Doctor of Philosophy (PhD)
Top-down (consumer) versus bottom-up (resource) control of food webs has long interested ecologists. Here, I take advantage of a full-factorial design of ecosystem-wide manipulations of nutrient additions (loading rates 10x above background) and the significant reduction (~60%) of a key predator, the killifish Fundulus heteroclitus, in the tidal creeks of the Plum Island Estuary, Massachusetts. Prior to manipulations, annelids numerically constituted 97% of the infaunal community and the largest scale (creeks) accounted for little spatial variability in annelid populations and diversities. Tidal creeks were similar based on diversity indices, abundance, and community patterns, suggesting the tidal creeks are appropriate replicates/experimental units for manipulations. Using data collected before (2003) and after (2004-2006) manipulations began, I observed little evidence of top-down or bottom-up control on infaunal densities, biomass, or community structure in four different habitats along an inundation gradient. Using exclusion cages to remove all predators (primarily killifish and the grass shrimp Palaemonetes pugio) within fish removal treatments (in non-nutrient creeks), I found top-down control of surface feeding polychaetes including Manayunkia aestuarina and Streblospio benedicti. Shrimp body size increased with killifish reduction but not shrimp density, suggesting that shrimp may alter their behavior and exert stronger top-down control on infauna when killifish are removed. No corresponding decrease in benthic microalgae (BMA) occurred when infauna abundance increased, suggesting a weak infauna-BMA interaction. For epifauna on the marsh platform, I found that hydrobiid snails increased in the creek bank Spartina alterniflora with fish removal and treatments interacted antagonistically on the amphipod, Uhlorchestia spartinophila. The interaction likely resulted from the parasite-induced movement of U. spartinophila to the creek wall habitat. This movement, in turn, made the amphipod more susceptible to predation by the semipalmated sandpiper, Calidris pusilla. Top-down and bottom-up control has been thought to operate independently on saltmarsh invertebrates. I demonstrate that food-web phenomena such as trophic omnivory, behavioral modification and indirect effects increase complexity and preclude simple predictions of trophic control on benthic invertebrates. If these trends are widespread, then long-term, large spatial-scale studies may be required to more completely understand the relative importance of top-down and bottom-up control on benthic invertebrates.
Document Availability at the Time of Submission
Student has submitted appropriate documentation to restrict access to LSU for 365 days after which the document will be released for worldwide access.
Johnson, David Samuel, "Trophic control of saltmarsh invertebrates" (2008). LSU Doctoral Dissertations. 2977.