• Alternative stable equilibria;
  • Belostoma flumineum;
  • Helisoma trivolvis;
  • Physella gyrina;
  • priority effects;
  • productivity;
  • size-structure;
  • trophic cascade


Both local and initial conditions have been implicated in causing variation in the strengths of predator effects among natural communities. In this study, I performed a controlled mesocosm experiment using two common snails, Physella gyrina and Helisoma trivolvis, and their insect predator, Belostoma flumineum, to explicitly quantify and understand the causes of previously observed variation in food web interactions in natural ecosystems. The results from a controlled mesocosm experiment showed that: (1) at low productivity, snails were dominated by small-vulnerable species (P. gyrina), predator effects were strong and predator effects caused a trophic cascade extending to producers; (2) at high productivity, snails were dominated by large-invulnerable species (H. trivolvis), predator effects were weak and there was no trophic cascade to producers; (3) at intermediate productivity, alternative stable equilibria were implicated, such that depending on initial conditions, snails were dominated by either small-vulnerable or large-invulnerable species and predator effects were either strong or weak. Thus, this study represents one of the first to quantify how variation in food web structure can result from the local environment, initial conditions and their interaction.