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.M. Spitzer et al. J. Exp. Mar. Biol. Ecol. 244 2000 67 –86
1. Introduction
Variation in complexity of habitat may have profound effects on biological interac- tions, 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, 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, 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
Fig. 1. Graphical comparison of the relationship between net-benefit growth and plant surface area per unit area of bottom for both an open and a closed system as suggested by Heck and Crowder 1991.
P .M. Spitzer et al. J. Exp. Mar. Biol. Ecol. 244 2000 67 –86
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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, 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.
2. Materials and methods
