Population Ecology Part Two: Population Growth Developed by Steven Taylor Wichmanowski based in part on Pearson Environmental Science by Jay Withgott

Remember that a population is the number of organisms of a particular species that live in one place at one time. As time passes, old organisms die, new organisms are born and some organisms migrate to other locations. All of these changes lead to changes in populations.

Populations change size according to the relative rates of birth/death and immigration/emigration: If the birth rate is greater than the death rate, a population will increase. If the death rate is greater than the birth rate, a population will decrease. If the rate of immigration is greater than the rate of emigration, a population will increase. If the rate of emigration is greater than the rate of immigration, a population will decrease.

If there are no limiting factors to control a population’s growth, it should grow exponentially. exponential growth: the phenomenon where a population grows at a fixed rate and, therefore, the larger a population gets, the faster it grows.

Here’s a graph of a population experiencing exponential growth:

At the early stages of an exponential growth curve, the population grows slowly.

As time goes by and the population gets bigger, it begins to grow at a very fast rate.

Populations sometimes grow exponentially, but only until the population reaches its carrying capacity. Carrying capacity: the maximum population that an ecosystem can support. Remember that population is a measure of individuals of one species. In any ecosystem, different species will have different carrying capacities.

In reality, exponential growth can only occur for a limited time because of finite resources that a population depends on for survival. Logistic growth: the pattern where the growth rate slows and stabilizes at a carrying capacity.

An Exponential growth pattern is sometimes called a “J curve” a Logistic growth pattern is then called an “S curve”

Ideally, as a population nears its carrying capacity, the growth rate slows until there is zero growth and the population stabilizes.   This pattern repeats itself as the population fluctuates above and below the carrying capacity.

In reality, populations often overshoot their carrying capacity and the population experiences a “dieback” which usually results in the population falling below the carrying capacity.

This pattern repeats itself as the population fluctuates above and below the carrying capacity.

Here’s a graph showing a population that has reached its carrying capacity.

Notice that a population that has reached its carrying capacity still fluctuates, but averages out at the carrying capacity.

Populations below the carrying capacity tend to increase. Populations above the carrying capacity tend to decrease.

At times, a population will go so far past their carrying capacity that they deplete the resources on which they depend on for survival. When this happens, the population crashes.

If the population survives the crash, it may stabilize at a new carrying capacity much lower than the original carrying capacity.

Carrying capacity is usually determined by one or more limiting factors. A limiting factor is some kind of environmental pressure that restrains the exponential growth of populations. Examples of limiting factors: availability of food availability of water availability of nutrients presence of predators presence of mates available habitat disease

Limiting Factors and Carrying Capacity

(e.g., disease, competition, predation, resource availability) Some limiting factors are related to population density. Density dependent factors are limiting factors whose influence depends on population density (e.g., disease, competition, predation, resource availability) 

Density independent factors are limiting factors that do not depend on population density and usually relate to natural disasters. (e.g., a tsunami would significantly affect the populations of a small coastal village as well as a large coastal city)

One important factor in understanding population growth is a species’ biotic potential. Biotic potential is the maximum ability to produce offspring in ideal conditions. (i.e., how many babies can you have?)

K-selection versus r-selection Ecologists describe low and high biotic potentials with the categories of K-selected and r-selected species. r-selected species tend to have high birth rates and high death rates.   They produce many offspring, have relatively small body size, short life- spans, and provide relatively little paternal care.

K-selection versus r-selection K-selected species tend to have lower birth rates, but the amount of parental care provided tends to result in lower death rates as well. Compared to r-selected species, they produce few offspring, have large bodies, long life spans, and invest significant time and energy to parental care.

K-selection versus r-selection Populations of K-selected species grow slowly and tend to fluctuate near their carrying capacity. Populations of r-selected species experience rapid bursts of exponential growth followed by rapid decline and thus tend to follow a “rise and crash” population pattern. The difference between between K and r strategies can be thought of as “Quality vs. Quantity”

This graph shows that the world human population is currently experiencing exponential growth.

The Earth’s human carrying capacity is not known, but is estimated to be around 10 billion.

The current human population is approximately 7 billion The current human population is approximately 7 billion. What limiting factors might determine the human carrying capacity of the Earth?

Historically, technology and agriculture have helped push the Earth’s human carrying capacity higher and higher. It is unknown what will happen when the human population reaches the Earth’s ultimate carrying capacity. We too might suffer some loss, and stabilize. Or we may see a cataclysmic population crash…

What do you think?