Long-term impacts of insect herbivores on plant populations and communities

As major consumers of plants, insect herbivores have strong potential to influence long-term patterns of plant abundance and community structure. Nonetheless, remarkably few manipulative experiments exclude insects for three or more years. Thus, how often, and under what conditions insects have enduring impacts remains unclear. Here we summarize the key conceptual issues and evidence from long-term studies on the impact of insects on plant populations and communities. Models that project effects of herbivores on plant population growth, based on demographic study, suggest that insects often limit plant asymptotic population growth. However, empirical estimations of insect impacts on plant abundance based on long-term experimental studies are few. As such, the strength and conditionality of compensatory mechanisms (such as density dependence and seedbanks) that may dampen (or exacerbate) the negative effects of insects on plant populations remains unclear. The influences of insects on plant range size and limits, invasion success and biocontrol are well-established research areas, but are also underexplored in long-term experiments. Insect herbivores can influence plant community structure and diversity, and change successional trajectories, often through modification of plant competitive ability. Nonetheless, effects are mostly known from mesic grasslands, take years to manifest and the extent of insect damage does not always predict their impact. The relative importance of specialist versus generalist herbivores is unclear, as is how feedbacks between plant productivity, herbivory and plant species richness contribute to community dynamics over time. Synthesis. The few existing long-term insect suppression experiments, and many short-term studies, suggest that insect herbivores can importantly affect plant populations and communities. Although challenging, experimental studies spanning multiple plant generations could greatly enhance our ability to develop a more predictive framework for how insect herbivores influence plant populations and communities.