Seed-predation and dispersal by ants determine plant-species dominance and diversity in a semi-arid grassland

Question

Granivorous ants may affect plants negatively by diminishing their numbers, or positively by dispersing seeds, reducing aggregation and thus competition. We assessed whether, and how, ants affect plant populations and community diversity.

Location

Semi-arid grassland in southern Mexico.

Methods

Over 5 years, using ant exclosures, we compared plant diversity in plots where ants were present or not using Hill numbers. Furthermore, we determined whether the effects of ants on plant species depend on the relative dominance or rarity of plants. We tested whether ants cause greater reductions in the abundance of the plant species they prefer most. We also determined whether plant species whose aggregation was reduced by ants increased in numbers.

Results

Six plant species increased their numbers when ants were present, and four experienced reductions. Ants reduced richness, although evidence was inconclusive; however, they clearly increased dominant species diversity (²D). Although the overall effect that ants exerted on plant species did not depend on their dominance or rarity, four of the five most abundant species were favored by ants. Ants’ preferences did not determine their effect on species’ abundance. The species that benefited from ants were those whose spatial aggregation increased when exposed to ants.

Conclusions

Ants do not maintain richness, but instead may reduce it by favoring a few dominant species. The lack of a 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 increase aggregation by enhancing seedling performance near their nests, where conditions may be improved, and by dispersing seeds into these areas. This is in line with our finding that common species are favored by ants, because, at our study site, they are more likely to be dispersed.