Population Growth
Population growth describes changes in population size over time. Learn how births, deaths, immigration, and emigration shape populations, and how exponential and logistic growth models help explain population change.
What Is Population Growth?
Population growth is the change in the number of individuals in a population over time. A population grows when additions to the population exceed losses. Additions can come from births and immigration, while losses occur through deaths and emigration.
Population growth is influenced by environmental conditions, resource availability, competition, predation, disease, and many other factors. Ecologists use population growth models to describe patterns and make predictions about how populations may change.
Population growth depends on the balance between individuals entering a population and individuals leaving it.
Births, Deaths, Immigration, and Emigration
The number of individuals in a population changes through four major processes. Births and immigration add individuals. Deaths and emigration remove individuals.
The Four Processes
These processes work together to determine whether a population increases, decreases, or remains relatively stable.
Exponential Population Growth
Under ideal conditions, a population can grow rapidly when resources are abundant and limiting factors have little effect. This pattern is called exponential growth. As the population becomes larger, the number of individuals being added per unit of time can become larger as well.
Exponential growth produces a characteristic J-shaped curve. The model is useful for understanding the potential of a population, but natural populations cannot continue increasing exponentially forever because resources and environmental conditions eventually impose limits.
Logistic Population Growth
In real ecosystems, populations encounter limits. Food, water, space, shelter, and other resources are not unlimited. As a population becomes larger, limiting factors can slow its growth. This produces a model called logistic growth.
Logistic growth is commonly represented by an S-shaped curve. The population initially grows rapidly, but growth slows as the population approaches the environment's carrying capacity, the maximum population size that the environment can sustain over time under particular conditions.
Exponential growth = J-shaped. Logistic growth = S-shaped and levels near carrying capacity.
Carrying Capacity
Carrying capacity is the largest population size that an environment can support over a particular period under its current conditions. It is not a permanent number. Changes in food, water, habitat, climate, disease, predators, or other limiting factors can cause carrying capacity to change.
Resource availability
Essential resource
Habitat availability
Can reduce survival
Density-Dependent and Density-Independent Factors
Some limiting factors become more important as population density increases. These are called density-dependent factors. Competition, predation, parasitism, and disease are common examples.
Density-independent factors affect populations regardless of their density. Severe storms, droughts, floods, fires, and extreme temperature events can affect populations whether they are large or small.
Density-Dependent
Effects often become stronger as organisms become more crowded.
Examples: competition, disease, predation, parasitism.
Density-Independent
Effects do not depend primarily on how crowded the population is.
Examples: drought, floods, fires, severe storms.
How to Read Population Growth Curves
Population growth graphs usually show population size on the y-axis and time on the x-axis. The shape of the curve provides information about how quickly the population is changing.
Common Misconceptions
If You See This on a Test, Think This
Quick Practice
1. What four processes directly change population size?
2. What type of curve represents exponential population growth?
3. Why does logistic growth slow down?
4. What is carrying capacity?
5. Is competition density-dependent or density-independent?
6. A flood reduces a population regardless of how crowded it was. What type of factor is this?
Key Takeaways
- Population growth is the change in population size over time.
- Births and immigration add individuals; deaths and emigration remove individuals.
- Exponential growth produces a J-shaped curve under ideal conditions.
- Logistic growth produces an S-shaped curve as growth slows near carrying capacity.
- Carrying capacity depends on environmental conditions and limiting factors.
- Density-dependent factors become more important as population density increases.
- Density-independent factors can affect populations regardless of density.
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