Top‐Down Effects in a Tritrophic System: Parasitoids Enhance Plant Fitness

We determined the direct and indirect top—down forces in a tritrophic system composed of a guild of three parasitoid species (Necremnus tidius, N. folia, and Aprostocestus sp.), a single weevil seed—predator species (Ceutorhynchus sp. nov.), and the host plant, Hormathophylla spinosa, a woody crucifer of the high—mountain elevations of the Sierra Nevada (southeast Spain). By Means of observational and experimental approaches, we focused on the possible regulatory effects of parasitoids on the weevil population as well as on the analysis of the sign and strength of the resulting indirect effects on plant fitness. Weevill attacks 21.7% of the fruit produced by H. spinosa during the study, selecting fruits having the most seeds. Seeds eaten by weevils were significantly bigger than uneaten ones. The number of seeds per fruit after being depredated by Ceutorhynchus was significantly smaller than for fruit not attacked. Thus, weevils affect the fitness of the host plant both by decreasing the number of seeds (quantitative effect) and by reducing the average size of surviving seeds per fruit (qualitative effect). When parasitoids were excluded experimentally, the percentage of attacked fruit (which corresponds to the number of weevil larvae) rose from 20 to 43%. We assume that the increase in the experimental weevil population may be because excluding parasitoids eliminates not only death by parasitoid oviposition for some Ceutorhynchus larvae, but also death by direct predation of parasitoids on weevil larvae. We have also experimentally verified that parasitoid activity benefits the entire plant, by way of parasitoids contributing to a higher average number of seeds dispersed per fruit in each plant, by influencing both the abundance of the weevis and the behavior of the weevil larvae.