Plant Inducible Responses to Damage: Evolution and Ecological Implications

Abstract Inducible plant responses to damage, i.e. inducible defence responses and compensatory growth responses, are theoretically examined in an evolutionary and ecological context. Inducible defences are generally considered to be beneficial under conditions of low risk of herbivory and when initial damage is moderate. This study indicate that under these circumstances, localized defence responses, systemic defences, or even interplant defence systems, might be optimal depending on type of herbivore as well as the efficiency of the defence. The effects of inducible plant defences on the herbivvore population is also examined and one cannot exclude that induced defences may cause fluctuations in herbivore density. One assumed mechanism behind compensation for lost tissue is the presence of dormant meristems which are activated after breaking of apical dominance. This trait may well have evolved as a result of competition for light, but breaking of apical dominance alone is not enough to explain observed overcompensation, i.e. the situation when damaged plants have higher reproductive output than undamaged ones, which has been proposed. Some other selective force is required and it seems as if the evolution of a good compensatory capacity is facilitated by damage that occurs with a high probability only once per season and, as suggested in this thesis, is predictable in time. In this thesis it is also suggested that a compensatory capacity may, if damage is continuous, increase the density of herbivores but may still be advantageous in competition with other plant strategies. Overcompensation is also discussed in terms of mutualistic and antagonistic coevolution and it is suggested that overcompensating plants and herbivores have not necessarily evolved into a mutualism.