Differential survival of two minnow species under experimental sunfish predation: implications for re‐invasion of a species into its native range

Abstract

Summary Predators have long been recognised as important drivers of community structure, and there is a growing literature that suggests predation can affect invasion success by limiting survival or recruitment of potential invaders. Effects of apex predators are well known, but there is increasing evidence that mesopredators may also affect invasion success. In southern Oklahoma (USA), red shiners (Cyprinella lutrensis, which are considered highly invasive outside their native range) disappeared from several streams in which they had historically been the most abundant species. Subsequent to a major flood, red shiners reappeared repeatedly in one stream (Brier Creek) but have not become re‐established, and several lines of evidence suggest that native predators, including sunfish mesopredators, may be inhibiting the re‐invasion of their native range. Over this same time course, another midwater minnow, the bigeye shiner (Notropis boops), has remained common in Brier Creek. In a mesocosm experiment conducted in 2009, red shiners and bigeye shiners exhibited differential survival, and the results of that experiment suggested differential predation by sunfish as the underlying cause. To test specifically for differential predation, we conducted another mesocosm experiment in 2010 in which we exposed equal numbers (n = 15 per mesocosm) of red shiners and bigeye shiners to two sunfish species: green sunfish (Lepomis cyanellus; n = 2 per mesocosm) and longear sunfish (L. megalotis; n = 1 per mesocosm). There was no differential survival of the two minnow species in units without predators in 2010, but significantly more bigeye shiners than red shiners survived in predation units. Survival rates in mesocosms with predators ranged from 80 to 100% (mean of 93.3%) for bigeye shiners and from 33.3 to 93.3% (mean of 74.1%) for red shiners. Feeding trials in aquaria showed that both predator species could consume both prey species, suggesting that the differential survival was not the result of inability of the predators to eat either prey species. Although red shiners historically occurred in natural streams along with sunfish, they have failed to become re‐established following their abrupt decline, despite their occasional occurrence in these creeks in recent years. Increased abundance of mesopredators in conjunction with differential survival of red and bigeye shiners in the presence of sunfish predators may underlie the seemingly paradoxical situation in which red shiners fail to re‐invade their native habitat, but bigeye shiners persist in the same system.