The study of the interacting factors that constrain resource use in organisms, and promote diversity, is an important task, especially in a mega diverse area, in which the increasing transformation of ecosystems modifies constantly the interactions between organisms. Among the hypotheses that explain resource use and diversity of insects are the Top-down and the Bottom-up hypotheses, in which experimental studies have shown trade-offs between these factors. In addition, adaptive responses of organisms result from the synergic, and some times, the antagonistic, interaction between the factors that influence resource use. Little is known about the influence of other selection factors such as host availability, parasitism, and competition that in a natural setting influence host use in herbivorous insects. Given that competition, natural enemies, and host plant affect simultaneously resource use, this study determined and compared the effects of host plant, density of eggs and density of larvae on the mortality due to parasitism and to competition in the seed beetle Stator limbatus (Horn, 1873). By collecting seeds of 7 populations adapted to one out of four hosts, I compared mortality of eggs due to parasitism, and mortality due to competition of larvae.
Results/Conclusions
The nested ANOVA performed to determine the effects of host plant, population origin, competition, and egg size in mortality of eggs by parasitsm and in mortality of larvae by competition, showed that populations that use Acacia greggii experienced the lowest mortality, and populations that use Parkinsonia florida suffered the highest mortality exhibiting no evidence of trade-offs between bottom-up and top-down factors. Interactions host by density of larvae, and host by number off eggs on seeds, showed variation among hosts in the mortality of beetles due to parasitism and to competition. In addition, there was no evidence of egg size affecting parasitism of eggs. These results post the necessity of including in the traditional bottom up-top down explanations, the study of factors that could be mediating their outcome such as the one examined here (competition); a more urgent necessity in the current time when we are exposing ecosystems to accelerated changes in structure, functioning, and composition.
