Plant protection services mediated by extrafloral nectaries decline with aridity but are not influenced by chronic anthropogenic disturbance in Brazilian Caatinga

Most terrestrial species occur in human-modified landscapes that are experiencing climate change. In addition to direct impacts on species, both anthropogenic disturbance and climate change can have important effects through changes in species interactions, including the disruption of ecological services provided by them. Here we investigate how chronic anthropogenic disturbance (CAD) and aridity affect the effectiveness of plant protection services provided by ants to plants bearing extrafloral nectaries (EFNs). The study was conducted across 13 01-ha plots distributed along CAD and aridity gradients in Caatinga vegetation of northeastern Brazil. We focused on Pityrocarpa moniliformis, the most abundant and widely distributed EFN-bearing tree species occurring in our study area, and we used experimental attack rates on termites as a measure of effectiveness of ant protection services. We investigated the relative roles of nectar production (volume and concentration) and ant species composition in mediating the effects of CAD and aridity on the effectiveness of protection services. Attack rates by ants declined with increasing aridity but were not related to CAD. The volume of extrafloral nectar declined with increasing CAD but was not affected by aridity, whereas the concentration was not related to either CAD or aridity. The composition of attendant ant species varied with aridity but not with CAD. Synthesis. Our findings suggest that CAD does not affect plant-protection services mediated by EFNs in Brazilian Caatinga. However, ant-protection services declined with increased aridity, and this occurred through changes in the composition of attendant ant species rather than by changes in the production of extrafloral nectar. Such a response to increasing aridity highlights the vulnerability of EFN-bearing plants to climate change through decreased predation of herbivores.