Predation as an Agent of Population Fragmentation in a Tropical Watershed

Abstract

Stream fish sometimes show mutimodal distributions, with high densities in the tributaries of a river but rarity or absence in the river itself. To assess if predation can produce such a fragmented distributional pattern on a large geographic scale, we determined the density and habitat use of a prey fish in two tropical stream watersheds, each with a barrier waterfall that split the drainage into a region with a strongly piscivorous fish and a region lacking a strong lacking a strong piscivore. In contrast to sites with the strong piscivore, the prey fish in areas above barriers showed a dramatic expansion into the main river, thereby spatially consolidating an otherwise fragmented distribution. Manipulation of piscivores in a third—order stream flanked by a series of first—order experimental streams also showed that the prey distribution expanded into the third—order stream when piscivores were excluded, and that the presence of the piscivores in the third—order stream reduced prey densities both by killing the prey and by inducing the prey to ascend cascades to enter the tributaries. Because the predator created spatial fragmentation of the prey population, we examined prey—fish dispersal in the experimental stream facility to ask whether the predator could reduce prey movement into and beyond the predator—occupied sites. As hypothesized, the experiment revealed that predators could block prey dispersal by killing prey. However, the experiment also suggested that predators may increase prey movement between tributaries by inducing shifts out of river sites. The results suggest that realistic models of dispersal by prey would need to account for effects of both predator consumption and prey behavioral responses to the predator.

Constraints on food chain length arising from regional metacommunity dynamics

“…The opposite pattern, albeit less documented, is also possible: there may be a stabilizing effect of predators on population dynamics [19] or 'hydra effects' [31], whereby predation increases average prey population density, potentially decreasing the extinction rate and/or increasing the colonization rate. Non-consumptive effects can also have contrasting impacts: colonization and extinction rates may increase owing to emigration from high-risk patches [32], but colonization may also decrease because of increased vigilance and/or decreased activity levels [33].…”

Factors Affecting the Distribution of Atyid Shrimps in Two Tropical Insular Rivers

“…She found that the effect of fish predation on benthic macroinvertebrates was dependent on season and habitat complexity. However, Fraser et al (1995) performed both inclusion and exclusion experiments in streams on the tropical island of Trinidad and found that in areas split by barrier waterfalls, predators can produce disjointed prey distributional patterns, by both consumption and by causing prey to ascend cascades. In addition, Reznick et al (1997) moved guppies from pools with many predators below barrier waterfalls into pools with only one predator above waterfalls in Trinidadian rivers.…”

Predator Effects in Predator-Free Space: the Remote Effects of Predators on Prey

“…thereby reducing the number of individuals available to disperse; 2) non-consumptive effects, whereby predators increase or decrease rates of prey flux by influencing prey movement (i.e., changing the likelihood that an individual will disperse or the mean phenotype of dispersers). For example, the dynamics of fish populations (prey) in predator-free tributaries are a function of predators in connecting streams not only because consumption by predatory fish reduces dispersal of smaller fish (Fraser et al 1995), but also because surviving prey are more likely to disperse from areas that contain predators (Fraser et al 1999, Gilliam and. Non-consumptive remote effects also arise when mobile prey choose to avoid otherwise suitable habitats that contain predators (e.g., Kats and Sih 1992, Resetarits 2001, Resetarits 2005, Resetarits and Binckley 2009).…”