Impacts of agrochemical intensification and spatial isolation on the assembly and reassembly of temporary pond metacommunities.
Abstract
Since the Green Revolution, the management of agrochemicals has become a cornerstone of agriculture, allowing it to expand over terrestrial ecosystems previously deemed unsuitable for cultivation. Such expansion altered the connectedness of aquatic habitats to organisms with complex life cycles by increasing the number of temporary ponds in interfluves. Here, we investigate how spatial isolation and intensification in the use of agrochemicals can interact to increase or decrease the permeability of agricultural fields to these organisms by affecting community structure in temporary ponds. We carried out a field experiment that simulates realistic scenarios of agrochemical contamination at different levels of spatial isolation. We constructed 45 1200-L artificial ponds in a Savanna landscape in Brazil at three distances from a source wetland (30, 120, and 480 m). The ponds were spontaneously colonized by aquatic insects and amphibians and treated with no agrochemicals (control), fertilizers (as in managed pastures) or fertilizers and a single pulse of the insecticide fipronil and the herbicide 2,4-D (as in sugarcane fields) following realistic dosages and application schedules. Communities treated only with fertilizers generally exhibited a higher abundance of aquatic insects at the peak of the rainy season, suggesting that fertilization caused bottom-up effects. However, because low spatial isolation and fertilization favoured different predatory taxa, their insect prey eventually became less abundant in less isolated fertilized ponds. Communities treated with fertilizers and pesticides diverged strongly from other treatments after the application of the insecticide, when a decrease in insect abundance indirectly benefitted amphibian populations. In this case, spatial isolation delayed the recovery of the abundance of several herbivore and detritivore insects in isolated ponds. Synthesis and applications. These results have direct consequences for the emergence of insects and amphibians, possibly affecting important ecosystem services. Such consequences can be controlled by avoiding agrochemical contamination at the peak of most taxa reproductive season or through changes in spatial isolation by deliberately constructing wetlands in interfluves. In all cases, the effects of agrochemicals and isolation must be addressed jointly for a successful management of biodiversity in agricultural landscapes.