The Nutrient Availability Hypothesis: A test of a unifying plant-herbivore hypothesis

Background/Question/Methods

There are many hypotheses that explain the interactions between plants and insect herbivores. The plant stress hypothesis (PSH) states that water stressed plants will raise herbivore performance due to increases in N containing compounds (White 1969). The plant vigor hypothesis (PVH) states that plants that are growing rapidly, or have shoots or leaves that are growing rapidly will support greater herbivore performance (Price 1991). The recently developed pulsed stress hypothesis (PLSH), states that plants under pulsed water stress are better hosts for herbivores than continuously stressed plants (Huberty and Denno 2004). Tests of these hypotheses have provided mixed support. However, these hypotheses have two principles in common: 1) they identify a plant’s physiological state as the reason for changes in herbivore performance and 2) the concentration of nutrients during the specified physiological state is the central explanation for changes in herbivore performance. We propose a single hypothesis that incorporates the principles of each. The aptly named “nutrient availability hypothesis” (NAH) simply states that when plants are under a physiological state that increases the concentration of nutrients, whether it be due to stress, vigorous growth, etc., there should be greater performance from herbivores that can access these nutrients. 

Results/Conclusions

We conducted a meta-analysis on relevant literature to illustrate that nutrient availability is central to predicting herbivore performance. We found that herbivore response to a hypothesis test was dependent upon the nutrient concentration in the plants. Studies in which herbivores responded positively to water stress had plants with increased nutrient concentrations. In contrast, studies that reported a negative response from water stress had plants with significantly lower nutrient concentrations. We found that water stress increases the concentration of allelochemicals. However, unfavorable plants only had increases in allelochemicals, whereas favorable plants had increases in nutrients as well, suggesting that herbivores may be able to overcome plant defenses with adequate nutrients. In conclusion, we illustrated that nutrient concentration explained the efficacy of these hypotheses. Thus, when trying to predict herbivore performance, we should turn away from debating which hypothesis is more accurate and instead use the plant’s nutrients to predict herbivore performance. We believe that the “nutrient availability hypothesis” greatly simplifies the predictability of herbivore performance.