<p>Turbulence causes anomalous transport of plasma and thus limits achieving high temperature and density of magnetically confined fusion plasmas. Recently, fast-ion-driven Alfvénic instabilities were found to influence plasma thermal confinement by regulating turbulence. However, both favourable and unfavourable effects coming from various mechanisms have been reported from experiments and simulations to date. Here, we report a key parameter (ratio between growth rate of Alfvénic instability and that of microturbulence) which can characterise changes in heat transport of thermal ions due to nonlinear interaction between the fast-ion-driven Alfvénic instabilities and microturbulence in tokamak plasmas. The transition from transport enhancement to reduction occurs at a particular value of the parameter as it gets higher, and is reminiscent of the bifurcation responsible for transport-barrier formation. This simple characterisation successfully covers the trend of experimental results, and can provide a useful guideline for realising high performance confinement of burning plasmas in which fast ions are abundant.</p>