But Turchin's model can only paint the broader picture and cannot pinpoint how bad things can get and what precisely triggers a collapse. Mathematician Safa Motesharrei also applied predator-prey models to human society, with the upper-class and lower-class being the two different types of "predators" and natural resources being the "prey." He found that either extreme inequality or resource depletion facilitates a collapse. But a collapse is only irreversible if a society experiences both at the same time, as they "fuel each other."