Insect herbivores play an important role in plant populations. The can determine population size by consuming and killing plants. They determine plant fitness, either by consuming seeds, reducing photosynthesis rates by consuming leaves, or by forcing plants to invest limited resources into defensive chemistry rather than seed production.
Warming, caused by climate change, affects both plants and herbivores. Rising temperatures increase plant growth rates, affect the production of plant defensive chemicals, and can also affect plant seed production. On top of that, warming often increases herbivore metabolic rates, causing herbivores to increase their consumption of plant material. Such increased consumption rates may strengthen the effect of herbivores on plants, but the direct effects of warming on plants (i.e. higher growth rates), could offset the increased herbivore damage. My co-authors and I just published a new paper in the journal Ecology, titled “Responses of plant phenology, growth, defense, and reproduction to interactive effects of warming and insect herbivory“, that examines whether rising temperatures might disrupt the ability of insects to influence plant life history.
What we found surprised us. At normal temperatures, plants were able to compensate for herbivory. Sure, plants exposed to insects were shorter and produced fewer fruits, but each fruit was packed with more seeds. As a result, total seed biomass was unaffected by insects (although seeds were smaller).
Warming changed all that. Plants were shorter and they produced more fruits. However, fruits were smaller, and packed with many small seeds. It was quite astonishing, however, because herbivores no longer had any effect. Despite high leaf damage, plant life history was unaffected by the presence of herbivores.
Why would this happen, why would warming negate any influence of insects on plants? We don’t know for sure, but we suspect that warming reduced soil moisture (a common effect of rising temperatures). Low soil water content appears to have induced a stress response, causing stunted growth in the plants and altering their life history strategy. Interestingly, the stress response to low soil moisture appears to have superseded any effect of herbivores.
Herbivores play an incredibly important role in plant ecology; they drive plant population dynamics and evolution. Our results suggest that climate warming might drastically alter the role of insects in plant communities.