Fish modulate how connectivity and local factors shape food webs of permanent ponds

The propagation of metacommunity processes through local trophic interactions has seldom been explored, particularly in empirical studies. This study examines how local environmental conditions and metacommunity connectivity influence community assembly and food web structure in pondscapes. We survey taxonomic richness, abundance, and isotopic niches across multiple assemblage, including zooplankton, macroinvertebrates, tadpoles, turtles, and fish. A gradient in the composition of the pond community representing the dominance of fish or other taxa and another gradient associated with macroinvertebrate diversity were evidenced. Pond size and connectivity promoted fish richness and abundance, representing the main path through which metacommunity processes cascade down within communities, determining the assembly of the whole system. The dominance of fish was associated with large food chains, a reduction in the width of the food web width, and the trophic spacing. Local conditions such as macrophyte coverage and pond size modulate the strength of these fish effects, determining the representation of macroinvertebrates and amphibians. Consequently, the effects of pond connectivity on community assembly may result as a consequence of isolation's influence on top predators' performance, rather than by the direct effect of dispersal on different species, as is usually assumed. Metacommunity theory has highlighted the direct effect of dispersal on species performance and diversity. Our study highlights the indirect influence of landscape mediated by keystone species, which connect metacommunity dispersal with local community processes more than previously thought.

Methods
The data were collected in two pondscapes located in the grassland landscape of Uruguay, where sampling was conducted in permanent ponds to assess taxonomic richness, abundance, and isotopic niches of different assemblages, including zooplankton, macroinvertebrates, tadpoles, turtles, and fish. The sampling was designed to explore how local conditions and metacommunity connectivity influence community structure and food web dynamics, as described in the forthcoming article titled 'Fish modulate how connectivity and local factors shape food webs of permanent ponds' (to be published in Oikos). Following data collection, the data were processed using statistical analyses in R, including data cleaning and correction for sampling effort, as well as analyses to evaluate trophic interactions and the effects of connectivity and local conditions on community composition and food web structure.