Legacy and live effects of ecosystem engineer differ across energy gradient, reversing net engineering outcome.

Ecosystem engineers influence abiotic and biotic properties of communities. However, environmental factors like stress or energy gradients could modify their influence. Furthermore, engineering effects include legacies that persist after the engineer's death that may interact with gradients differently than the live engineer. Here, we show that live and legacy effects of the oyster Crassostrea virginica, a reef-building ecosystem engineer, act in opposing directions across a physical energy gradient, reversing oysters' net engineering effect. When energy is low, live oysters primarily buffer erosional forces and increase habitat availability for cordgrass (Spartina alterniflora). When energy is high, legacy effects of oysters on cordgrass are negative, because high energy piles dead oyster shells onto cordgrass and smothers it. Thus, with increasing energy, net effects of engineering on cordgrass shift from positive live effects to negative legacy effects. Estuary wide, oysters have a net positive effect on cordgrass, because high energy coastlines - where negative effects predominate - are rare. Live and legacy engineering can have disparate, context-dependent effects; combined, they determine the overall effects of engineers on communities.