Comparing lab, field and mesocosm experiments with intertidal seaweeds to examine the biological basis of species complementarity and diversity effects

Background/Question/Methods

While consistent effects of biodiversity on ecosystem productivity and stability occur in many ecosystems, the biological mechanisms underlying these are rarely clearly identified. Statistical partitioning of effects into sampling and complementarity can distinguish whether one or many species are responsible for diversity effects, but cannot elucidate the biological traits or interactions responsible for these effects. Many of the commonly cited mechanisms (such as resource partitioning) can require time (multiple generations) and environmental heterogeneity to be adequately expressed, yet the number of experiments that include both of these are limited. When such experiments are combined with an understanding of the basic biology or natural history of the component species, specific biological mechanisms become more evident.  Here we report the results of a 7 year long field manipulation of intertidal seaweed diversity and compare results with shorter term mesocosm and field experiments to assess the importance of possible biological mechanisms underlying diversity effects.  We specifically focus on examining the strength of mechanisms that operate over the short term at the individual level vs over the long-term at the population level. 

Results/Conclusions

We found that seaweed species richness increased biomass accumulation in field experiments in both short (2 month) and long (3 year) experiments, although effects were stronger in the long-term experiment.  In contrast, richness had no effect in mesocosm experiments, where biomass accumulation was completely a function of species identity.  Furthermore effects of seaweed richness on community resilience were greater in magnitude than the effects of herbivory, a known important driver of seaweed biomass and diversity in this system.  We do find evidence for individual-based complementarity in nutrient uptake or photosynthetic rates in air vs water, but the effect of these differences on total biomass accumulation were not sufficiently strong to produce diversity effects.  Instead, the major mechanisms underlying diversity effects appear to be largely population level phenomena, including complementarity in microhabitat preferences that are manifest in enhanced recruitment and survival in diverse assemblages. Greater buffering of environmental conditions and different behaviors of herbivores in diverse plots also likely contribute to diversity effects on biomass and resilience, respectively.  Comparing short and long-term experiments and field vs mesocosm experiments afforded insight into the specific biological mechanisms by which complementarity does (and does not) affect ecosystem functioning in this system.