Good Friends: Growth and Flowering Responses of Eelgrass to Grazing by Brant Geese

Eelgrass makes up nearly all of the diet of brant geese on west coast of North America and there are several lines of evidence indicating that this is a coevolved plant-animal relationship. Brant could therefore affect the ability of an eelgrass bed to support juvenile crabs and rockfish. But what does the plant think about losing photosynthetic surface area to a bird? Our first experiment in Humboldt Bay, California tested the hypotheses that eelgrass growth is altered by grazing induced canopy thinning, brant feces, or the interaction between these two factors. The treatment that combined grazing (i.e. leaf clipping) and brant feces resulted in the fastest leaf growth and eventually the most flowering. The second experiment tested the hypothesis that an intermediate combination of clipping and feces would result in faster eelgrass growth relative to 'no brant' or 'intense grazing' treatments. This hypothesis was supported. Implications of this study are that, so long as brant do not damage shoot apical meristems, their grazing increases plant productivity and likely sexual reproduction, which in turn means that an eelgrass bed with brant should have a more productive detrital pathway than a bed without the bird.

Speaker: Frank Shaughnessey, Humboldt State Univ.