Top predator introduction changes the effects of spatial isolation on freshwater community structure

Abstract

Current conceptual metacommunity models predict that the consequences of local selective pressures on community structure increase with spatial isolation when species favored by local conditions also have higher dispersal rates. This appears to be the case of freshwater insects in the presence of fish. The introduction of predatory fish can produce trophic cascades in freshwater habitats because fish tend to prey upon intermediate predatory taxa, such as predatory insects, indirectly benefiting herbivores and detritivores. Similarly, spatial isolation can limit dispersal and colonization rates of predatory insects more strongly than of herbivores and detritivores, thus generating similar cascading effects. Here we tested the hypothesis that the effect of introduced predatory fish on insect community structure increases with spatial isolation by conducting a field experiment in artificial ponds that manipulated the presence/absence of fish (the redbreast tilapia) at three different distances from a source wetland. Our results showed that fish have direct negative effects on the abundance of predatory insects but probably have variable net effects on the abundance of herbivores and detritivores because the direct negative effects of predation by fish may offset indirect positive ones. Spatial isolation also resulted in indirect positive effects on the abundance of herbivores and detritivores but this effect was stronger in the absence rather than in the presence of fish so that insect communities diverged more strongly between fish and fishless ponds at higher spatial isolation. We argue that an important additional mechanism, ignored in our initial hypothesis, was that as spatial isolation increases fish predation pressure upon herbivores and detritivores increases due to the relative scarcity of predatory insects, thus dampening the positive effect that spatial isolation confers to lower trophic levels. Our results highlight the importance of considering interspecific variation in dispersal and multiple trophic levels to better understand the processes generating community and metacommunity patterns.

Community reorganization stabilizes freshwater ecosystems in intensively managed agricultural fields

“…and 480 m from a source wetland (Pelinson et al, 2021). We found that a 120-m isolation distance was sufficient to impose a significant dispersal limitation on pond communities, preventing colonization by several insect taxa.…”

“…As a result, the hostile abiotic condition experienced in the waterbody is also experienced in many, if not all neighbouring waterbodies, creating strong spatial effects tied to land use that manifest as a limited pool of species capable of colonization. That strong spatial effects in addition to local unfavourable conditions are linked to these biodiversity declines is suggested by another field experiment in which we followed freshwater community assembly in mesocosms constructed at 30, 120 and 480 m from a source wetland (Pelinson et al, 2021). We found that a 120‐m isolation distance was sufficient to impose a significant dispersal limitation on pond communities, preventing colonization by several insect taxa.…”

Impacts of agrochemical intensification and spatial isolation on the assembly and reassembly of temporary pond metacommunities

“…Furthermore, wide interfluves (i.e. >500 m) generally benefit all herbivore and detritivore taxa (Pelinson et al, 2021), being more effective stepping stones for them. However, such an effect can be attenuated or even interrupted for some non-predatory taxa if ponds are contaminated by insecticides as these taxa can take longer to recover from insecticide pulses in isolated communities.…”

“…dragonflies and aquatic beetles), they have smaller populations and longer generation times than many herbivores and detritivores (i.e. midges and mosquitoes); thus, predators have fewer events of dispersal, which result in lower abundances in isolated habitats when compared to more connected ones (Chase & Shulman, 2009; Pelinson et al, 2021). Additionally, because pesticides are usually applied in seasonal pulses, their direct acute effects, albeit strong, can be temporary—provided that connectedness allows full or partial recovery of the original community structure (i.e.…”

“…Each isolation-by-agrochemical treatment combination was replicated five times. Selected isolation distances effectively limit the abundances of most taxa colonizing artificial ponds, and well-drained soils prevented the formation of other ponds at our study site, which could have confounded our manipulation of isolation distances(Pelinson et al, 2021).Agrochemical treatments followed a 'crop cycle' design (Arts et al, 2006) in which molecules, doses and application schedules F I G U R E 2 (a) One of the naturally colonized mesocosms during the experiment. (b) Experimental set-up comprising three rows of 15 artificial ponds at 30, 120 and 480 m from a source water body (a stream and its floodplain).…”