Kenneth L. Heck

SEAGRASS HABITATS: THE ROLES OF HABITAT COMPLEXITY, COMPETITION AND PREDATION IN STRUCTURING ASSOCIATED FISH AND MOTILE MACROINVERTEBRATE ASSEMBLAGES

Seagrass meadows represent a distinct habitat in shallow coastal and estuarine ecosystems. We examine the role of seagrass meadows as an important habitat for fishes and large mobile invertebrates. In particular, we emphasize the importance of the structural complexity of the vegetation and associated algal components. Based on data from a variety of geographical localities we consider how vegetation density, plant morphology and associated sessile colonial animals can influence abundance and diversity of predator and prey species in vegetated areas on both local and regional geographical scales. In so doing we generate hypotheses that lead to predictions concerning: size of populations and the amplitude of their fluctuations in vegetated habitats at different latitudes; success rate of predators using different foraging strategies in vegetation of different densities; and resultant diversity and abundance of invertebrate prey, juvenile fish and adult fish in different densities of vegetation.

Structural components of eelgrass (Zostera marina) meadows in the lower Chesapeake Bay—Fishes

The structure of the fish community associated with eelgrass beds in the lower Chesapeake Bay was studied over a 14 month period. A total of 24,182 individuals in 48 species was collected by otter trawl with Leiostomus xanthurus (spot) comprising 63% of the collection, Syngnathus fuscus (northern pipefish) 14%, Anchoa mitchilli (bay anchovy) 9%, and Bairdiella chrysoura (silver perch) 5%. The density and diversity of fishes were higher in vegetated areas compared to unvegetated areas; fishes were more abundant in night collections Fish abundance and species number increased in the spring and early summer as both water temperature and eelgrass biomass increased and decreased in the fall and winter as temperature and eelgrass biomass decreased. Gill netting revealed some of the top predators in the system, especially the sandbar shark, Carcharhinus milberti. The fish community in the Chesapeake Bay was quite different from North Carolina eelgrass fish communities. Most notable was the rarity of the pinfish, Lagodon rhomboides, which may be a very important predator in the structuring of the epifaunal communities.

The nursery role of seagrass meadows in the upper and lower reaches of the Chesapeake Bay

A two-year trawling and gill-netting study of vegetated and unvegetated bottoms near Parson’s Island, Maryland and near the mouth of the York River, Virginia was carried out to assess the nursery function of submerged vegetation for populations of fishes and decapod crustaceans in the Chesapeake Bay. Results revealed that vegetated bottoms supported substantially larger numbers of decapods, but not fishes, than unvegetated substrates. The lower Bay grassbed was an important nursery area for juvenile blue crabs, although neither of the grassbeds functioned as a nursery for commercially or recreationally valuable fishes. Our results suggest that: (1) further decreases in lower Bay Seagrass biomass would result in reduced numbers of adult blue crabs, but should not substantially affect populations of valuable fish species; (2) additional decreases in Upper Bay submerged vegetation should not produce dramatic change in the population sizes of either adult blue crabs or fishes.

Experiments on Competition and Predation Among Shrimps of Seagrass Meadows

A series of experiments was conducted to evaluate the distribution and abundance patterns of two species of macrophyte—inhabiting caridean shrimps found in Apalachee Bay, Florida, USA. Experiments using Palaemonetes vulgaris and Palaemon floridanus included microhabitat association and behavioral interactions. Predator—prey relationships, as they apply to habitat complexity and interspecifc interactions, were also examined using the pinfish, Lagodon rhomboides, a natural predator. Pt. vulgaris individuals were excluded from the experimental habitats by P. floridanus over a range of densities. We conclude that their nonoverlapping microgeographical distribution is primarily a consequence of interspecific competition rather than just microhabitat selection. Predation success on single species was inversely related to the physical complexity of the experimental habitat. However, when placed together, a significantly greater proportion of Pt. vulgaris is consumed than P. floridanus. We posit that the displacement of Pt. vulgaris from cover by P. floridanus significantly increases the formers risk of detection and capture. The importance of refuges in regard to: (1) seagrass predator strategies; (2) prey tactics, and (3) species abundance and richness patterns in macrophyte—dominated communities are discussed.

Biogeography of Corals, Seagrasses, and Mangroves: an Alternative to the Center of Origin Concept

McCoy, E. D., and K. L. Heck, Jr. (Department of Biological Science, Florida State University, Tallahassee, Florida 32306) 1976. Biogeography of corals, seagrasses, and mangroves: an alternative to the center of origin concept. Syst. Zool. 25:201-210.-A center of origin explanation for the distribution and diversity patterns of the organisms comprising the principal shillow-water habitats in the tropics-hermatypic corals, mangroves, and seagrasses-is highly unlikely, based on fossil data and the dispersal capabilities of the organisms. Instead, these biogeographic patterns are better explained by the existence of a previously widely-distributed biota which has since been modified by tectonic events, speciation, and extinction, in accordance with modern geological and biogeographical theory. [Corals; seagrasses; mangroves; biogeography; vicariance.] The center of origin concept-that a taxons place of origin is the region in which it is most diverse-is nearly axiomatic (Rotramel, 1973). Briggs (1961, 1966, 1967), Darlington (1957), and Mayr (1965) have all been active proponents of this assumption, although Darlington has warned against its application when a thorough fossil record does not exist to verify it. Other biogeographers (e.g. Cain, 1944, Croizat, 1958; Croizat et al., 1974) also have strongly questioned the uncritical application of the center of origin concept. Despite these warnings and criticisms the concept has a facile appeal, and remains viable (for a recent example, see Muller, 1973). The center of origin concept was originally conceived on the assumption that the positions of the continents remained constant through time. This implied that closely-related organisms inhabiting widelyseparated land masses or bodies of water must have somehow dispersed over long distances. Of course, it can no longer be accepted that the continents have remained stationary, and as a result, it is necessary to critically reevaluate the validity of earlier conclusions involving the center of origin concept. In place of the notion that 1 Order of authorship was determined by the toss of a coin. centers of origin can be identified by centers of diversity, we advocate acceptance of the following ideas which take into account current biogeographical, ecological, and geological reasoning: (1) Longterm climatic and tectonic events cause environmental changes with important biotic consequences; hence, present centers of diversity may not have always been so (cf. Carson, 1970); and (2) localized ecological processes, such as extinction and species-area relationships, are significant determinants of species diversity independently of where the taxon originated; thus, present distributions and diversity patterns represent more than long-term, monotonous radiation of species from some point of origin. An important ancillary point is that the evidence for long-distance dispersal of widely-distributed organisms is often tenuous, though it is rarely presented as such. To illustrate how these newer ideas can be applied in explaining biographical patterns, we use three important groups of marine organisms and compare results obtained from them with those generated by the center of origin concept. We have chosen hermatypic corals, seagrasses, and mangroves for a number of reasons: (1) They are the principal shallow-water sessile assemblages throughout the worlds tropics; (2) they presently co-occur throughout the tropics, and their diversity patterns and

Structural components of eelgrass (Zostera marina) meadows in the lower Chesapeake Bay—Decapod crustacea

Otter trawl collections of eelgrass habitats in the lower Chesapeake Bay during 1976–1977 produced 14 species of decapod crustaceans. These collections were dominated by palaemonid shrimp (Palaemonetes spp.), blue crabs (Callinectes sapidus), and sand shrimp (Crangon septemspinosa), each of which exhibited unimodal seasonal abundance curves with large summer peaks. Decapod abundance was positively correlated with plant biomass throughout the year. Decapod densities on vegetated bottoms were greater than on unvegetated bottoms, and nighttime abundance on each bottom type was greater than corresponding daytime abundance. Total decapod abundances in Chesapeake Bay eelgrass meadows appear to be much greater than those reported in North Carolina eelgrass or Gulf of Mexico turtlegrass habitats.

Long-Distance Dispersal and the Reef-Building Corals of the Eastern Pacific

It is currently widely accepted that the hermatypic coral fauna in the Eastern Pacific Ocean underwent massive extinction during the mid-Tertiary, with subsequent transoceanic colonizatiion by planulae from the Indo-West Pacific region during periods of favorable conditions. We suggest that the available evidence does not strongly support this biogeographic hypothesis; moreover, we contend that it is untestable in its present form. In its place we propose an alternative hypothesis based upon modification of a previously widespread, pan-Tethyan coral biota which has since been modified by tectonic events, speciations, and extinctions.

Some observations on the use of taxonomic similarity in large-scale biogeography

Measurement of taxonomic similarity is common in large-scale biogeography. Of the many indices of similarity, those with a probabilistic basis appear to be superior to others in their ability to recognize high and low levels of relationship. Probabilistic indices often are difficult to apply, however, because they require knowledge of the complete species pool from which taxa came to inhabit the various locations and careful formulation of the hypotheses which are to be tested. It cannot be expected that the results of any similarity analysis, no matter how well conceived, lead directly to a unique biogeographic