What factor significantly contributes to high populations of rats and mice?

High reproductive potential is a key factor that significantly contributes to the high populations of rats and mice. These rodents have an impressive ability to reproduce rapidly; a female can have several litters a year, with each litter containing multiple offspring. This means that populations can grow exponentially in a relatively short period under favorable conditions. The capacity for fast reproduction allows these species to quickly recover from population declines and establish themselves in various environments, even when faced with predation or environmental hazards.

In contrast, low food availability would hinder their ability to thrive, high predator presence would decrease their populations, and environmental hazards could lead to mortality, all of which are not conducive to the maintenance of high population numbers. Therefore, it is their high reproductive potential that ensures both rats and mice can maintain and often increase their populations even in challenging circumstances.