The nutrient availability hypothesis: A test of a unifying plant-herbivore hypothesis

There are many hypotheses that explain the interactions between plants and insect herbivores (plant stress, plant vigor, pulsed stress, etc.). Empirical tests of these hypotheses have provided mixed support, but 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.

We conducted a meta-analysis on relevant literature to test that nutrient availability is central to predicting herbivore performance. We found that herbivore response to stressed plants was dependent upon nutrient concentration. When herbivores performed better on water stressed plants (increased performance), the stressed plants had increased nutrient concentrations. In contrast, when herbivores did not perform well, the stressed plants had lower nutrient concentrations. We found that water stress also increases the concentration of allelochemicals. However, unsuitable plants also had higher concentrations of allelochemicals in addition to lower nutrients. We conclude that nutrient concentration may predict herbivore response to plant stress more effectively than physiological state. . We believe that the “nutrient availability hypothesis” greatly simplifies the predictability of herbivore performance on stressed plants.