Exploring the role of large predators in marsh food webs: evidence for a behaviorally-mediated trophic cascade

Abstract

The influence of large predators on lower trophic levels in oligotrophic, structurally complex, and frequently disturbed aquatic environments is generally thought to be limited. We looked for effects of large predators in two semi-permanent, spikerush-dominated marshes by excluding large fish (>12 mm body depth) and similarly sized herpetofauna from 1 m 2 cages (exclosures) for 2 weeks. The exclosures allowed for colonization by intermediate (in size and trophic position) consumers, such as small fish, shrimp, and crayfish. Exclosures were compared to control cages that allowed large fish to move freely in and out. At the end of the experiment, intermediate-consumer densities were higher in exclosures than in controls at both sites. Decapod crustaceans, especially the riverine grass shrimp (Palaemonetes paludosus), accounted for the majority of the response. Effects of large fish on shrimp were generally consistent across sites, but per capita effects were sensitive to estimates of predator density. Densities of intermediate consumers in our exclosures were similar to marsh densities, while the open controls had lower densities. This suggests that these animals avoided our experimental controls because they were risky relative to the surrounding environment, while the exclosures were neither avoided nor preferred. Although illuminating about the dynamics of open-cage experiments, this finding does not influence the main results of the study. Small primary consumers (mostly small snails, amphipods, and midges) living on floating periphyton mats and in flocculent detritus (''floc'') were less abundant in the exclosures, indicative of a trophic cascade. Periphyton mat characteristics (i.e., biomass, chlorophyll a, TP) were not clearly or consistently affected by the exclosure, but TP in the floc was lower in exclosures. The collective cascading effects of large predators were consistent at both sites despite differences in drought frequency, stem density, and productivity.

A synthesis of long-term research by the Florida Coastal Everglades LTER Program

“…The Taylor Slough/ENP Panhandle transect (TS/Ph) is anchored at two main canal inflow points, and extends through the oligohaline ecotone and Florida Bay estuary to the same coastal ocean endpoint. This is a smaller, more localized drainage basin (see Trexler & Gaiser, 2006, for a more detailed description of the transects and a site map). Because the freshwater Everglades is a highly oligotrophic, phosphorus (P) limited system (Noe et al, 2001), freshwater inflow to both estuaries is very nutrient-poor.…”

“…Dorn et al (2006) reported on top down effects in SRS freshwater marshes. They found that [experimental] removal of large fish predation increased the numbers of smaller fish and decreased numbers of invertebrate meiofauna within periphyton mats.…”

Influence of Experimental Sheet Flow on Aquatic Foods Webs of the Central Everglades

“…An aliquot of the periphyton was taken from each cage at the inception of the experiment to determine nutrient content, ash-free dry mass (AFDM), FA relative abundance, and algal composition. Flow velocity measurements were taken at the initiation of the experiment and then weekly using a SonTek After 3 weeks, the fish and shrimp were removed from the cages by lifting enclosure bags and allowing the water to drain out (Geddes and Trexler 2003;Dorn et al 2006;Chick et al 2008). Animals were picked out of periphyton, euthanized in MS-222, and immediately placed on ice.…”

“…Individual specimens were pooled to meet minimum dry weight requirements for both nutrient analysis and FAME analysis. At the end of each experimental trial, thirty strips of the artificial vegetation and a subsample of periphyton were collected from each cage and frozen for later processing (Geddes and Trexler 2003;Dorn et al 2006;Chick et al 2008). In the laboratory, biofilm was scraped from each blade of artificial vegetation, the area scraped was recorded, and samples were homogenized.…”

Spatial and Temporal Patterns in the Distribution, Behavior, and Activity of Fishes in Canals of the Everglades

“…In the Everglades, small fishes feed on invertebrates that live in periphyton mats and flocculent benthic detritus (Dorn et al 2006), which are abundant along the canal-marsh edge. This may explain the reverse direction of the horizontal migration.…”