Final answer:
The logistic growth model describes population growth in three phases: exponential growth when resources are abundant, slowed growth as resources get depleted, and stabilization at the carrying capacity. Examples include yeast in a test tube and wildlife populations like sheep and harbor seals. Density-dependent and density-independent factors can interact after events like mass extinctions, influencing a population's recovery and growth dynamics.
Step-by-step explanation:
In the logistic growth model, population expansion follows an S-shaped curve which consists of three distinct phases. Initially, when resources are plentiful, the growth rate is exponential, akin to a J-shaped curve. This is followed by a phase where growth slows down as resources begin to diminish; during this time, the population experiences density-dependent factors such as competition for resources. Finally, the growth stabilizes at a point called the carrying capacity, where population size fluctuates around an equilibrium but does not increase indefinitely.
For example, yeast grown in a test tube follows this classical S-curve by eventually leveling off due to nutrient depletion, which is a density-dependent factor. In natural settings, populations of sheep and harbor seals have been observed to exceed carrying capacity temporarily, then dip below it, exhibiting the characteristic oscillation around the carrying capacity of the logistic growth model.
Density-dependent and density-independent factors can interact in complex ways. A mass extinction event caused by a density-independent factor, such as a volcanic eruption, could drastically reduce a population's size, eliminating the density-dependent limitation of resources temporarily. As the population begins to recover, density-dependent factors would again come into play, first allowing a period of exponential growth followed by a slowdown as the carrying capacity is approached.