A unified framework for herbivore-to-producer biomass ratio reveals the relative influence of four ecological factors

Abstract

The biomass ratio of herbivores to primary producers reflects the structure of a community. Four primary factors have been proposed to affect this ratio, including production rate, defense traits and nutrient contents of producers, and predation by carnivores. However, identifying the joint effects of these factors across natural communities has been elusive, in part because of the lack of a framework for examining their effects simultaneously. Here, we develop a framework based on Lotka-Volterra equations for examining the effects of these factors on the biomass ratio. We then utilize it to test if these factors simultaneously affect the biomass ratio of freshwater plankton communities. We found that all four factors contributed significantly to the biomass ratio, with carnivore abundance having the greatest effect, followed by producer stoichiometric nutrient content. Thus, the present framework should be useful for examining the multiple factors shaping various types of communities, both aquatic and terrestrial.

Fig. 1. Ponds used in the experimental test.

Pond 217 (a) and Pond 218 (b) in the Cornell University Experimental Ponds Facility divided into four sections by vinyl-canvas curtains and partially shaded by floating mats to regulate primary production rate. Floating docks were placed at the center of the ponds for sampling.

Fig. 2. Relationships between zooplankton biomass and phytoplankton biomass, and between H/P mass ratio and photosynthetic active radiation.

Biplots of zooplankton biomass (μg C L⁻¹) and phytoplankton biomass (mg C L⁻¹) (a) and H/P mass ratio and photosynthetic active radiation (PAR, mol photon m⁻² d⁻¹) (b) in the water column during the experiment in no-shade (blue), low-shade (orange), mid-shade (red), and high-shade (gray) treatments in pond 217 (circles) and 218 (squares). In each panel, small symbols denote values at each sampling date, and large symbols denote the mean values among the sampling dates. Bars denote standard errors on the means (n = 7 sampling date in each section). Correlation coefficients (r) with p values between the mean values are inserted in each panel.

Fig. 3. Relationships between H/P mass ratio and environmental variables.

H/P mass ratio plotted against edible phytoplankton fraction (a), seston carbon to phosphorus (C:P) ratio (b), specific production rate (c), and fish abundance (d) during the experiment in no-shade (blue), low-shade (orange), mid-shade (red), and high-shade (gray) treatments in pond 217 (circles) and 218 (squares). In each panel, small symbols denote values at each sampling date, and large symbols denote the mean values among the sampling dates. Bars denote standard errors on the means (n = 7 sampling date in each section). Correlation coefficients (r) with p values between the mean values are inserted in each panel.

Fig. 4. Partial effects of edible phytoplankton fraction, seston C:P ratio, specific production rate, and fish abundance on H/P mass ratio.

Partial regression leverage plots showing relationships between H/P mass ratio (the response variables), and log-transformed C:P ratio of seston (a), fraction of edible algae (b), specific daily production (c), and relative fish abundance (d) (the explanatory variables) without interfering effects from other explanatory variables. The vertical axis represents the partial residuals of H/P mass ratio, and the horizontal axis represents the partial residual of the specific explanatory variable. Dashed and dotted lines in each panel represent the partial regression line and its 95% confidence curves. Partial correlation coefficients with p values are also inserted in each panel. Data from four different treatments of pond 217 (circles) and 218 (squares) are denoted by different colors.