The relative importance of plant production (bottom-up) and consumption (top-down) effects on ecosystem functioning is an important issue. This debate becomes more important as climate change may shift the relative importance of bottom-up and top-down effects for ecosystem functioning. Grasslands, grasshoppers and avian predators are a useful model system to address these questions for several reasons. First, grasslands are important productive terrestrial systems for humans and wildlife; grasshoppers are major (often the most important) herbivores in these systems; birds, as predators, have the potential to limit grasshoppers. Second, this system is amenable to experimental manipulation of water, nutrients and numbers of plants, grasshoppers and birds using enclosures, exclosures, and mesocosms. We present results from 39 years of monitoring and experiments designed to address this issue in a Montana (USA) bunchgrass prairie, where we document climate change and how our experimental results have changed in response.
Over 39 years, we documented:
- Climate has become cooler and wetter during the spring plant growing season, but warmer and drier afterwards, so increased spring primary production rapidly senesces.
- Grasshopper numbers have increased in response to the greater spring primary production (food), but grasshopper life expectancy has declined with the more rapid plant senescence.
- When food is less abundant (fewer grasshoppers), avian predation is able to limit grasshoppers, but when food is more abundant (more grasshoppers), avian predation cannot limit grasshoppers and grasshoppers are food limited.
- Therefore, the intensity of herbivory has changed due to greater numbers of grasshoppers that have shorter life expectancy, and this increases the likelihood of predator limitation.
These shifts in trophic dynamics impact ecosystem functioning. In some habitats, primary production is enhanced as grasshopper herbivory increases nitrogen availability (limiting nutrient) for plants. In other habitats, primary production is diminished as grasshoppers decrease nitrogen availability. In addition, the changes in predator limitation diminish positive effects of grasshopper herbivory in the former habitats, and diminish negative effects in the latter habitats.
Therefore, climate change has complex impacts on ecosystem functioning by changing the responses of plants, herbivores and predators, and their biotic interactions.