2.07 Population Dynamics

Remember those dandelions we saw in the last lesson? Why were there so many? The field was just full! If we'd stuck around and observed quietly we would probably have noticed a whole slew of animals that were reaping the benefits of that field full of delicious dandelions.

Let's consider how populations--meaning the number of any one species in an area--of organisms respond to changes in limiting factors in their habitat.

Imagine that you went to a park right after school let out for summer. You walked by one single dandelion seed pillow and picked it. Then you decided to blow real hard to see if you could blow all the floating things off and get your wish for 4 months of summer vacation instead of two. Then, as you expected, the wish didn't come true, and as you make a final trip at the end of the summer right before school starts, you notice dandelions everywhere. "Where did they come from?"

At first, there was just one dandelion. You helped spread the seeds, they grew, and their seeds were probably spread the natural way, by wind. Regardless of how the seeds spread, the original plant spread seeds that grew into new dandelions that then spread mores seeds, and so on. Before you know it, the park is full of dandelions.

Organisms like the dandelions have a special way of seed dispersal that helps them to be successful in populating open, grassy areas. Their little seeds have tiny feathers attached so the seeds can travel in the wind. This helps them spread out and grow in areas where there are abundant resources, like sunlight and water.

Let's Chart It!

Because the number of reproducing organisms (Dandelions) is small at first, the increase starts off slow. As more and more seeds start to grow and produce seeds themselves, the growth rate becomes much faster. If we were to graph the growth rate of several different living things (dandelions, flies, mold, birds, and even humans), their chart would be roughly the same and look very similar to the graph to the right. The growth is exponential. It's very fast!

Exponential Growth This growth rate is typical of many organisms just beginning to populate an area.

A World Full of Flowers

Think about it: why isn't the Earth covered in dandelions or over run with roaches or rats or any other organism for that matter? Look at the graph to the right now. Notice that there comes a time when the upward slope of the line representing the number of organisms begins to level off - population growth has slowed down and then stays roughly the same over time. When this happens in an ecosystem, we say that the population has reached its carrying capacity. The maximum number of organism that the ecosystem can support is called the carrying capacity.

Exponential growth followed by the leveling off point due to limiting factors.

Those resources or conditions in the environment that limit the number of organisms that can live in a certain area are called limiting factors. These factors include:

• The requirement for living space
• The available supply of food and water
• Predator/prey relationships
• Ability to reproduce successfully
• Competition for resources between similar species of organisms
• Special physical adaptations.

Awesome Adaptations!

Not only do the numbers of organisms in a population change and vary over time in response to environmental factors, but the characteristics of an organism can change over time in response to slow, small changes in the environment. The organisms that live and survive through environmental change pass their characteristics to their offspring, and the species "adapts" to the environment; though normally these changes, or adaptations, occur very slowly over time. We call this natural selection. If the species, as a group, cannot adapt to the change, extinction may occur.

Consider the Crocodile

Let's look at crocodiles. They were present on the planet at the same time as the dinosaurs. Why did they survive and not the dinosaurs? Here, let me grab my video camera. I have some great crocodile footage from my last eco tour. Let's explore the world of the crocodile to see if we can find some possible clues.

• Open the Video Viewing Guide; be sure to save a copy as an .rtf file.
• Print a copy to work on as you view the video.
• You'll use the information in this lesson and the Video Viewing Guide for the lesson assessment; so be sure to read the lesson material completely and complete the Video Viewing Guide.

Text-only version

Crocodiles (04:05) Science Investigations Life Science: Investigating Animals Ectothermic Crocodiles (01:36)

As you can see, it all comes down to survival of the fittest. Crocodiles have special adaptations that make them deadly and able to survive year after year in good times and bad! Over time, natural selection has produced in the crocodile favorable characteristics for their environment. Their adaptations allow them to survive and pass their traits on to their offspring. The process of natural selection can help us understand how populations evolve, or change over time.

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