Size-dependent vulnerability to herbivory in a coastal foundation species.

Ecologists have long wondered how plants and algae persist under constant herbivory, and studies have shown that factors like chemical defense and morphology can protect these species from consumption. However, grazers are also highly diverse and exert varying top-down control over primary producers depending on traits such as body size. Moreover, susceptibility of plants and algae to herbivory may vary across life stages and size classes, with juveniles potentially the most vulnerable. Here, we focus on diverse grazing communities within giant kelp forests and compared consumption on two size classes of juvenile giant kelp (Macrocystis pyrifera) across four herbivore species ranging in size. We also integrated field and literature densities to estimate impacts on populations of juvenile kelp. We found that purple sea urchins, a species known for exerting strong control over adult M. pyrifera, had weak per capita impact on microscopic kelp, on par with a much smaller crustacean species. While urchin consumption increased with macroscopic juvenile kelp, it never surpassed the smaller brown turban snail, suggesting that feeding morphology, in addition to herbivore body size, is a predictor of consumption at these small size classes. The smaller herbivores also occurred in high densities in the field, increasing their predicted population-level impacts on juvenile kelp compared to urchins and perhaps other larger, but less abundant, herbivores. This study highlights the variation in species' roles within an herbivore guild and the importance of age-related changes in grazing vulnerability to better understand herbivore control on plant and algae population dynamics.

Short-term effects of hypoxia are more important than effects of ocean acidification on grazing interactions with juvenile giant kelp (Macrocystis pyrifera).

Species interactions are crucial for the persistence of ecosystems. Within vegetated habitats, early life stages of plants and algae must survive factors such as grazing to recover from disturbances. However, grazing impacts on early stages, especially under the context of a rapidly changing climate, are largely unknown. Here we examine interaction strengths between juvenile giant kelp (Macrocystis pyrifera) and four common grazers under hypoxia and ocean acidification using short-term laboratory experiments and field data of grazer abundances to estimate population-level grazing impacts. We found that grazing is a significant source of mortality for juvenile kelp and, using field abundances, estimate grazers can remove on average 15.4% and a maximum of 73.9% of juveniles per m2 per day. Short-term exposure to low oxygen, not acidification, weakened interaction strengths across the four species and decreased estimated population-level impacts of grazing threefold, from 15.4% to 4.0% of juvenile kelp removed, on average, per m2 per day. This study highlights potentially high juvenile kelp mortality from grazing. We also show that the effects of hypoxia are stronger than the effects of acidification in weakening these grazing interactions over short timescales, with possible future consequences for the persistence of giant kelp and energy flow through these highly productive food webs.