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Abstract
Ecosystem engineers have a central role in modifying key attributes of the environment that affect species throughout their community. The outcome of their environmental modification, however, depends on the degree of modification to the existing physical conditions. That is, the magnitude and sometimes direction of engineering outcome often depends on the environmental context in which it occurs. Environmental context can vary spatially often due to gradients in energy flow or baseline structure. Thus, quantifying the strength and sensitivity of ecosystem engineering to spatially variable physical factors like energy or structure can inform their system wide effects, as well as their responses to environmental change. However, engineering effects are not limited to the presence of the living engineer, because ecosystem engineering can also affect change through legacy effects, which persist after the engineer’s death. Additionally, multiple ecosystem engineers may coexist, and because of their dominant influence, how they interact can affect the entire system. In the southeastern US, the eastern oyster, Crassostrea virginica (Gmelin) and smooth cordgrass, Spartina alterniflora (Loisel) are two prominent ecosystem engineers that affect estuarine ecosystem functioning. Here we look at how baseline structural context affects oyster engineered structure, and the outcome of that structure on cordgrass habitat. We look at the effect of wave energy on the effects of oyster live and legacy engineering of cordgrass habitat. And we look at how both species engineer the others habitat. We show that reef building by oysters increases intertidal bank slope which increases habitat for neighboring cordgrass, but that the magnitude of the engineering effect is highly dependent on unengineered slope, or structural context. We show live engineering effects of reefs formed by oysters primarily buffer erosional forces and increase habitat availability for cordgrass. In contrast, the legacy effect of oysters has the opposite effect. The negative effects of oyster legacy engineering dampen the positive effects of live oyster engineering in high wave energy areas. Increasing wave energy ultimately tips the net effect of oyster presence on cordgrass from positive to negative. We emphasize the importance of quantifying underlying environmental context, for both live and legacy engineering.