Oak bodyguards

Scientific context

Oaks are among the European tree species associated with the greatest diversity of insects herbivores. Consequences of defoliation can simply consist in growth loss, but massive and repeated defoliations can result in tree mortality. Over the last decades, it became clear that there are large scale graidents in insect damage, being higher in tropical regions and decreasing towards the poles. Yet, despite many research on large scale gradients in insect herbivory, underlying mechanisms are still unclear.

Several hypotheses have been proposed. Some are centred on the herbivores, others on the plant. The activity of herbivorous insects may be reduced in cold regions with long and harsh winters as compared to milder regions. Another hypothesis suggests that in warmer regions plants would be less restricted by climatic conditions for their growth and would be more likely to tolerate insect herbivory. Large scales changes in herbivory could also be driven by mechanisms controlling herbivore performances, namely plant defenses and predation.

Herbivorous insects have many predators that contribute to indirect plant defenses against herbivores. This is referred to as a trophic cascade. The logic is simple: no more predation, less herbivores; Less herbivores, less damage, and therefore more predation, less damage.

Several studies suggest that the intensity of trophic cascades varies along large scale climatic gradients. Other studies show that, on the contrary, the climate has no effect on top-down control of herbivores by predators. The question remains open.