Community reorganization stabilizes freshwater ecosystems in intensively managed agricultural fields.
Abstract
Sustainable intensification may depend on associating precision farming with the harnessing of ecological principles in crop fields, and integrating farms and non-farmed land in productive landscapes. Small wetlands could play an important role in both pursuits for having high per-unit-area rates of element cycling and species richness while deeply penetrating crop fields. However, their potential for ecosystem service provisioning is unlikely to be met if land management intensification promotes ecosystem destabilization in biomass production. We tested the consequences of land use intensification on various dimensions of freshwater ecosystem stability by means of a large-scale field experiment converting extensive pastures to intensive pastures and sugarcane plantations in Southeastern Brazil. Nested within experimental plots were 4000 L mesocosms simulating ponds and puddles commonly found in productive landscapes. Mesocosms were monitored for basic physico-chemical parameters, nutrients, pesticides, phytoplankton standing crop, and the spontaneously colonizing biodiversity. Despite severe environmental change, the stability of sugarcane communities was no different from that of extensive and intensive pastures. This occurred because the local extinction of a sensitive top dragonfly predator following the application of vinasse and insecticide was compensated by colonization of a suite of more tolerant invertebrate mesopredators such as beetles and bugs. Community stability tended to increase with biomass asynchrony and species richness, evidencing a portfolio effect of biodiversity. Unfortunately, the species richness necessary to stabilize biomass production is unlikely to be available in many sugarcane fields and several other row crops. Synthesis and applications. Ponds and puddles could be effective centers of irradiation of ecosystem service provisioning in agricultural fields in terms of pest control; nutrient accumulation, cycling and export back to fields; and habitat and stepping stones for freshwater biodiversity. However, the impoverished biodiversity that results from a combination of harsh local conditions and spatial isolation renders pond communities inherently unstable. Given the unlikely, immediate reduction in agrochemical use in much of the intensively managed crop area, a combination of large, protected source wetlands at the margin of fields and small constructed or naturally forming ponds and puddles in plantations could contribute to sustainable intensification.