What conditions can cause a population to decrease?

if the population is too high and food is scarce.

high population attract predators.

high competition for food.

mutations.

A reduction over time in a region's population can be caused by sudden adverse events such as outbursts of infectious disease, famine, and war or by long-term trends, for example sub-replacement fertility, persistently low birth rates, high mortality rates, and continued emigration.

helicase, i think thats it