Abstract
As the human population grows, there is an influx of nutrients to aquatic systems due to urbanization, agricultural practices and other sources of pollution. Therefore resource availability, which drives productivity, also changes. Whether species diversity is a function of productivity has eluded researchers; however, aquatic systems show a trend towards a unimodal relationship. There have been several hypotheses which attempt to describe the underlying mechanisms of a unimodal curve. The More Individuals Hypothesis (MIH) predicts that communities supporting more individuals will have higher species richness since extinctions due to stochastic events are less likely within any given trophic level. The Oksanen, Fretwell, Arruda, and Niemala (or OFAN) hypothesis states that new trophic levels can only be added once a biomass threshold controlled by resources is reached. The OFAN model is unique in that it uses a “bottom-up” approach to predict how species within each of the trophic levels will respond to an increase in productivity. While the patterns predicted by the OFAN hypothesis have been observed in many systems, it is unknown whether these patterns and mechanisms operate in the threatened ecosystem of California vernal pools. Therefore this experiment assessed how species richness and abundance changes over a nutrient gradient and investigated whether the patterns were consistent with the OFAN model. Mesocosms lined with vernal pool soil received one of five treatments, a control and four nutrient addition treatments, creating a gradient for nitrogen and phosphorus levels. Macro-invertebrates, microorganisms, and abiotic variables were collected and recorded every two weeks for three months. Repeated measures ANOVA were used to compare treatments to test for the effects of resource treatments on the abundance and species richness. Organismal abundance increased species richness in accordance to the MIH, however abiotic variables affected species richness. The prey trophic level was not affected by nutrients while the predator trophic level increased with nutrients as predicted by the OFAN model. Nutrient addition did not affect abundance of vernal pool organisms, nor did nutrient treatments significantly affect the total number of trophic levels. The effects of nutrient addition were partially consistent with the predictions made by the OFAN model in vernal pool mesocosms.