Final answer:
A decrease in available resources or an increase in predation can lead to a decrease in the size of the greenleaf population.
Step-by-step explanation:
The change that led to the decrease in the size of the greenleaf population was likely a decrease in available resources (such as food or water) or an increase in predation. For example, if the greenleaf population relied on a certain type of plant for food and that plant became scarce, it would lead to a decrease in the greenleaf population size because the population wouldn't have enough food to sustain itself. This change would cause competition among the greenleaf individuals and could potentially lead to a decrease in overall population size.
If the degree of predation increased, it would also lead to a decrease in the greenleaf population size. For instance, if there were more predators preying on the greenleaf individuals, it would result in a higher mortality rate and fewer individuals surviving to reproduce and sustain the population.