Conclusion
In this study, the effect size of fish abundance was greatest when our
linear model was applied to the ponds, indicating that abundance of
herbivores relative to that of primary producers depends highly on
abundance of carnivores as classically and repeatedly pointed out since
Hairston et al. (1960). We also found that stoichiometry, defense
traits, and production rate of producers play substantial roles in
regulating the H/P biomass ratio. Thus, this study revealed that
herbivore biomass relative to producer biomass was regulated
simultaneously by the abundance of carnivores, stoichiometric nutrient
content and defense traits of producers, and primary production rate in
natural ecosystems. In this study, we considered the size and phosphorus
content of phytoplankton as the defense trait and nutritional quality of
primary producers, respectively. However, it is also possible to
consider effects of other chemical and physical defenses such as toxins
and thorns, and nutritional substances such as protein and fatty acid
contents in eq. (4). Since our theoretical framework (eq. (4) and (9))
can incorporates multiple factors, application of the model to various
terrestrial and aquatic communities is possible and thus would serve to
generalize the relative importance among production rate, defense traits
and stoichiometric nutrient content of producers, and predation rate by
carnivores in various ecosystems.