Cascading Impacts of Insect Herbivory in Aquatic Ecosystems
Insects consume more plant tissue than vertebrates, making understanding how they impact the plants they feed upon an important goal of ecology. However, few studies of aquatic food webs focus on the cascading impacts of insect herbivory on higher trophic levels in freshwater ecosystems. Herbivory not only mediates the abundance of primary producers, but also controls nutrient fluxes across multiple trophic levels, with significant implications for ecosystem functioning. Herbivory-induced changes in nutrient dynamics can therefore drive direct reductions in host plant abundance, but have indirect effects on diverse species interactions in the aquatic system. These diverse outcomes may be mediated by certain plant traits that influence the magnitude of herbivory impacts observed across different plant-herbivore interactions. Studying how top-down and bottom-up effects of herbivory vary with different macrophyte-herbivore interactions is therefore critical for developing a holistic understanding of freshwater trophic cascades to improve invasive species management.
Using a factorial design, we manipulated herbivory pressure by the waterlily leaf beetle (Galerucella nymphaeae) to investigate herbivory’s direct effects on native (water shield, Brasenia schreberi; and yellow water-lily, Nyphar lutea) and and introduced host plant (water chestnut, Trapa natans). We measured insect and leaf abundance as well as its indirect effects on algal growth and green frog (Lithobates clamitans) tadpole performance. We randomly assigned each mesocosm one of three levels of herbivory pressure (0, 25, 75% defoliation) and included in each treatment one plant individual, a clay tile to assess periphyton performance, and 10 tadpoles. To monitor the outcomes of our different treatments throughout the growing season, we analyzed leaf turnover and percent defoliation, dissolved oxygen and pH, inorganic nitrate and phosphate, and chlorophyll-a densities. We then assessed bottom-up impacts of nutrient loading on amphibian larvae development by evaluating survival rate in each mesocosm and the Gosner stage of each tadpole individual.
The experiment is still ongoing.
Who’s involved: Katie Lee, Wade Simmons, Stacy Endriss
Funders: NYISRI, Cornell hatch