The consequences of declining biodiversity remain controversial, in part because many studies focus on a single metric of ecosystem functioning and fail to consider diversity's integrated effects on multiple ecosystem functions. We used tide pool microcosms as a model system to show that different conclusions about the potential effects of producer diversity on ecosystem functioning may result when ecosystem functions are measured separately vs. together. Specifically, we found that in diverse seaweed assemblages, uptake of either nitrate or ammonium alone was equal to the average of the component monocultures. However, when nitrate and ammonium were available simultaneously, uptake by diverse assemblages was 22% greater than the monoculture average because different species were complementary in their use of different nitrogen forms. Our results suggest that when individual species have dominant effects on particular ecosystem processes (i.e., the sampling effect), multivariate complementarity can arise if different species dominate different processes. Further, these results suggest that similar mechanisms ( complementary nutrient uptake) may underlie diversity - functioning relationships in both algal and vascular-plant-based systems.

Although nutrient availability and oceanographic context can influence top-down processes in intertidal communities, the reciprocal effects of consumers on bottom-up processes, such as nitrogen uptake by macroalgae, are less well-documented. We used a combination of field observations, mesocosm experiments, and laboratory nitrate-uptake measurements to evaluate the interaction between the kelp crab Pugettia producta and its preferred food source, the kelp Egregia menziesii, in northern California, USA. We found that R producta fed selectively on E. menziesii, removing tissues with a high surface area to volume ratio (SA:V), which resulted in a 66% decrease in overall SA:V. Because these high SA:V tissues are disproportionately more responsible for nitrate uptake, selective herbivory by P. producta decreased E. menziesii's biomass-specific uptake of nitrogen, the primary limiting nutrient in this ecosystem, by 65% per gram of remaining algal tissue. In field surveys, where P. producta abundances were high, E. menziesii thalli more frequently exhibited signs of intense grazing and thus had lower SA:V ratios, suggesting that grazing is sufficiently intense in the field to generate differences in algal morphology and nitrate uptake. P. producta therefore influences E. menziesii biomass and productivity from the top down, by removing substantial amounts of biomass, and from the bottom up, by reducing the kelp's subsequent ability to acquire nutrients. Because of selective grazing on specialized tissues, the effects of mesoherbivores, like P. producta, on the structure and dynamics of marine communities may often be greater than predicted simply by the biomass they consume.

Although human-mediated extinctions disproportionately affect higher trophic levels, the ecosystem consequences of declining diversity are best known for plants and herbivores. We combined field surveys and experimental manipulations to examine the consequences of changing predator diversity for trophic cascades in kelp forests. In field surveys we found that predator diversity was negatively correlated with herbivore abundance and positively correlated with kelp abundance. To assess whether this relationship was causal, we manipulated predator richness in kelp mesocosms, and found that decreasing predator richness increased herbivore grazing, leading to a decrease in the biomass of the giant kelp Macrocystis. The presence of different predators caused different herbivores to alter their behaviour by reducing grazing, such that total grazing was lowest at highest predator diversity. Our results suggest that declining predator diversity can have cascading effects on community structure by reducing the abundance of key habitat-providing species.

We combined field monitoring and laboratory experiments to examine the population ecology of both the microscopic and macroscopic stages of a new invasion of Undaria pinnatifida in California. Over the course of 1 yr, we observed 2 distinct recruitment pulses of individuals in the Santa Barbara harbor; the appearance of these pulses was strongly correlated with a 4degreesC drop in ocean temperature approximately 2 mo prior to recruitment, Cultures of zoospores and successive microscopic stages revealed thermal tolerances consistent with field recruitment data; individuals grown at 13degreesC had significantly higher survivorship than individuals grown at higher temperatures (harbor temperatures annually ranged from 12 to 21degreesC). The 2 cohorts also differed greatly in individual size, growth rate, and survival to maturity. Grazing by herbivores, predominately the native kelp crab Pugettia producta, effectively prevented nearly all individuals in the second cohort from reaching reproductive maturity. Grazer control was effective despite far higher rates of recruitment during the second recruitment pulse. Our results highlight the potential for extreme variability in U. pinnatifida demography mediated by local oceanographic and biotic factors. Understanding controls on U. pinnatifida demography helps to explain variation in the spread and impact of this invader worldwide, and allows better prediction of when and where U. pinnatifida may continue its invasion along the west coast of North America.