13.1 A growing Human population
Ecology is the study of interactions between organisms and their surroundings.
A population is a group of individuals of the same species that are living in the same area.
Population structure
There are several methods for estimating the population of a species:
- Direct counting: counting the number of trees in a certain area, or using a census for the human population.
- Mark-recapture: often used for birds and other animals. A sample is captured and marked, then released back into the population. A while later another sample is collected, and the percentage of individuals that were previously marked is recorded.
- Sampling small areas, then multiplying by the total area. For example to estimate the number of alligators in the Florida Everglades.
Patterns of dispersion
Individuals can be spaced in three main ways: Fig 13.1
1) clumped eg fish in a school, or cattails growing in a pond.
2) uniform eg birds in a nesting colony, or houses in a subdivision.
3) random eg fruit trees in the rain forest.
Population growth
Under ideal conditions, populations grow exponentially: Fig 13.2. Graph Eventually in nature the population growth is limited (by lack of food, space etc), so the population stops growing: Fig 13.4 Logistic growth. This is an "S" shaped curve. Graph
Before about 1800 human birth rates were high, but death rates were also high, so population growth was very slow. After 1800 medicine slowly improved, so the death rate dropped but birth rate was still high, so many populations grew rapidly. Fig 13.3. This period of time is called the demographic transition. Later, the birth rate falls so you end up with a low birth rate and a low death rate, and population growth drops back to near zero (like in Europe at the moment).
13.2 Limits to Population Growth
The maximum population that can be supported indefinitely in an area is called the carrying capacity.
Two main types of factor can limit growth:
Density dependent factors become more important as the population grows larger eg food supply, disease and pollution. Fig 13.5.
Density-independent factors are not affected by the size of the population eg
freezing weather or drought.
Earth's carrying capacity
When a population approaches carrying capacity the growth rate of the population slows. Fig 13.4. Human population growth rates worldwide were at 2.1% per year in the 1960s, but have now declined to 1.2% per year. However it is still too soon to accurately estimate whether the world population will level off by 2100. Fig 13.6. The fact that death rates are still declining suggests that humans are not yet close to the carrying capacity of the earth. On the other hand when nonrenewable energy sources, such as oil, run out it will be harder to maintain our current food supply.
13.3 The future of the Human Population
Some populations may grow beyond their carrying capacity. As food becomes scarce the poulation crashes, and a boom-then-bust population cycle may continue. Fig 13.7. One example of that was the human population of Easter island, that dropped from about 7000 people down to only a few hundred. Fig 13.8.
Countries that have a rapidly increasing population have an age structure that looks like a pyramid, with a wide base and narrow top. Fig 13.9. Countries where population is pretty stable have a more even age pyramid, which is about the same width the whole way down.
Overall, the world population is still increasing, with a total world population of more than 6.9 billion people:
Census estimates
To find the estimates for different countries, look at:
Census bureau.
Notice that population is growing rapidly in some countries eg Bangladesh, it is growing slowly in others eg US, and it is falling in a few eg Italy (after 2010), Germany and Russia.
Generally as people earn more money, and women have more access to education and jobs the birth rate falls. Fig 13.10. Some European countries are starting to give incentives to people to have more children. Germany.
Last edited November 2011, by David Byres, dbyres@fscj.edu