Final answer:
An S-shaped population growth curve describes logistic growth, where the population size levels off at the environment's carrying capacity after an initial period of exponential growth. Option a is correct.
Step-by-step explanation:
Logistic growth occurs when a population's expansion decreases as resources become scarce, and it levels off when the environment's carrying capacity is reached. This pattern of growth is represented by an S-shaped curve, which is distinct from the J-shaped curve of exponential growth, where the population growth rate continues to increase when resources are plentiful.
Examples of logistic growth include yeast in a test tube or natural populations in an environment with limited resources. Initially, in logistic growth, the population experiences exponential growth with few individuals and ample resources.
However, as the population size increases, the resources start to become limited, which slows down the growth rate. Finally, the population growth levels off at the carrying capacity of the environment because the birth rate and death rate become equal, stabilizing the population size.
Hence, the correct option is a.