19.2 Population Growth and Regulation

Curves show the distribution of people in a population.

Humans and most mammals have a type I survivorship curve, because death occurs in the older years.
Birds have a curve of death at any age.
People are more likely to survive after a certain age, which is why trees have a type III curve.

A variety of methods are used to model population dynamics.

Predicting future changes should be possible with an accurate model.

deterministic equations are used to describe the rate of change in the size of a population.

The first model to describe population growth without limits is called exponential growth.
Limits to reproductive growth that become more intense as the population size increases are introduced in the second model.
Both models give points of comparison, but neither adequately describes natural populations.

The English clergyman Thomas Malthus influenced Charles Darwin in developing his theory of natural selection.

Malthus wrote a book in 1798 in which he stated that populations with abundant natural resources limit further growth by using up their resources.

The best example of rapid growth in organisms is the bacterium.

Prokaryotes are prokaryotes that reproduce.
The division takes about an hour.
If 1000bacteria are placed in a large flask with an abundant supply of nutrients, the number ofbacteria will double within an hour.
Each of the 2000bacteria will divide in an hour.
There should be 8000bacteria in the flask after the third hour.
The This OpenStax book is available for free at http://cnx.org/content/col11487/1.9 population size.
The population would have increased from 1000 to 16 billionbacteria after 24 cycles.

The real world with limited resources is not the same as the one depicted in the bacteria-in-a-flask example.

When a species is introduced into a new environment, it may show rapid growth.
The growth rate is lowered from a maximal rate in which there is no mortality because somebacteria will die during the experiment and not reproduce.

Natural resources are not always available in the real world.

In his description of the struggle for existence, Charles Darwin states that individuals will compete for limited resources.
Natural selection shows that the successful ones are more likely to survive and pass on their success to the next generation.

In the real world, with limited resources, growth cannot continue indefinitely.

When the number of individuals gets large enough, resources will be exhausted and the growth rate will slow down.
In real populations, a growing population often overshoots its carrying capacity, and the death rate increases beyond the birth rate, causing the population size to decline back to the carrying capacity or below it.
Most populations change around the carrying capacity in an undulating fashion.

The carrying capacity is added to the growth rate in the formula used to calculate it.

The carrying capacity available for further growth is the fraction.

The model of population growth is more realistic.

The S-shaped curve has three different sections.
Growth is exponential because there are few individuals.
The growth rate decreases as resources become limited.
The growth rate is off at the carrying capacity of the environment, with little change in population number over time.

Population growth is shown in a J-shaped curve when resources are unlimited.

Populations grow when resources are limited.

When the carrying capacity of the environment is reached, population expansion decreases as resources become scarce.
The curve is S-shaped.

The model assumes that every person in the population will have the same chance for survival.

In animals, important resources include food, water, shelter, and mates, whereas in plants, important resources include water, sunlight, and space to grow.

Some people will be better adapted to their environment than others in the real world.

Populations that are below their carrying capacity may not be affected by competition because resources are plentiful and everyone can get what they need.
The competition increases as the population grows.
Carrying capacity can be reduced by the amount of waste products.

The classical S-shaped curve can be seen when yeast is grown in a test tube.

The population depletes the vitamins that are necessary for growth.
There are variations to the idealized curve in the real world.
sheep and harbor seals are examples of wild populations.
The population size exceeds the carrying capacity for a short time and then falls below it after a while.
As the population fluctuates around its carrying capacity, the population's size continues to change.
The model is confirmed even with this oscillation.

The yeast is visualized using a light microscope.

The seal population would decrease.

The seal population would not change even though the carrying capacity of seals would decrease.

Logistic model of population growth is a simplification of real-world population dynamics.

The model states that the carrying capacity of the environment does not change.
Each year the carrying capacity varies.
The carrying capacity during the winter is lower than it is during the summer in many areas.

Natural events such as earthquakes, volcanoes, and fires can change the environment.

Populations do not usually exist in isolation.

They compete with each other for the same resources in the same environment.
Understanding how a population will grow is important.

There are a variety of ways in which population growth is regulated.

Wildlife biologists want to understand both types because it helps them manage populations and prevent extinction.

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During interspecific competition, the reproductive rates of the species will usually be lower, reducing their populations' rate of growth.
The mortality of its predator is increased by low prey density.

The mortality rate is affected by diseases spreading more rapidly among the members of the population.

Density dependent regulation was studied in a natural experiment with wild donkey populations.

The population on one site was reduced by a population control program, while the population on the other site did not.

The high density plot was denser than the low density plot.
The low density plot saw an increase in donkey density from 1986 to 1987 while the high density plot did not.
The growth rates of the two populations were different because of mortality.
The number of offspring born by each mother was unaffected by density.
The growth rates in the two populations were different because of juvenile mortality caused by the mother's lack of high-quality food in the dense population.

The age-specific mortality rates for wild donkeys are shown in the graph.

There is a shortage of high-quality food in the high-density population that leads to a higher rate of juvenile mortality.

Mortality of a population is caused by many factors that are physical in nature.

The factors include weather, natural disasters, and pollution.
Regardless of how many deer are in the area, an individual deer will be killed in a forest fire.
The chances of survival are the same whether the population density is high or low.
For cold winter weather, the same holds true.

Population regulation in real-life situations is very complex and can be influenced by density and independent factors.

A dense population that suffers from mortality from a density-independent cause will be able to recover differently than a sparse population.

If there are more deer remaining to reproduce, the deer population will recover faster.

A 1916 mural of a mammoth herd from the American Museum of Natural History is one of the three images.

About 10,000 years ago, woolly mammoths went extinct after humans began to colonize North America and northern Eurasia.

We know a lot about the animals that were found frozen in the ice of Siberia.

Climate change and human hunting are thought to have led to their extinction.

Humans hunted these animals through archaeological evidence.

During the last ice age 20,000 years ago, the migration of human hunters across the Bering Strait to North America was an important factor in the extinction of the mammoth.

Stable populations are very complex with many interacting factors.

Humans are also part of nature.
We contributed to a species' decline using primitive hunting technology.