|
If a population has abundant space and food, and is protected from predators and disease, then organisms in that population will multiply and the population size will increase. Let's conduct an imaginary investigation to understand how growth under ideal conditions might occur. Suppose you put a single bacterium in a petri dish. Supply it with enough nutrients and incubate the culture with the right amount of heat, moisture, and light. How will the population change over time?
| |
|
Bacteria reproduce by splitting in half. If the bacteria have a doubling time of 20 minutes, then within 20 minutes the first bacterium will divide to produce 2 bacteria. Twenty minutes later, the 2 bacteria will divide to produce 4. After another 20 minutes, there will be 8 bacteria. In another hour, there will be 64 bacteria; and in just one more hour, there will be 512. And in just one day, this colony of bacteria will grow to an astounding size of 4,720,000,000,000,000,000,000. What would happen if this growth pattern continued for several days without slowing down? Bacteria would cover the planet!
| |
|
The figure at right shows a graph with the size of the bacterial population plotted against time. As you can see, the pattern of growth is a J-shaped curve. The J-shaped curve indicates that the population is undergoing exponential (eks-poh-NEN-shul) growth. Exponential growth occurs when the individuals in a population reproduce at a constant rate. At first, the number of individuals in an exponentially growing population increases slowly. Over time, however, the population becomes larger and larger until it approaches an infinitely large size. 
Under ideal conditions with unlimited resources, a population will grow exponentially.
|
Exponential Growth
|
|
With a doubling time of 20 minutes, some bacteria have the fastest rates of reproduction among living things. Populations of other species grow more slowly. For example, a female elephant can produce an infant only every 2 to 4 years, and then the offspring take about 10 years to mature. But as you can see in the graph at right, in the unlikely event that all the offspring of a single pair of elephants survived and reproduced for 750 years, there would be nearly 20 million elephants!
| |