The effects of vegetation density on the relative growth rates of juvenile pinfish, Lagodon rhomboides (Linneaus), in Big Lagoon, Florida

Abstract

The efficiency of visual predators may often be affected by complexity of habitat. For example, an increase in complexity of habitat may lead to an increase in search and/or pursuit times, while decreasing habitat complexity may reduce search and/or pursuit times and result in elevated feeding success. If true, it might be expected that predator rates of growth would be greater in small complexity habitats and decline with increasing complexity of habitat. In shallow turtlegrass (Thalassia testudinum) meadows, we used field enclosures to evaluate the effect of increasing turtlegrass density (small=0–133 shoots/m², intermediate=177–267 shoots/m², and large=>267 shoots/m²) on the growth rates of juvenile pinfish, Lagodon rhomboides (Linneaus). Experiments were carried out five times during the growing season, with individual experiments lasting from 21 to 28 days. Benthic core samples were taken to examine differences among treatments and possible caging effects on plant surface area, faunal abundance and epiphyte coverage. We found a consistent pattern of decreasing growth with increasing density of vegetation, although due to low power, ANOVA showed no significant differences among treatments. However, the probability of obtaining the same treatment rankings we observed by chance was very low (P=0.003). The effects of vegetation density on annual growth rates were also examined, and we estimated a 20% decrease in growth rates between small and large vegetation density treatments. These results suggest that vegetation density can have an impact on growth rates that is biologically significant.

Introduction

Variation in complexity of habitat may have profound effects on biological interactions, and the influence that changes in density of vegetation may have on the efficiency of actively foraging predators has been a subject of considerable interest for aquatic ecologists (see reviews by Orth et al., 1984, Heck and Crowder, 1991). The rate at which a visual predator grows is primarily a function of the amount of energy obtained from captured prey minus the cost of obtaining the prey. Increased complexity of habitat leads to longer periods of search and/or pursuit per item of prey, which in turn increases the energetic costs of foraging (Crowder and Cooper, 1979). It follows then, that a visual predator’s growth-rate should be negatively related to complexity of habitat.

In a well-known study, Crowder and Cooper, 1979, Crowder and Cooper, 1982 used three experimental ponds of uniform composition of vegetation and prey, and subsequently cropped the vegetation to create ponds with small, intermediate and large density of plants to examine the effects of vegetation on the growth-rates of Bluegills, Lepomis macrochirus. Crowder and Cooper, 1979, Crowder and Cooper, 1982 concluded that density of macrophytes affected growth of fish significantly, with growth fastest at intermediate density of plants, where invertebrate prey were provided sufficient hiding places to avoid overexploitation by the predator. They hypothesized that at small density of vegetation, predators were overexploiting their prey due to insufficient hiding places, but at large density of vegetation, predators were not able to forage effectively, resulting in reduced growth of predators. From their observations, Crowder and Cooper generalized the relationship between net benefit (intake-cost) and density of vegetation for a closed system in a graph similar to that in Fig. 1.

Independently, Heck and Orth (1980) considered the role that density of vegetation might play in influencing predator effectiveness in an open, marine system. They suggested that predator success would be inversely related to plant surface area, and hypothesized that net benefit (intake-cost) to marine predators would be greatest at small density of seagrass (Fig. 1). When comparing the mechanistic relationships for fresh water (closed) and marine (open) vegetated environments (Fig. 1), the omission of the relatively small growth rates at small density of vegetation in Heck and Orth’s model is due to the fact that they did not hypothesize overexploitation at small densities (Heck and Crowder, 1991). Their rationale for not expecting overexploitation was based on the concept of marine environments as ‘open’ systems that allow for constant immigration of possible items of prey in the form of larvae, juveniles and adults. To date, very little has been done to test Heck and Orth’s hypothesized relationship between complexity of habitat and the efficiency of actively foraging predators in marine environments.

Efficiency of predators may be assessed by many factors (e.g. growth, strike rate, rate of ingestion, and selectivity of food). In the work described below, we chose growth-rate as an indicator of predator efficiency primarily because of the ease of comparing our results with those of Crowder and Cooper, 1979, Crowder and Cooper, 1982, but also because it is very difficult to quantify many of the other measures of predator efficiency using field experiments.

The objective of this study was to assess the role of vegetation density on growth rates of juvenile pinfish, Lagodon rhomboides (Linneaus), a common predator in seagrasses, in various densities of seagrass. Specifically, we tested Heck and Orth’s (1980) hypothesis of a decrease in growth-rates with an increase in complexity of habitat in a marine environment. We expected to find faster growth-rates in the smallest complexity of habitat and slower rates where complexity was largest. We also investigated the possibility of overexploitation of prey in the smallest complexity of habitat, as was found by Crowder and Cooper (1982). Using an experimental field enclosure, we explicitly tested the hypothesis that there was no difference in growth rates of pinfish among small, intermediate or large density seagrass beds.