Natural selection could not occur without:
a. genetic variation in species
b. environmental changes
c. competition for unlimited resources
d. gradual warming of the earth

The rate that would change would be the  searching efficiency/attack rate. there are only two other rates in this model: the  growth rate of prey, and the  predator or consumer mortality rate.the  searching efficiency/attack rate is what indicates how effectively can a predator capture its prey whether in the searching process or actual capturing/attacking process. 

Other organisms can change their behavior to take the place of the plants in the ecosystem.