Hydrological disturbance diminishes predator control in wetlands

Abstract

Effects of predators on prey populations can be especially strong in aquatic ecosystems, but disturbances may mediate the strength of predator limitation and even allow outbreaks of some prey populations. In a two-year study we investigated the numerical responses of crayfish (Procambarus fallax) and small fishes (Poeciliidae and Fundulidae) to a brief hydrological disturbance in replicated freshwater wetlands with an experimental drying and large predatory fish reduction. The experiment and an in situ predation assay tested the component of the consumer stress model positing that disturbances release prey from predator limitation. In the disturbed wetlands, abundances of large predatory fish were seasonally reduced, similar to dynamics in the Everglades (southern Florida). Densities of small fish were unaffected by the disturbance, but crayfish densities, which were similar across all wetlands before drying, increased almost threefold in the year after the disturbance. Upon re-flooding, juvenile crayfish survival was inversely related to the abundance of large fish across wetlands, but we found no evidence for enhanced algal food quality. At a larger landscape scale (500 km2 of the Everglades), crayfish densities over eight years were positively correlated with the severity of local dry disturbances (up to 99 days dry) during the preceding dry season. In contrast, densities of small-bodied fishes in the same wetlands were seasonally depressed by dry disturbances. The results from our experimental wetland drought and the observations of crayfish densities in the Everglades represent a large-scale example of prey population release following a hydrological disturbance in a freshwater ecosystem. The conditions producing crayfish pulses in the Everglades appear consistent with the mechanics of the consumer stress model, and we suggest crayfish pulses may influence the number of nesting wading birds in the Everglades.

Transient recovery dynamics of a predator–prey system under press and pulse disturbances

“…by hunting [31]. For example, it was previously observed that the prey population release following the hydrological disturbance in a freshwater ecosystem was due to the reduced abundance of large sized predators [16]. A similar pattern has been also observed in an island ecosystem following a hurricane which reduced the abundance of top predators and caused herbivore outbreak [15].…”

“…2b, d). This is a clear sign of prey release [16, 30]. Effectively, disturbance had uncoupled the two interacting species, such that the prey population was no longer relevantly affected by its predator.…”

“…Traits such as large body size, slow growth rate and low population size make top predators more vulnerable than other trophic levels. Studies have been shown that a reduced top-down control allowed prey outbreaks with cascading changes in ecosystem structure and function [14–16]. Similarly, in a microcosm study, increasing temperature led to increasing invasion success of a bacterial prey species due to the increased prey release from protozoan predation stress [17].…”

Tradeoffs between fine‐scale site measurements and coarse sensory data for long‐term monitoring of pulsed wetlands

“…Since crayfish can burrow into the substrate, the crayfish population is not only better equipped than fish to survive periods of drought, but already spread across the landscape once water levels increase (Dorn & Trexler, ; Dorn & Volin, ). As a result, crayfish populations can rapidly increase after severe dry‐downs (Dorn & Cook, ), whereas certain fish species are poor dispersers and may take several years to both recolonise and repopulate areas that have completely dried (Ruetz et al ., ; Goss, Loftus & Trexler, ). However, crayfish populations may be more constrained by predation and food availability (Jordan et al ., ; Kellogg & Dorn, ), both factors that were not represented in our models.…”

Drought attenuates the impact of fish on aquatic macroinvertebrate richness and community composition

“…The specific mechanisms through which drought altered the effects of fishes on invertebrate communities could involve several direct and indirect pathways. Although the locations and identities of fish species observed over the sampling period remained consistent (i.e., there was no evidence of local extirpations or colonisations of fish), it is possible that drought‐driven decreases in fish abundance or size distribution could have dampened the intensity of predation risk on sensitive macroinvertebrate taxa (Dorn & Cook, ). Alternatively, evaporation and reduction in pond size are associated with numerous physiochemical changes that can have significant effects on fish behaviour and microhabitat use, such as warmer temperatures, higher salinity and reduced areas of vegetated shoreline (Crowder & Cooper, ; Magoulick & Kobza, ; Webster, Atton, Ward, & Hart, ).…”