Predation and resource abundance influence the diversity-productivity relationship in herbivores and primary producers

Background/Question/Methods

The relationship between competitor diversity and community productivity is well studied in ecology.  Hypotheses that address this relationship generally recognize that “bottom-up” environmental factors, such as the availability of resources, frequently influence diversity and productivity simultaneously.  However, it is unclear if a primarily “bottom-up” framework applies to systems with strong “top-down” control. Here consider the strength and direction of the diversity-productivity relationship in a system where variation in predation is one of the strongest drivers of competitor biomass and community composition. 

We focus on zooplankton and phytoplankton communities in Coastal New England Lakes. In New England lakes, the presence and life history of a zooplanktivorous fish, Alewife (Alosa pseudoharengus), has a strong influence on crustacean zooplankton communities and a weaker but observable influence on phytoplankton communities.  We use data from two studies addressing the effect of intraspecific variation in alewife on lower trophic levels; (i) a mesocosm experiment that manipulated the life history form of alewife, and (ii) a survey of coastal lakes.  We use AICc values to compare models of competitor biomass as a function of competitor richness, predation, and resource abundance.

 Results/Conclusions

In the mesocosm experiment, zooplankton biomass and richness differed significantly among alewife treatments, resulting in a significant positive correlation between zooplankton richness and biomass.  The best-fit model of zooplankton biomass included terms for zooplankton richness and alewife treatment.  In the survey, zooplankton richness differed significantly among lakes differing in alewife presence and life history, but zooplankton biomass was best predicted by phytoplankton biomass.  Phytoplankton richness and biomass in both data-sets were best predicted by nutrient concentrations.

We conclude that bottom-up and top-down factors can simultaneously influence the diversity productivity relationship at multiple trophic levels.  In the mesocosm experiment, where variation in nutrient levels and initial competitor composition were limited, variation in predation was the primary driver of the diversity biomass relationship in zooplankton.  In the survey, predation was the best predictor of zooplankton richness, but plankton biomass was correlated with nutrient quantity.