Bryozoan overgrowth interactions—the interdependence of competition for space and food

Abstract

Space has been demonstrated repeatedly to be a limiting resource to sessile suspension-feeding invertebrates1–7,15, whereas the question of whether food resources are ever limiting has long been a controversial topic8,9. On the basis of previous research and in the virtual absence of any mechanistic understanding, it is commonly assumed that interactions can be interpreted as competition for spatial resources alone. This assumption is clearly unacceptable. Whereas a mobile organism might feed in one location rich in prey, spawn in another safe from predators of its young, and hide in yet another inaccessible to its own predator, a sessile organism cannot separate, spatially or temporally, its demands for food to eat and space on which to live. For the sessile organism, access to food implies access to space and vice versa. Consequently, the empirical observation of competition for space does not justify the assumption that competition for food is unimportant. I report here that competition for food among suspension feeders can occur and may, in fact, provide a mechanism by which sessile organisms compete for space.