Resistance and toleranceare two plant defence strategies against natural enemies like herbivores. Substantialevidence shows that plants allocate resources simultaneously to both toleranceand resistance. Both strategies are considered adaptive and are assumed toimply fitness costs to a plant’s phenotype. As a result, the evolution andmaintenance of each defensive strategy in plant populations should be affectedby the relative fitness costs and benefits that each strategy involves. Authorsreviewed different models for the joint evolution of tolerance and resistance,to test the hypothesis whether a mixed pattern of defence allocation totolerance and resistance constitute evolutionary stable phenotype equilibrium.
Defensive traitsexpressed by plants to herbivory damage may also serve other functions andhence be selected by agents other than herbivores. Resistant traits are often correlatedwith other fitness-enhancing traits, which represent a constraint to theevolution of increasing levels of resistance. This cost of allocating resourcesto defence may be expressed as a direct reduction in fitness in anherbivore-free environment, or as an indirect effect of altering the phenotypicexpression of other traits that also increase fitness. Changes in the physicaland biotic environment can also affect the phenotypic expression of plantdefensive traits. Putative traits of tolerance are often associated with plantgrowth. Thus, any selective agent acting on growth could select potentially toincrease or decrease tolerance. To date, a combinationof resistance and tolerance was favoured, however population differences in thecost/benefit ratio for tolerance and resistance can vary, promoting the evolutionof either tolerance or resistance but not both.
In conclusion, selective forcesconstraining the evolution of maximal resistance or tolerance that leads to themixed pattern of allocation to both strategies appear to be evolutionarilyunstable.I will cite this information to supportmy argument that there is a correlation between two strategies. By exploringthe strategies plant use to defend themselves and their links with other traitsand environmental conditions, I will address the ecological factors that mayconstrain the simultaneous evolution of resistance and tolerance.