What is a Population?

A population is a group of organisms of the same species, living in the same place, at the same time, which can successfully interbreed. For example, the number of Rana pipiens (green grass frogs) in a local park, or the number of bacteria under a microscope are each populations.

There are three important characteristics of populations: geographic distribution, density, and growth rate.

How Do Populations Grow?

There are two types of growth we typically see in nature. One is exponential growth and the other is logistic growth. Read below to compare the two:

Stop and Think: Below is a graph of the human population. Examine it and try to answer the questions that follow:

Are humans displaying exponential or logistic growth thus far?  (Answer: yes)

The Black Plague killed 25 million people around the year 1350. Can you find this on the graph?

What might account for some of the other dips in the graph?  (Answer: wars, disease)

How much longer will earth’s resources support a rapidly doubling population?

What Limits Growth at Carrying Capacity? Limiting Factors

So if a population has reached carrying capacity and slowed population growth – what might be the cause? Things that slow growth are called limiting factors; they may reduce the birth rate and/or increase the death rate. Limiting factors can be placed in one of two categories:

Limiting Factor	Description	Examples
Density-Dependent Limiting Factor	Limits population based on population size. These factors have the most effect when a population has reached or exceeded its carrying capacity.	competition predation parasitism disease
Density-Independent Limiting Factor	Limits population regardless of population size.	unusual weather natural disasters seasonal cycles human disturbance (damming rivers or clear cutting forests)

Species Have Different Life Strategies

By now you have probably realized that it’s a tough world out there for a species when it comes to dealing with the environment: Eat or be eaten, try not to starve, be outcompeted, or even infested with parasites and disease. Then you have to worry about passing on your genes before you die!

Although organisms don’t necessarily “think” about how to best survive in a changing environment, there are two patterns, or strategies seen commonly in nature: k-strategists and r-strategists.

r-strategist	k-strategist
live in chaotic or unstable environments invest little in many young (more young, many die- no caretaking) short life span Examples: insects, fish, spiders	live in stable environoments invest more in fewer young (few offspring- more caretaking) long life span Examples: birds, cows, elephants

If you’re pretty perceptive, you’ve noticed that the names of each strategy relate to each type of growth curve. Nice work! R-strategists are so named because they focus on r, the growth rate: rapid reproduction in unstable environments, knowing that many young will die. K-strategists focus on reaching K the carrying capacity without a population crash: make it to old age and focus on high quality young.