Population FluctuationsPopulations rarely, if ever, remain static for long periods of

time. What does static mean? What causes population sizes to change?

Four factors cause populations to change:BirthsDeathsImmigrations Emigrations

What is the difference between immigration and emigration?

Births and Immigrations add to a population, whereas Deaths and Emigrations take away from one.

The equation for population change is therefore:Pop. Change = (Births + Immigration) – (Deaths + Emigrations)

Reproductive PatternsThe reproductive patterns

of different species affects how their populations grow.

Some species have lots of young as early and as often as possible.

Do these species tend to take care of their young?

This first reproductive strategy is successful when very few individuals are likely to survive to reproductive maturity (more offspring die than survive to adulthood).

Reproductive PatternOther organisms tend

to have long maturation rates and few young at a time. The parents spend more time taking care of their offspring

Offspring of organisms that follow this second pattern are more likely to mature to adulthood and even reach old age.

Population FluctuationsHow can you explain the population

fluctuations shown in this graph? (Why does the mouse population increase and decrease?)

A more complete storyWhat do you notice about the population of mice in relation to the population of weasels? What does the additional information reveal about the relationship between mice and weasels?

Examine the graph. Write 3 to 5 sentences describing the information about the rabbit population provided by the graph.

Nice Curves!Organisms that employ the

first reproductive pattern often have population growth that resembles this J-curve, where you have exponential growth over a short period of time.

If there were not environmental pressures to stop this kind of growth, some organisms would literally cover the earth.

Why do you think the population increases so quickly over such a short period of time?

In 1 to 2 sentences describe the change in the sparrow population. Generation 1-14. In 3-5 sentences describe the change in the sparrow population from generation 15 to24. What do you think caused the population to decrease then increase again?

Even Nicer Curves!Fortunately, ecosystems have

environmental resistance that prevents exponential growth (J-shape) from continuing indefinitely.

What happens to the population as it reaches the carrying capacity? What is carrying capacity?

A population’s carrying capacity is a measure of the maximum population that can live in an ecosystem and maintain a stable population.

The population stabilizes due to a decreased in the amount of resources available to the increased number of individuals in the population. This is called logistic growth, and is represented by an S-curve

This is what happened to the sparrows!!!

Oftentimes, you’ll see a population fluctuate around the carrying capacity.

In this instance, a population overshot the carrying capacity (more organisms than resources available) so the population dropped due to starvation. Fewer organisms in turn leads to an “abundance” of resources, allowing for a resurge in the population

This cycle can go on for generations

Populations Crash!When a population far

overshoots its carrying capacity, the population crashes.

This is due to reproductive lag, where the death rate hasn’t caught up with the birth rate. The increased number of organisms leads to all the resources being used up.

Other Factors That Contribute to Crashes

Aside from overshooting the carrying capacity, crashes can also be caused by diseases.

This little brown bat has white-nose syndrome, a fungus that kills hibernating bats. These bats are expected to go extinct in less than 10 years, and their declining numbers already cost farmers $3.7 BILLION per year, and their complete disappearance may cost $53 billion per year. Why?

Invasive predators can also contribute to population crashes, as can habitat destruction.

StabilityLong established ecosystems generally have had

more time to stabilize.In stable populations the number of individuals

within the population remain level or fluctuate at regular intervals

Human actions like habitat destruction and introducing non-native species can throw off these balances, sometimes with disastrous consequences.

Do you know any examples?

Don’t Forget About Us…

Humans are not immune to these pressures.

What is suspiciously (or ominously) missing from this graph?