28.5
Population Growth
Population size is dynamic, increasing with birth rates and immigration, and decreasing with death rates and emigration. In ideal conditions with unli…
Population growth is an increase in the number of individuals in a population, and many factors can limit this growth.
For example, the size of a puffer fish population changes over time. Individuals move into an area through immigration and leave through emigration. The population also changes when new fish are born, and others die.
If resources are unlimited, a population can grow exponentially over time, producing a J-shaped curve. But in nature, populations face environmental limits on growth. These limits define the habitat’s carrying capacity, or the maximum number of individuals the environment can support.
As the puffer fish population uses more food and space in the habitat, competition between individuals increases because they share the same limited resources. Other factors, such as predation, disease, and environmental conditions, also contribute to limiting population size. As a result, the population approaches carrying capacity, growth slows, and follows a more realistic logistic growth pattern, producing an S-shaped curve.
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Q1: What factors cause population size to change over time?
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.
Q2: What is carrying capacity and how does it affect population growth?
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.
Q3: How does exponential growth differ from logistic growth?
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.
Q4: What environmental factors limit population growth besides resource scarcity?
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.
Q5: Why do invasive species and recovering populations show J-shaped growth curves?
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.
Q6: How does population density affect the rate of population growth?
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.
Q7: Why do island populations grow more slowly than mainland populations?
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.