Pre-dispersal seed predators boost seed production in a short-lived plant

Pre-dispersal seed predation diminishes fitness and population growth rate of many plant species. Therefore, plants have developed multiple strategies to reduce the harmful effects of this type of herbivory. The present study aims to determine the effect of pre-dispersal seed predators (PSPs) on the fitness of a short-lived herb, and to discern the mechanisms allowing the plants to reduce the impact of pre-dispersal seed predation. Knowing that the interplay between pre-dispersal seed predators and plants is strongly shaped by the presence of other co-occurring organisms, we tested whether detritivores modulate plant responses towards pre-dispersal seed predators. To do so, we experimentally manipulated in the field pre-dispersal seed predators and detritivores interacting with the short-lived herb Moricandia moricandioides. We found that detritivores did not alter the response of plants to PSPs. Strikingly, the plant overcompensated for pre-dispersal seed predation, almost doubling the number of seeds produced. Plant response to PSPs led to substantial changes in shoot architecture, reproductive traits, chemical defences in leaves and seeds and in seed nutrient content. The overcompensating mechanism seems to be meristem activation, which allowed plants to produce more reproductive tissue, and increasing the proportion of ovules that became seeds, a response which specifically compensates for pre-dispersal seed predation. As far as we know, this is the first experimental evidence of a positive effect of PSPs on plant lifetime fitness as a consequence of plant overcompensation.