個体数の増加

Population size changes through four key processes: birth rate increases population, death rate decreases it, immigration adds individuals moving into an area, and emigration removes individuals leaving. These factors work together to determine whether a population grows, shrinks, or remains stable in a given habitat.

Carrying capacity is the maximum number of individuals an environment can support. As a population approaches carrying capacity, resources become limited and competition and predation increase. This causes growth to slow from exponential to logistic, producing an S-shaped curve where population size levels off or fluctuates around the carrying capacity.

Exponential growth occurs when resources are unlimited, producing a J-shaped curve where population doubles each generation. Logistic growth is more realistic, starting exponentially but slowing as resources become limited. The population eventually stabilizes near carrying capacity, creating an S-shaped curve characteristic of natural populations.

Predation, disease, and environmental conditions all limit population growth. As populations increase and competition and competition exclusion principle intensifies for shared resources, these factors combine to slow growth. Predators reduce population size, disease spreads more easily in dense populations, and unfavorable environmental conditions stress individuals.

Invasive species and rebounding populations exhibit J-shaped exponential growth because they initially encounter abundant resources and minimal competition. With few natural predators or diseases present, populations increase rapidly at their biotic potential. This unchecked growth continues until environmental resistance eventually limits further expansion.

Population growth rate is proportional to available carrying capacity. As population density increases and more individuals occupy the habitat, the fraction of available carrying capacity decreases. This causes growth rate to decline as resources become more limited, eventually stabilizing the population near carrying capacity.

Island populations experience lower growth rates because islands have reduced access to resources and space compared to mainland habitats. This limitation lowers the carrying capacity on islands, meaning fewer individuals can be supported. Consequently, populations reach their environmental limits more quickly and grow at slower rates.