Modulation of different kelp life stages by herbivory: compensatory growth versus population decimation

Partitioning the effects of herbivory on different life stages of primary producers is key to understanding the population-wide consequences of herbivory. We assessed the performance of microscopic (MiS <1 mm) juveniles, macroscopic (MaS) juveniles and adult kelp (Laminaria ochroleuca) under contrasting herbivory regimes through a herbivore exclusion field experiment. The abundance of MiS and the survival of MaS decreased by 67 and 63%, respectively, when herbivorous fishes and sea urchins were present. Blade growth (linear and area) of adult kelp displayed contrasting patterns under herbivore pressure: a 60% increase and a 46% decrease, respectively. These results indicate that while herbivory severely reduces juvenile survival, it may also induce compensatory growth (measured as linear growth) in adult kelp. In summary, we here demonstrate how herbivory affects all sporophyte life stages of the kelp L. ochroleuca. This is likely to have important implications for situations where historical patterns of herbivore presence and herbivory are changing, such as is increasingly the case in many temperate regions due to warming around the world.