Overgrazing can deplete macroalgal communities and lead to reduction of habitat complexity and species diversity.

This study aimed to quantify the effects of sea urchins and fish herbivory on the rocky reefs of the North Aegean Sea, through a six-month grazer exclusion experiment undertaken in the Lesvos Island. It was tested whether, i) algal growth is hampered by the combined grazing activity of sea urchins and fish, and ii) algal growth is suppressed by the foraging of fish alone.

The findings of this experimental study suggest that grazing pressure plays a significant role in shaping the structure of the shallow rock reef communities in the northeastern Aegean Sea. Herbivore exclusion revealed that erect algae are substantially impacted by the grazing pressure of herbivores, and are almost completely eradicated form most rocky reefs, while algal turf development is also significantly impeded.

The authors conclude that sea urchin populations are primarily responsible for the maintenance of this overgrazed and degraded state in the region, while overfishing, as well as illegal destructive fishing practices, are possibly the drivers that have generated, and further maintained, the current conditions.

https://www.sciencedirect.com/science/article/pii/S0022098117302836

Tsirintanis K, Sini M, Doumas O, Trygonis V, Katsanevakis S (2018) Assessment of grazing effects on phytobenthic community structure at shallow rocky reefs: An experimental field study in the North Aegean Sea. Journal of Experimental Marine Biology and Ecology. 503:31-40