How Populations Grow

What is a population?

Since 1930, the world’s population has nearly tripled. What factors determine how fast and how much a population can grow in a span of time?

A population consists of all the individuals of a species that Live together in one place At one time

Every population tends to grow because individuals tend to have “multiple offspring” over their lifetime.

demographics

Eventually, limited resources in a community limit the growth of a population.

Not enough food, water, shelter or medicine will hold down the population growth.

The statistical study of all populations is called demography.

Demographers study the composition of a population; the demographics, and try to predict how the size of the population will change.

Population size

One of the most important features of any population is it’s size.

The number of individuals in a population, the population size, can affect the population’s ability to survive.

Studies have shown that very small populations are the most likely to become extinct.

Random events; such as a fire or flood, endanger small populations more than they endanger larger populations.

Populations and inbreeding

Small populations also tend to experience more inbreeding (breeding with relatives) than larger populations because only relatives are available.

Inbreeding produces a more genetically identical (less diverse) population and is therefore likely to reduce the population’s fitness.

More individuals in an inbred population will be homozygous for harmful recessive genetic traits.

For example, the world population of cheetahs is very small and the individuals are almost genetically identical. Scientists predict a disease could easily wipe out the population.

Population density

A second important feature is it’s density.

Population density is the number of individuals that live in a given area.

If the individuals of a population are few and spaced far apart, they may seldom meet each other, making reproduction of the species a rare event.

Today’s panda population in China is very small and thus new panda births are very rare.

Patterns of population dispersion

The third feature of a population is the way the individuals are arranged in space. This feature is called dispersion.

Three main patterns of dispersion are possible within a population. Random dispersion Even dispersion (or regular dispersion) Clumped dispersion

Patterns of population dispersion

Regularly spaced individuals have an even spacing between them, equal distances from each other.

Randomly spaced individuals have a spacing determined by chance.

Clumped individuals are in bunches or clusters, located in groupings close to each other.

Population models

When demographers want to predict how a population will grow, they make a model of the population.

A population model is a hypothetical population that attempts to exhibit the key characteristics of a “real” population.

By making a change in the model, and observing the outcome, demographers might predict what might happen in a real population.

To learn how demographers might study a population, we will look at a model of population growth in three stages of complexity

Population models: growth rate

Growth rate:

A population grows when more individuals are born than die in a given period.

A simple population model describes the rate of population growth as the difference between the birth rate and the death rate.

Exponential growth curve

When population growth is plotted against time on a graph, the population growth curve resembles a J-shaped curve and is called an exponential growth curve.

An exponential growth curve is a curve in which the rate of population growth stays the same.

As a result, the population growth increases steadily in an unrestricted growth.

Exponential growth curve

Figure (a) at left shows an exponential growth curve.

A single bacteria cell that divides every thirty minutes will produce more than 1 million bacteria after only 10 hours. Every thirty minutes the population doubles.

However, most populations do not grow unchecked. Their growth is limited by predators, disease, food supply and availability of other resources.

Eventually growth slows and a population stabilizes. The population size that an environment can sustain is called the carrying capacity.

Exponential growth curve

As a population grows, resources such as food supply eventually become depleted. Their may not be enough food supply to get all the animals through a winter in a given area.

When this happens the rate of population acceleration slows. It has reached the point of maximum growth.

Resources such as food or water are called density-dependant factors because the rate at which they become depleted depends on the population density of the area.

Exponential growth curve

The population model that takes into account the declining resources available to a population is called the logistic model of population growth.

The logistic model of population growth is a population model in which exponential growth (constant doubling of the population) is limited by a density dependant factor (not enough food or water)

The logistic model takes into account real world factors such as how much food is available in an area to feed a population.

Logistic growth curve

Let us look at a deer population in a 100 acre forest as an example.

When the deer population is small, there is enough food for all easily and the population grows exponentially, unchecked by any factors. There are far more births than deaths.

When the deer hit the point of maximum growth, they have used up much of their food and the birth rate slows as the death rate starts to rise.

When the death rate equals the birth rate, the deer population has hit it’s carrying capacity. There is enough food for only so many deer to exist and the weak and old die off and are replaced by just enough young to keep the population in balance. This is the carrying capacity for this population.

Logistic growth curve

The logistic model of population growth provides excellent estimates for how populations grow in nature.

Competition for food, shelter, mates, and limited resources tends to increase as a population approaches it’s carrying capacity. The accumulation of wastes also increases.

Demographers try to make logistic models based on current population sizes and resources available to predict how world populations will increase.

Growth patterns in real populations

Many species of plants and insects reproduce quite rapidly.

Their growth is limited not by density-dependant factors but by environmental conditions known as density-independant factors.

Weather and climate are the most important density-dependant factors.

For example, mosquito populations increase in the summer when it is warm but decrease in the winter when it becomes cold.

Growth patterns in real populations

The growth of many bacteria, plants and insects is often described by an exponential growth model.

The growth of slower growing organisms, such as humans and other mammals, is best described by a logistic growth model.

Most species have a pattern of population growth somewhere between the two models.

Rapidly growing populations

Many species, including bacteria, some plants, and many insects, are found in rapidly changing environments.

Such species, called r-strategists, grow exponentially when environmental conditions allow.

This strategy allows temporarily large populations. When environmental conditions worsen, the population size drops quickly.

In general, r-strategists have a short life span.

Rapidly growing populations

R-strategists reproduce early in life and have many offspring each time they reproduce.

Their offspring are small, mature rapidly with little or no care from the parents.

Mosquito populations are a prime example of an r-strategist species.

Rapidly growing populations

Organisms that grow slowly, such as whales, often have small population sizes.

These species are called K-strategists because their population density is usually near the carrying capacity of their environment.

K-strategists are characterized by a long life span, few young, a slow maturing process, and reproduction later in life.

K-strategists often provide extensive care for their young and tend to live in stable environments.

Many endangered species such as tigers, gorillas, and whales are K-strategists.