Natural Selection Image Map

The Wars of Succession

Have you ever wondered where forests come from? Within an ecological environment, populations of different species will compete with each other. Given enough time, this competition will change the community structure. This natural process is known as succession. If you had a time machine, it would be a simple matter to see succession in action. Just find a small, grass field and visit it every decade for about 100 years. If left undisturbed by humans, the field would gradually become populated with shrubs, and then trees would begin to appear.

Eventually, assuming there was enough rain, the presence of many tall trees come to characterize the oldest forest communities. From your knowledge of ecosystems, why would trees replace smaller plants like shrubs and grasses, assuming there is enough rain? 

Another example familiar to farm children is that if cattle overgraze pasture, the pasture will eventually become dominated by brush and brambles. Why do you suppose that is? Hint: cows won't eat everything.

Do all grasslands become forests eventually? Not necessarily! Remember, a grassland or forest is nothing more than a collection of various populations and the growth of populations is limited by several factors. Water and sunlight are the most important resources for plant populations. Go here, and take a look the distribution of plant life in Texas. Now, you should have an idea of where the most resources are available. To check yourself, take a look at the Texas Average Rainfall Map.

Predators and Prey

Photo copyright Ross Warner


The graceful cheetah silently stalks the group of gazelles. Its small, rounded head slowly orients to a smaller member. Using the surrounding terrain to keep it hidden from view, the cheetah edges as close as possible to the unaware antelopes. Suddenly, the gazelle notices the unwelcome visitor and attempts to escape the impending danger. But in a few seconds, the cheetah accelerates to 70 miles per hour and closes the gap that exists between its next meal. Almost before it begins, it’s over. 

Regardless of body mass or numbers in a population, a given mass of prey will support a given mass of predator. 

If the numbers of prey go down, the numbers of predators go down. On the other hand, if the numbers of predators go down, the numbers of prey will go up.

Is this how you envision the relationships between predator and a prey? True, this is the usual example of this process, but it is a specific form of predation called carnivory. Let us define "predation" to include any attack, either direct or indirect, with or without eating the prey, on another organism. Can you think of some less obvious instances of predation?

If you’ve ever had a pet dog, you may not think that it experiences any danger from predators. Not true. Most domestic dogs suffer the attacks of numerous, almost invisible predators -- FLEAS! This is a special type of predation called parasitism in which the parasite (a flea) feeds on the host (a dog). Although this can be very harmful to the host, death seldom results. 

Of course, you usually think of dogs as predators, hunting rabbits or other small mammals, but in the previous paragraph, you saw how dogs are also preyed upon. Because of the many relationships in an ecosystem, many organisms have more than one role to perform. 


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