Direct herbivory by biological control agents, and the consequent disruption of plant-nutrient-feedback cycles, combine to reduce the invasiveness of Melaleuca quinquenervia (Myrtaceae).
Herein we present the results of a 5-year herbivore exclusion experiment in which the suppression of two successful biological control agents revealed a nutrient feedback cycle that promoted the growth and dominance of the exotic tree Melaleuca quinquenervia. When herbivory was restricted, plants produced greater quantities of aboveground biomass, which transferred a greater volume of litterfall and a larger load of nutrients to the soil surface. The resulting organic layer on the surface of the soil decomposed more quickly releasing nutrients and resulting in a larger pool of total phosphorus in the surficial soil layer. These mineralized-soil-nutrients were available for uptake by the exotic plant, which produced a greater quantity of surficial roots in the restricted-herbivory plots creating a positive nutrient-feedback cycle to biomass production. Our data suggest that, in addition to the direct suppression of exotic plants through biomass consumption, biological control agents may provide indirect suppression of weeds by preventing the establishment or altering the magnitude and/or direction of an invasion-promoting feedback cycle.