I present a simple model of a community in which filter-feeders consume spores of their algal space competitors.  There is a positive feedback loop in this system by which higher densities of filter-feeders reduces settlement of algae and thus increases free space available for their own population growth.  This feedback leads to thresholds in the system’s response to smooth environmental gradients.  If filter-feeding is strong it can even cause the system to have alternate stable states. 

Experimental evidence from a temperate reef system in the Santa Barbara Channel demonstrates all interactions required for this feedback to produce sharp state transitions: (1) the groups are competing, (2) there is strong spore predation, and (3) there is an appropriate spatial gradient in at least one demographic rate.  Field surveys reveal the sharp spatial transitions predicted by this model.

The requirements for filter-feeding to change the outcome of competition are present in many marine systems, but the potential importance of these interactions depends crucially on the efficiency of filter-feeding at preventing settlement.  I review existing measurements of this efficiency and assess in which systems strong positive feedbacks are likely.