Ant taxonomic and functional diversity show differential response to plantation age in two contrasting biomes

Abstract

The increasing global demand for lumber and pulp has led to the conversion of natural habitats into monocultures of fast-growing tree plantations. The environmental filtering model proposes that both environmental characteristics of anthropogenic habitats and biotic interactions act as a filter that can be passed through by some species of the regional pool, driving the formation of local assemblages. Therefore, environmental filtering promotes the selective loss of species and convergence of functional traits, resulting in assemblages of species that are functionally more similar than expected by chance. In Argentina, pine monocultures have expanded in both subtropical forest and grassland biomes. Typically, environmental similarity between plantations and natural habitat decreases in the grassland and increases in the subtropical forest with increasing plantation age (time since pine stands were planted). Then, we predict that changes in biological diversity with plantation age will be opposite when plantations develop in environmentally contrasting biomes. To test the prediction, we studied taxonomic (species richness) and functional (based in morphological functional traits) diversity of epigeal ant assemblages in pine plantations of different ages (from 1 to 12 years old), developing in contrasting biomes that determined different contexts of the main natural (i.e., native) habitat: subtropical forest and grassland. Temperature, humidity and vegetation cover were recorded, and ants were collected using pitfall traps. For plantations and natural habitats of both biomes, we estimated functional diversity in sets of randomized communities and compared them with the observed functional diversity throughout plantation age. As expected, results showed opposite environmental similarity gradients between natural habitats and plantation ages. In the subtropical forest, ant species richness remained similar but functional diversity increased with increasing plantation age, associated with the presence of predatory ants. In the grassland, species richness showed a maximum at intermediate ages, but functional diversity remained similar with increasing plantation age. Null model analyses showed lower functional diversity than expected by chance in young plantations developing in the subtropical forest and at all ages in grassland, indicating convergence of functional traits. Our findings support environmental filtering as the primary mechanism driving the formation of ant assemblages along pine plantation cycle. To our best knowledge, this is the first study analysing patterns of diversity along tree plantation cycles developing in contrasting biomes. Our results show that the biome effect should be taken into consideration to predict diversity responses to commercial forestry and to unify a theory of assemblage formation in monoculture plantations.