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How do species become extinct and what can humans do to prevent this loss of biodiversity?

We lose biodiversity when species become extinct. In this section you will explore some of the causes of extinction and investigate some ways conservation biologists are helping to preserve biodiversity.

“. . . twilight is a time for sharing-and a time for remembering-remembering the things of beauty wasted by our careless hands-our frequent disregard of other living things-the many songs unheard because we would not listen-listen tonight with all the wisdom of your spirit-listen too with all the compassion of your heart-lest there come another night-when there is only silence-a great and total silence-”

Winston Abbott

“Have You Heard the Cricket Song?”

quoted in The Earth Speaks

Disappearing Acts: Extinction

A first step in preventing the loss of biodiversity is to figure out how we are losing biodiversity. For this reason, conservation biologists study species that are about to become extinct. They also try to determine what happened to a species that has already become extinct. Remember that a species is said to be extinct when all individuals of that species have died.

The first step biologists take in analyzing how a species becomes extinct is to identify the ultimate and proximate causes of extinction. Those two words-ultimate and proximate-may be unfamiliar to you. But they are fairly simple ideas. An ultimate cause is an action that starts a series of events. In this case, the events lead up to the extinction of a species. A proximate cause is an action that happens right before an event that causes the event to happen at that moment. To understand the difference between a proximate and ultimate cause, think about this example: Your little brother is crying. The proximate cause is that you yelled at him. However, the ultimate cause is that he stole your candy bar, which led up to you yelling at him. So is your brother crying because you yelled or because he took your candy bar without permission? The answer is both!

Did You Know?

Rain forests are one of the most biologically diverse, natural communities in the world. But they are being destroyed at a rate of 74,000 acres per day!

The story of the brown pelican demonstrates a good example of ultimate and proximate causes of a near-extinction. The brown pelican almost became extinct in d1e 1970s. Biologists discovered that the pelicans were laying eggs that had extremely thin shells. The shells were so soft that they crushed under the weight of the mother pelican. The proximate cause for the pelican's near-extinction was that the mothers were crushing the eggs during incubation before any chicks could be hatched.

Did You Know?

You can get rid of insect pests naturally without using pesticides such as DDT. Instead, you can use insects, such as ladybugs, which are natural predators of the pests you're trying to get rid of.

The biologists then had to track down why the pelicans were laying eggs with such thin shells. They found out that the pelicans' food was contaminated with DDT, a pesticide used by farmers. DDT stopped the production of the calcium needed to make the eggshells thick. As a result, the pelicans laid eggs with shells that were too thin. Therefore, the ultimate cause for the near-extinction of the pelican was the use of DDT by farmers!

Apparently farmers sprayed DDT on their fields and it washed off the fields into ponds and lakes. Eventually the DDT ended up in the ocean. There, producers called phytoplankton (FI-toh-plank-tun) absorbed the DDT. The phytoplankton were eaten by herbivores called zooplankton (ZOH-oh-plank-tun), which were eaten by small fish. The little fish were eaten by bigger fish, which were eventually eaten by the pelicans. The DDT simply traveled up the food chain!

Luckily this story has a happy ending. The federal government banned the use of DDT. Since farmers no longer used DDT, it no longer washed off the fields and ended up in the ocean. Eventually DDT no longer contaminated the fish the pelicans ate. The pelicans then began laying eggs with thicker shells. More pelican chicks were hatched. Finally, the population increased. The biologists had to discover the ultimate cause of the population decline-DDT use-to solve the problem of the proximate cause-thin eggshells.

What Do You Think?

The federal government banned the use of DDT in the United States. But the government did not ban its manufacture or exportation. Several U.S. companies still make and sell DDT to farmers in other countries. Should the U.S. ban the manufacture and sale of DDT to people in other countries or should those people have the right to buy DDT if it helps them grow food?

Figure 12.1 DDT worked its way through the food chain and almost caused the extinction of brown pelicans and other fish-eating birds.

When a species such as the brown pelican becomes endangered, how much money, time, and effort do you think humans should spend trying to save it from extinction? Do you feel it is as important for governments to spend money on endangered species as on health care, homeless people, education, military and weapons, roads and transportation? Why or why not?

Designing a Nature Reserve

One way that conservation biologists preserve biodiversity is by creating protected areas called nature reserves. Nature reserves are one strategy for keeping species from becoming extinct. Establishing nature reserves makes sure that the habitat of a species will not be changed or disturbed by humans for the foreseeable future.

You can create a simple wildlife refuge in your backyard. Find out from a local nursery which plants will attract local animals and help provide a habitat for them.

Designing a reserve can be a tricky task. Sometimes nature reserves are too small. Sometimes they are inappropriately shaped. Sometimes they have to be managed to keep conditions suitable for the different species that live there. Sometimes things happen outside of the reserve, such as acid rain or global warming. These factors can't be controlled within the reserve. The reserve's conservation biologists, planners, and managers do their best to consider all these factors and more.

Reserve planners ask themselves a lot of questions when they are designing a reserve. These are some of the most important questions planners ask, questions to which they seek answers:

  • What are we trying to protect? Are we protecting a particular species? A community? A population of a species? Asking these questions helps planners create a goal for their reserve.)
  • What resources does the species, population, or community need? What types of food, shelter, and plants are required?
  • How big of a population do we need to preserve the species? (In general, the bigger the population, the better chance there will be for survival. Very small populations tend to become extinct more quickly.)
  • What is the minimum area required by a population of a specific size? (Reserve planners know that all organisms need a certain amount of space and they have to decide how much space will be required.)
  • Are there any species upon which the species we are trying to save depends? Are any species interactions crucial for its survival?
  • What is happening on neighboring lands and how does it affect the reserve?
  • How much will it all cost? (Often the amount of money a planner has to spend is the biggest limitation for reserve planners and designers.)
  • Where will we obtain the money to support the reserve?
  • Will the reserve need active management or will it be able to take care of itself?
  • Last of all, do we need one large reserve or several small ones? Is it better to put all of the eggs in one basket (so to speak)?

After planners and designers have answered all of these questions, they begin their planning phase. They discuss locations and map out the reserve. They decide issues such as the reserve's future management and budgets. Then, they purchase the land. Their hope is that they designed a reserve that will successfully preserve some facet of biological diversity!

Activity 12-1: Design a Nature Reserve


How can we prevent species from becoming endangered? How can we keep endangered species from becoming extinct? One way is to design and maintain a nature reserve. In this activity you design a nature reserve for the Kirtland's warbler, an endangered songbird.


  • Butcher paper
  • Marking pens or colored pencils
  • Resource 1
  • Resource 2


Step 1 Imagine that you are a conservation biologist for the Fish and Wildlife Service. You have been asked to make recommendations for the new nature reserve for the Kirtland's warbler. Your job is to design a plan for the set-up and continuing management of the reserve. Your plan must include:

  • your recommendation for the kind of habitat in which you will place the reserve so that the species will survive and reproduce in a healthy population. (Include food, water, space, and nesting requirements.)
  • an explanation of the threats to this species. (Include human activities as well as natural events that threaten this species.)
  • your recommendations for the management practices (strategies) you will use to protect the reserve's habitat, keep the population of the warblers at a stable level, and any other long-range plans for maintenance of the reserve.

Step 2 Prepare a plan for the nature reserve and submit it to the Nature Conservancy, which is the organization that will buy the land for the reserve. Prepare a presentation to give to the Nature Conservancy board members (your teacher and classmates). Include drawings, maps, and other graphics if needed.

Review Questions

  1. What is the difference between a proximate and an ultimate cause? How does this apply to the causes of extinction of a species?
  2. What are nature reserves? How do they help prevent the loss of biodiversity?
  3. If the Department of Parks put a wire fence around an empty lot in your town and claimed that it was a reserve for grizzly bears, would you say that the reserve designers had done a good job? Why or why not?

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Date Created:

Feb 23, 2012

Last Modified:

Jul 16, 2014
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