Ant-plant interactions: Migration in herbivore fauna and conditional outcomes related to plant sequential flowering

Background/Question/Methods Plant-animal interactions may vary in their results, from mutualistic to antagonistic, depending on spatial and temporal variations. Interactions between interactions can have strong influence in these associations, considering that the composition of associated species change spatially and temporally. We tested the outcomes of interactions in a system involving four species of extrafloral nectary bearing Malpighiaceae, their herbivores and associated ants in the Brazilian Tropical Cerrado savanna. Plant species have sequential flowering and we hypothesized that: a- different plant species will share a similar guild of associated herbivores and ants; b- herbivores will move on between plants depending on species flowering period; c- the outcomes of ant-plant-herbivore associations will change seasonally depending on species composition and due the action of particular species. Field work was conducted between June 2008 and 2009 in a Cerrado reserve nearby Uberlândia, SE, Brazil. We tagged 30 shrubs of each Malpighiaceae: Peixotoa tomentosa, Banisteriopsis laevifolia, B. campestris and B. malifolia; all shrubs in same phenological state and high. By the flip of a coin we divided plants of each species in two groups, control (without any manipulation) and treatment (ant-excluded). Weekly we sampled all ants and herbivores present on plants. Phenology and foliar herbivory were sampled monthly and reproductive success (buds, flowers and fruit production) during the flowering season.

Results/Conclusions Our three main hypotheses were confirmed. There was great similarity in the associated herbivore fauna (almost 50%) between plant species. In dry season, similarity was almost 70% between P. tomentosa and B. laevifolia, and in the wet season 65% between B. campestris and B. malifolia. Ants had a significant impact in the reduction of foliar herbivory in all species: P. tomentosa (F= 616,8; p<0,001); B. laevifolia (F=604,8, p<0,001); B. campestris (F= 603,6; P<0,001) e B. malifolia (F=544, p<0,001), but had no significant impact on plant reproductive success: P. tomentosa (U=49, p=0,67); B. laevifolia (U=97, p=0,52); B. campestris (U=101, p=0,61) e B. malifolia (U=89,5, p=0,34). The florivory by specific herbivore species was the most important factor against ant effectiveness in differential fruit production. Our study shows that plant phenological variation, indeed, affect the composition of associated herbivores guild in the case of cerrado Malpighiaceae. Sequential flowering between similar species is probably crucial to the maintenance of high diversity of herbivore fauna observed in this savanna. Variation in species composition in these relationships directly affect the outcomes of these interactions in the Tropical savanna.