Bottom-up (host plant quality) and top-down (natural enemies) forces both influence the fitness and population dynamics of herbivores. However, the impact of pollutants acting on these forces has not been examined, which prompted us to review the literature to test hypotheses regarding this area of research. In the first null hypothesis, the fitness, or population density, of a herbivorous insect due to bottom-up factors in a polluted environment is equal to the fitness, or population density, of a herbivorous insect due to bottom-up factors in a non-polluted environment. The alternative hypotheses provide that bottom-up forces are acting differentially on the fitness, or population density, of an insect herbivore in polluted vs. non-polluted environments. In the second null hypothesis, the fitness, or population density, of an herbivore due to top-down factors in a polluted environment is equal to the fitness, or population density, of a herbivore due to top-down factors in a non-polluted environment. The alternative hypotheses provide that top-down forces are acting differentially on the fitness, or population density, of herbivores.
Results/Conclusions
A comprehensive literature search found 126 references which examined fitness components and population dynamics of 203 insect herbivores. One-hundred fifty-three of the 203 herbivores (75.4%) had fitness impacted due to bottom-up factors in polluted environments. In contrast, only 20 of the 203 (9.9%) had fitness significantly impacted due to top-down factors in polluted environments. In regards to heavy metals and metalloids, 26 references examined fitness components and population dynamics of 33 insect herbivores. Twenty-four of the 33 herbivores (72.7%) had fitness or population density impacted negatively due to bottom-up factors in heavy metal and metalloid polluted environments. In contrast, only three of the 33 herbivores (9.1%) had fitness significantly impacted due to top-down factors in heavy metal and metalloid polluted environments. Heavy metals acting on host plant quality reveal a pattern of primarily negative effects on fitness for herbivores. Studies reported reductions in weight, growth, survival, fecundity, and eclosion success. A similar pattern was found for metalloids, most notably selenium (Se): exposure to Se-contaminated plants significantly decreased growth, larval weight, population size and survival. The paucity of results for top-down factors impacting fitness does not necessarily mean that top down factors are less important, but rather that fewer studies include natural enemies. We provide a synthesis of available data, guild effects, and suggest future areas of research.
