Benthic Pond Macroinvertebrates Coexist with Nearby Potentially Predatory Fish

AbstractIn ponds of the northeastern United States, benthic macroinvertebrates can dominate the local biomass. Isopods, predatory leeches, and fingernail clams can attain dense populations and suffer heavy predation pressure by fish. We predicted that pond benthic macroinvertebrates would recognize the proximity of predatory fish and avoid or vacate that area as an inducible behavioral defense. We deployed cages with and without predatory fish (sunfish and golden shiners) in a naturally fishless pond in October and November of 2020 and 2021. After at least 2 days, we collected leaf packs from directly under the cages and compared the number of invertebrates residing within. Surprisingly, the population densities of the dominant taxa (isopods, leeches, and clams) suggested that they did not avoid fish. Leeches and isopods may even reside in higher numbers near live sunfish, perhaps because feces from the fish augment the locally available food and nutrient levels. Our present field results support earlier laboratory findings: benthic macroinvertebrates in ponds may not respond to fish cues. Bottom-up control may dominate in ponds, providing important implications for conservation of these threatened ecosystems.