Randall E Cross

  Feeding pits created by rays may directly or indirectly affect the numbers of meiofauna. We examined the recovery rates of meiofauna inside feeding pits in both intertidal and subtidal areas. Cores were taken inside and immediately outside each pit. Numbers of total meiofauna, nematodes, and other meiofauna found within intertidal pits were significantly reduced following pit formation; the numerically dominant nematodes were reduced by 64%. Recovery to background levels occurred by 48 hours. Numbers of meiofauna in subtidal pits were either unaffected by the feeding activity of rays or differences were undetected due to rapid recovery within 24 h. This is the first study in which comparisons have been made concurrently in adjacent subtidal and intertidal areas. [PUBLICATION ABSTRACT]

Benthic meiofauna are important food resources for marine fishes and crustaceans, some of which have important commercial value. Ray feeding activities produce pits that disturb intertidal and subtidal sediments. Our previous research showed that feeding pit formation reduced meiofaunal abundance inside intertidal pits. The purpose of the present study was to evaluate the recovery time of meiofauna as the summer season progressed. In July, meiofauna recovered by 4 tidal cycles (within 48 h) after pit formation. In August, meiofaunal numbers took 6–14 tidal cycles (72–168 h) to recover. The longer recovery time later in the summer may be due to the continuous disturbance of the sediments by ray feeding activity, as 16–19% of the intertidal surface area was disturbed by this activity.

Competition and predation are major biotic interactions that influence the structure of aquatic communities. The relative importance of each type of interaction is usually a function of the relative size of the organisms involved. This study used field experiments to assess potential size-dependent interactions among small juvenile killifish (
Fundulus heteroclitus) and grass shrimp (
Palaemonetes pugio) and larger adult killifish. Density, species combinations, and predation risk were manipulated within field enclosures on an intertidal salt marsh on Sapelo Island, Georgia. Intraspecific and interspecific competitive effects between small juveniles (larval killifish and postlarval grass shrimp) were examined in the absence and presence of predators (larger adult killifish) to determine interactions and modifications of interactions due to predators. In the absence of predators both negative intraspecific and interspecific effects occurred in the form of reduced growth of juvenile grass shrimp. Survival of juvenile shrimp was the same in the presence or absence of juvenile killifish. However, when larger adult killifish were present, the interspecific competitive effect of reduced growth in juvenile grass shrimp was not significantly different from controls. Growth and survival of juvenile fish were unaffected by increased density of conspecific juveniles and the presence of juvenile grass shrimp and adult fish predators. These results provide field evidence for interspecific competitive interactions between juveniles of two of the most abundant species of nekton utilizing salt marsh habitat along the eastern coast of North America. However, when larger adult killifish predators were present these interspecific competitive effects among similar sized juvenile killifish and grass shrimp disappeared.

The presence and significance of interspecific interactions between larval killifish ( Fundulus heteroclitus Linnaeus) and postlarval grass shrimp ( Palaemonetes pugio Holthuis) were studied in laboratory microcosms. Densities and species combinations were manipulated in microcosms containing salt marsh sediments and naturally occurring metazoan food resources. Shrimp exhibited reduced growth in the presence of fish, but fish survival and growth were unaffected by the presence of shrimp, indicating an asymmetrical competitive interaction. Negative effects on shrimp growth were evident when the ratio of fish to shrimp was 2:1, but effects were not apparent at a 1:1 ratio. The results of the laboratory studies provided useful baseline information for the design of subsequent field experiments on size-dependent interactions between larval fish and shrimp in a salt marsh environment.

We tested the hypothesis that species diversity of birds is related to structural diversity of vegetation by using annual controlled burns to reduce the crown volume of the ground vegetation and shrub layer and the basal area of small trees in a pine forest in the Piedmont region of North Carolina. Birds were mist-netted on the treatment and control sites to determine changes in bird species diversity. Diversity of birds on the treatment site declined relative to that on the control site during a 5 year period. This is the first experimental evidence supporting findings of earlier observational studies of the hypothesis.