Which of the following is NOT a major factor affecting population size?

The option indicated as the correct answer is not traditionally viewed as a major direct factor affecting population size in the same way that the others do. Predation has a clear impact on population dynamics, as it directly reduces the numbers of prey species. Available resources, such as food, water, and shelter, are fundamental for the survival and reproduction of species, heavily influencing population growth rates. Pollution, while it can be complex, often leads to significant negative impacts on population size by harming the health of organisms or degrading their habitats.

Climate change, though, is typically considered an overarching factor that indirectly influences population sizes through alterations in habitats and available resources, rather than an immediate population control factor like the others. While it can have profound effects over time, it may not be seen as a direct cause of immediate fluctuations in population sizes when compared to more direct interactions like predation or resource availability.