How the anadromous fish alewife affect the whole food-web in it’s ecosystem is studied by Jerome J. Weis and David M. Post in their new Early View paper “Intraspecific variation in a predator drives cascading variation in primary producer community composition”. Below is Jerome’s presentation of the paper:
We know that predation can have a strong influence on the richness, biomass, and composition of prey communities. We also know that these effects can cascade down a food web to lower trophic levels, including primary producers. Often, when we design studies to ask how these top-down effects of predators influence lower trophic levels, we focus either on differing densities of a single predator species, or on differences among multiple species. However, in some cases, variation within a predator species can be an important component of these top-down effects.
In coastal New England lakes, a species of zooplanktivorous fish, alewife, has a strong influence on zooplankton biomass, diversity, and composition. Among lakes that support alewife, populations show one of two distinct life histories, an anadromous life history, where ocean residing adults spawn in coastal lakes and young-of-the-year alewife have a substantial impact on zooplankton communities from approximately June until November, and a landlocked life history, where alewife populations are isolated from the ocean and are present through the year. Differences in the behavior, morphology, and seasonal timing between anadromous and landlocked alewife drive distinct differences in their zooplankton prey communities across the landscape.
In this study we tested the hypothesis that alewife presence and life history will have cascading top-down impacts on phytoplankton density and community composition. We analyzed phytoplankton communities from a mesocosm experiment that manipulated the presence and life history form of alewife and observed lower zooplankton biomass density, average size, and species richness in the anadromous treatment than the landlocked and no-fish treatments.
We observed a statistically significant shift in phytoplankton community composition among treatments that was consistent with lower zooplankton densities in the anadromous alewife treatment. The biovolume density of two common single-celled phytoplankton genera, Chlamydomonas and Gymnodinium, was significantly higher in the anadromous treatment than the landlocked and no-fish treatments. Both of these genera are considered vulnerable to herbivory by zooplankton. However, these differences in community composition did not result in statistically significant differences in overall phytoplankton biovolume density nor richness among treatments, suggesting that the cascading effect of alewife was relatively small in this study.