Final answer:
Fluctuations at carrying capacity occur due to changing environmental conditions, natural events that affect resources, and competition with other species for resources, causing population sizes to oscillate around the carrying capacity.
Step-by-step explanation:
We see fluctuations at carrying capacity due to a variety of environmental and biological factors that affect the resources available to a population. The logistic model of population growth reflects an idealized curve, yet it is an oversimplification. In reality, the carrying capacity is not static; it can change annually with varying seasonal conditions, and can be impacted by natural events like earthquakes, volcanoes, and fires.
This variation in carrying capacity makes real population sizes fluctuate above and below this threshold. Additionally, interspecific competition, wherein different species vie for the same resources, can also affect a population's growth and its stabilization around the carrying capacity.