Keystone communities can rescue aquatic metacommunities influenced by pesticide contamination.

Pesticide contamination in freshwater habitats is a major global issue, affecting water quality, biodiversity, and ecosystem services. Uncontaminated habitats embedded in agricultural landscapes might act as keystone communities, with the ability to restore diversity and ecological processes in contaminated sites through dispersal. Despite their potential relevance, the role of keystone communities in mitigating agrochemical contamination remains untested. We asked if pristine habitats embedded in agricultural landscapes can act as keystone communities and drive the recovery of contaminated habitats. To answer this question, we conducted a mesocosm experiment to simulate zooplankton metacommunity dynamics under three treatments: uncontaminated, fully contaminated, and partially contaminated metacommunities. We examined communities over time following dispersal and pesticide contamination to analyze their trajectories, diversity, and recovery capacity. Analyses were conducted for all species, as well as for Cladocera and Copepoda separately, at both local (individual communities) and regional scales (three communities linked by dispersal-i.e., metacommunities). Taxon-specific population trajectories indicated that cladoceran densities increased across treatments irrespective of contamination, whereas copepods exhibited species-level declines or increases under local pesticide exposure. These taxon-specific population responses to contamination altered community trajectories, resulting in a greater loss of species in completely contaminated metacommunities. Metacommunities with uncontaminated habitats partially recovered from contamination and showed compositional and gamma diversity patterns comparable to uncontaminated metacommunities. Recovery patterns differed across Cladocera and Copepoda, with recovery being more evident at the regional scales. Keystone communities had a greater influence on the recovery of Cladocera community composition and on Copepoda gamma diversity. Our results supported the prediction that keystone communities play a fundamental role in local and regional dynamics of aquatic metacommunities inserted in landscapes with a heterogeneous structure of contamination. Positioning preserved habitats well connected to impacted sites could allow a quick colonization after pesticide contamination, recovering the system until the next crop management cycle. However, taxon-specific trajectories underscore the need to consider functional and dispersal traits when designing mitigation strategies. We thus suggest a metacommunity perspective for better predictions of risks associated with pesticide use in nature and ways of mitigating them.