Farmerfish hold small territories on reef flats and attack almost anything that enters, including much larger grazers. Inside the territory the fish repeatedly bites off and carries away algal species it does not eat, depositing them outside the boundary. What remains is a turf shifted heavily toward palatable filamentous reds.
In certain Indo-Pacific populations this goes further. Territories are dominated by a single species of the red alga Polysiphonia, and surveys have found that species largely absent from the surrounding reef, suggesting the crop depends on the fish's protection to persist. Removal of the fish is followed by rapid loss of the crop to competing algae and to grazing.
The practice is not uniformly beneficial to the reef. Farmerfish territories can suppress coral recruitment and, at high densities, contribute to algal dominance on degraded reefs, while also excluding grazers that would otherwise crop the turf. Whether an individual reef gains or loses from a large farmerfish population depends on the wider grazing community rather than on the damselfish alone.