Effects of increasing aridity and chronic anthropogenic disturbance on seed dispersal by ants in Brazilian Caatinga

1. Anthropogenic disturbance and climate change are the main drivers of biodiversity loss and ecological services around the globe. There is concern that climate change will exacerbate the impacts of disturbance and thereby promote biotic homogenization, but its consequences for ecological services are unknown. 

2. We investigated the individual and interactive effects of increasing chronic anthropogenic disturbance (CAD) and aridity on seed dispersal services provided by ants in Caatinga vegetation of north-eastern Brazil. 

3. The study was conducted in Catimbau National Park, Pernambuco, Brazil. Within an area of 214 km ² , we established nineteen 50 × 20 m plots that encompassed gradients of both CAD and aridity. We offered diaspores of six plant species, three myrmecochorous diaspores and three fleshy fruits that are secondarily dispersed by ants. We then quantified the number of interactions, seed removal rate and dispersal distances, and noted the identities of interacting ant species. Finally, we used pitfall trap data to quantify the abundances of ant disperser species in each plot. 

4. Our results show that overall composition of ant disperser species varied along the gradients of CAD and aridity, but the composition of high-quality dispersers varied only with aridity. The total number of interactions, rates of removal and mean distance of removal all declined with increasing aridity, but they were not related to CAD. These same patterns were found when considering only high-quality disperser species, driven by the responses of the dominant disperser Dinoponera quadriceps. We found little evidence of interactive effects of CAD and aridity on seed dispersal services by ants. 

5. Our study indicates that CAD and aridity act independently on ant-mediated seed dispersal services in Caatinga, such that the impacts of anthropogenic disturbance are unlikely to change under the forecast climate of increased aridity. However, our findings highlight the vulnerability of seed dispersal services provided by ants in Caatinga under an increasingly arid climate due to low functional redundancy in high-quality disperser species. Given the large number of plant species dependent on ants for seed dispersal, this has important implications for future plant recruitment and, consequently, for the composition of Caatinga plant communities.