Population ecology examines how groups of organisms of the same species change over time and space. Population density measures the number of individuals of a species per unit area or volume of their habitat. Each environment has a carrying capacity, denoted K, which is the maximum population size that the environment can sustain indefinitely given the available resources such as food, water, shelter, and space. When populations approach this limit, growth slows and stabilizes.
Under ideal conditions with unlimited resources, populations can undergo exponential growth, increasing at a constant per-capita rate and producing a characteristic J-shaped curve. In reality, populations more often experience logistic growth, which slows as the population approaches carrying capacity and produces an S-shaped, or sigmoid, curve. The difference between exponential and logistic growth reflects the influence of limiting factors that become more intense as populations expand.
Limiting factors are classified as either density-dependent or density-independent. Density-dependent factors, such as competition, predation, disease, and parasitism, intensify as population density increases. Density-independent factors, such as natural disasters, extreme weather, and pollution, affect population size regardless of density. Species have evolved different reproductive strategies in response to these pressures. R-selected species are characterized by high reproductive rates, small body size, short lifespan, and little parental care, allowing them to thrive in unstable environments. K-selected species, in contrast, have low reproductive rates, large body size, long lifespan, and extensive parental care, enabling them to thrive near carrying capacity.