Description: Small herbivores are abundant on large marine macrophytes, but their impact on their hosts is poorly understood relativeuto large grazers such as urchins and fish. To limit the risks of predation, many marine mesograzers live within nests or burrows,upotentially causing more damage to plants than predicted from consumption alone. To test whether the growth ofularge primary producers can be affected by modification of plant structures by small herbivores, we quantified the effect ofuthe nest-building amphipod Pseudopleonexes lessoniae on blades of the giant kelp, Macrocystis pyrifera in New Zealand.uAmphipods create their nests by rolling the blade margin in close proximity to the meristem. Blades with nests were 40%ushorter than blades lacking nests and reduced in area by 55%. We examined the composition of amphipods inhabiting eachunest to assess the temporal persistence of grazer aggregations. Nests were occupied by a single female or male–female pairs, and their newly hatched offspring. Analysis of offspring size distributions suggested that offspring dispersed from the maternal nest and did not remain to breed themselves. By concentrating physical damage and feeding on valuable tissues, these results indicate that even low numbers of small herbivores can cause localized impacts on the morphology and size of fast-growing algal blades. Predicting the consequences of this damage on larger scales will require understanding the spatial and temporal distribution of amphipod nests on giant kelp.

Description: Damage by small herbivores can have disproportionately large effects on the fitness of individual plants if damage is concentrated on valuable tissues or on select individuals within a population. In marine systems, the impact of tissue loss on the growth rates of habitat-forming algae is poorly understood. We quantified the grazing damage by an isopod Amphoroidea typa on two species of large kelps, Lessonia spicata and Macrocystis pyrifera, in temperate Chile to test whether non-lethal grazing damage could reduce kelp growth rates and photosynthetic efficiency. For L. spicata, grazing damage was widespread in the field, unevenly distributed on several spatial scales (among individuals and among tissue types) and negatively correlated with blade growth rates. In field experiments, feeding by A. typa reduced the concentration of photosynthetic pigments and led to large reductions (~80 %) in blade growth rates despite limited loss of kelp biomass (0.5 % per day). For M. pyrifera, rates of damage in the field were lower and high densities of grazers were unable to reduce growth rates in field experiments. These results demonstrate that even low per capita grazing rates can result in large reductions in the growth of a kelp, due the spatial clustering of herbivores in the field and the selective removal of photosynthetically active tissues. The impacts of small herbivores on plant performance are thus not easily predicted from consumption rates or abundance in the field, and vary with plant species due to variation in their ability to compensate for damage.