Tracing system dynamics when herbivore-plant interactions are net positive

Background/Question/Methods

We study herbivore-plant interactions with a positive feedback from the herbivore to the plant via nutrient cycling. First, we present evidence that herbivory by the Tupelo Leafminer, Antispila nysaefoliella Clemens (Lepidoptera: Heliozelidae), on its host plant, Black Tupelo, Nyssa sylvatica Marsh, augments nutrient decomposition and increases the supply rate of soluble nitrogen in the soil. Second, we model the system dynamics using ordinary differential equations that trace the flows of nitrogen between the plant, the herbivore, and soil.

We postulate that A. nysaefoliella plays the functional role of a detritivore by initiating the first step of decomposition. Because their emergence is timed near leaf fall, we also expect that the costs typically associated with herbivory are minimized (or neutralized) because the plant has completed growth and reproduction and nutrient resorption is well underway. Subsequently, the nutrients that are not resorbed would then be recycled via decomposition. Herbivory can augment this process by accelerating the conversion of organic nitrogen to a monomer, which then is converted to soluble nitrogen through microbial action. 

Results/Conclusions

We present: (1) a positive association between tree level leafminer loads and supply rates of soluble nitrogen in the soil; (2) higher supply rates of soluble nitrogen from decomposition of leaves with than without leaf-mining damage; and (3) a model of system dynamics when the relationship between an herbivore and its host plant shifts from an antagonism to a neutral or net positive state.

The negative effects of leaf-mining are likely to be similar to other forms of herbivory. However, if the nutrient cycling pathway is included, then the net effect of herbivory may be positive. The specialization between a leaf-mining species and its host plant also suggests a tightly linked evolutionary process and the potential importance of selection in stabilizing dynamics. We propose that the herbivore-plant relationship can evolve to one of mutual benefit when other ecological pathways are considered.