred by ants to experience reductions in their populations, and that those species whose seeds are dispersed by ants, reducing their spatial aggregation, increase their populations. To do so, we compared, over five years, the plant community in plots where ants were present or excluded. We measured changes in plant diversity using Hill numbers, which take into account the relative abundances of species. To understand these results, we determined if the effects of ants on species depend on their relative dominance or rarity. We tested if ants affect negatively the species they prefer by determining if ants reduced their abundance. We determined whether plant species whose aggregation is reduced by ants are the same ones whose abundance increases in the presence of ants. Out of 32 species, we found that six increased their numbers when ants were present, and four experienced reductions. We found that ants had no effects on species richness and common-species diversity (order 1 diversity), but reduced dominant-species diversity (order 2 diversity). While the overall effect that ants exerted on different plant species did not depend on their dominance or rarity, three of the six species that were favored by ants were some of the most abundant in the grassland. The effect of ants on the abundance of species did not depend on the ants’ preferences. Contrary to what we expected, the species that benefited from ants were the ones whose spatial aggregation increased when exposed to ants. We found that ants do not maintain diversity, but reduce it by favoring a few dominant species in the community. The lack of a simple relationship between ants’ preferences and their effect on plant populations may arise from a mixture of positive and negative effects on the preferred seeds. Ants may be causing seeds to aggregate, either by dispersing them near the nest entrance or by depositing them in nearby refuse piles. This is known to improve seedling performance because soil in the vicinity of the nest is richer. The fact that common seeds are more likely to be dispersed by ants at the study site, presumably aggregating them near nests, may explain why ants favor dominant species.
