Hochberg and van Baleen (1998) proposed in a theoretical model that geographic variation in the productivity of the victim in antagonistic interactions can determine the intensity of coevolution and hence account for the occurrence of hot and cold spots. Their model predicts that in those populations of the victim species that show a greater potential population growth, the effect of predation on the population dynamic and evolution of the victim will be more intense. This would generate the presence of coevolutionary "hot" and "cold spots" determined by geographical variation in the productivity of the victim. Furthermore, it is expected that the level of resistance of the victim will be positively correlated with the intensity of consumption of the exploiter. To our knowledge there has been no attempts yet to explore the predictions of this theoretical model to understand the causes of geographic variation in the presence of hot and cold spots among antagonistic interactions.The aim of our study were (1) to correlate populations estimate of productivity for the victim with the magnitude of the consumption of the exploiter using a tritrophic system (plant-seed predator-parasitoid), and (2) to determine if levels of resistance of the victim are positively correlated with the amount of consumption of the exploiter.We sampled 32 populations of the host plant (Datura stramonium), its pre-dispersal seed predator (Trichobaris sp.), and its parasitoid (Nealiolus sp.) in Central Mexico, and estimated populations productivity for the host plant (using the total number of fruits), proportion of infested fruits, abundance of seed predators, proportion of parasitized seed predators and resistance of the host plant against seed predation.
We found a significant positive correlation between the estimated productivity of the host plant and the proportion of infected fruits, as well as a positive correlation between the abundance of the seed predator and the proportion of parasitized insects. This pattern supports the theoretical expectation and suggests a possible cascade effect among the antagonistic interactions promoting the presence of multispecific hot spots. In addition, levels of resistance of the host plant against the seed predator were also positively correlated with the intensity of seed consumption indicating that coevolutionary host spots may be associated with highly productive host populations. Overall, our results support previous theoretical expectations and suggest a possible cascade effect upon the intensity of antagonistic interactions in multitrophic systems.