Final answer:
When a population exceeds carrying capacity, the sharp decline is caused by the supply of resources no longer being able to support the population.
Step-by-step explanation:
When a population exceeds its carrying capacity, there is a sharp decline in that population because the supply of resources can no longer support the population. As resources become scarce, competition for those resources increases, leading to a decrease in the population growth rate. Ultimately, this results in a decline in the population size.