Ecosystem ecology is founded on the idea that ecosystems are primarily driven by bottom-up. Yet emerging research is showing that ignoring the cascading top-down effects of carnivores can mean that we are failing to consider a potentially important driver of species diversity and ecosystem functioning. Field experiments conducted by Schmitz show that two kinds of top-predator species in the same hunting spider guild exhibit different hunting behaviors (actively hunting, sit-and-wait) on grasshoppers, grasses and herbs. These difference results in different direct effects on grasshopper prey. Grasshoppers demonstrate significant behavioral shift in the presence of sit-and-wait spiders but not when faced with actively hunting species. The sit-and-wait spider then causes indirect effects on plant by changing grasshopper foraying behavior (trait-mediated indirect effect) and the actively hunting spider has strictly density-mediated indirect effect on plant. The questions come up with how different predator hunting modes affect plants diversity and the level of key ecosystem functions such as plants biomass, primary productivity and nutrient cycling through density-mediated indirect interaction (DMII), trait-mediated indirect interaction (TMII) and the combination of DMII and TMII. Based on field experiments, we establish mathematical models to describe the dynamic behaviors of each species in ecosystems with actively hunting, sit-and-wait and combination of the two hunting modes of spider, respectively. By qualitative theory of differential equations we offer mechanistic insight into how predator hunting modes can determine plant species composition and accordingly ecosystem functioning.
Results/Conclusions
Grass and herb can not coexist under the actively jumping spiders, while they can coexist under the sit-and-wait spiders or the combination of the two spiders. Actively hunting spiders enhance aboveground net primary production and nitrogen mineralization rate, whereas sit-and-wait spiders had opposite effects under certain conditions. These effects arise from the different responses to the two different predators by their grasshopper prey—the herbivore species that controls plant species composition and accordingly ecosystem functioning. Predator hunting mode is thus a key functional trait that can help to explain variation in the nature of top-down control of ecosystems.
