NATURAL CONTROLS OF POPULATIONS: 3 CASE STUDIES

Size: px

Start display at page:

Download "NATURAL CONTROLS OF POPULATIONS: 3 CASE STUDIES"

Caitlin Richardson
5 years ago
Views:

the amount of disease and parasites infecting the population Because of these limitations a certain area can only support a certain size population of each type of organism.

1 NATURAL CNTRL F PPULATIN: 3 CA TUDI Population size in a community is regulated in many ways. ome of the controls acting on populations are: the amount of food and water available to feed the population the amount of sites available to support raising young the size of the predator population the amount of disease and parasites infecting the population Because of these limitations a certain area can only support a certain size population of each type of organism. The population size of each creature that the environment can support is called the carrying capacity of that community. This limit represents how many of a certain species can survive in that area without destroying the area for the future. In this activity, we will study three real life examples of populations, their natural controls, and the carrying capacity of their community. Read each case study, and then answer the questions on your answer sheet. CA TUDY #1: TH RINDR F PRIBILF ILAND Background Information: In 1911, 25 reindeer 4 males and 21 females were introduced onto t. Paul Island, one of the Pribilof Islands in the Bering ea near Alaska. t. Paul is approximately 106 km 2 (41 mi 2 ) in size and is more than 323 km (200 miles) from the mainland. No hunting was allowed on this island, nor were there any predators. By 1950, 8 reindeer remained. tudy the graph illustrating the change in the population of reindeer on t. Paul Island. No one counted reindeer during 1941 and 1942 due to World War II. This gap in data is represented by a dotted line. The Reindeer Population of Pribilof Island P P U L A T I N I Z YAR

2 CA TUDY #2: TH KAIBAB DR In the early 1900s, the Kaibab plateau, north of the Grand Canyon in Arizona, supported a population of about 4000 deer on over 700,000 acres. Predators, such as coyotes, wolves, and pumas helped to keep the deer population in check. It was estimated at the time that the plateau had a carrying capacity of about 30,000 deer, so there seemed to be plenty of food for the population that existed. n November 28th, 1906, President Theodore Roosevelt created the Grand Canyon National Game Preserve to protect the "finest deer herd in America." In an effort to increase the deer population for tourists, the Forest ervice tried to exterminate the predators of the deer. Between 1907 and 1939, 816 mountain lions, 20 wolves, 7388 coyotes and more than 500 bobcats were killed. With the successful removal of many of the predators, the deer herd increased dramatically. But signs that the deer population was out of control began to appear as early as 1920 and the range (the deer habitat) was beginning to deteriorate rapidly. The food supply was being over-grazed by the high population of deer. Trees and grass were being damaged to the point they couldn t recover for next year s growing season. oon there wasn t enough food to feed the growing population. By 1923, the deer were reported to be on the verge of starvation. Range conditions were described as "deplorable." The Kaibab Deer Investigating Committee recommended that the number of deer be cut in half as quickly as possible. Hunting was allowed, and during the fall of 1924, 675 deer were killed by hunters. However, these deer represented only one-tenth of the number of deer that had been born that spring. ver the next two winters, it is estimated that 60,000 deer starved to death. Today, the Arizona Game Commission carefully manages the Kaibab area. Hunting permits are issued to keep the deer in balance with their range. Predators were re-introduced and are now protected to help keep herds in balance with food supplies. Tragic winter losses are now reduced by keeping the number of deer near the carrying capacity of the range. Table 1 gives the historical data for the Kaibab deer population. Graph the data on your answer sheet and then answer the summary questions for this case. Table 1: Kaibab Deer Population from Year Population ize Comments ,000 Predators maintain control of the deer population ,000 Predator removal program begins , , , , ,000 40,000 deer die of starvation and disease , , , , , , , ,000

3 CA TUDY #3: TH M F IL RYAL Isle Royale is a forested island in the middle of Lake uperior in Michigan. The island is about 50 miles long and about 8 miles wide and is a protected forest reserve. A herd of moose lives on the island, but there were no moose-predators, like wolves, living on the island with them. In 1970 the moose population of the island was about 2000 animals. Although the island had excellent vegetation for feeding, the food supply obviously had limits. Thus the forest management personnel feared that overgrazing might lead to mass starvation. ince the area was too remote for hunters, the wildlife service decided to bring in natural predators to control the moose population. It was hoped that natural predation would keep the moose population from becoming too large and also increase the moose quality (or health), as predators often eliminate the weaker members of the herd. In 1971, ten wolves were flown into the island. The results of this program are shown in the following table. Table 2: Wolf and Moose Populations on Isle Royale from Year Wolf Population Moose Population , , , , , , , , , ,972 Graph the moose and wolf populations on the graph on your answer sheet. Use the left axis for the moose population (count by 100 s) and the right axis for the wolf population (count by 2 s). kip a line in between for each year on the X axis. Plot each line using a different color and make a key.

4 Name: Date: Per: CA TUDY #1: TH RINDR F PRIBILF ILAND 1. What was the size of the population at the beginning of the study? 2. What was the size of the population in 1920? 3. During which of the three periods ( , , ) was the increase in the population of reindeer greatest? 4. What was the greatest number of reindeer found on the island? When was this? 5. How many reindeer remained in 1950? 6. Could emigration or immigration have played a major role in determining the size of the reindeer population? xplain. 7. If you look at the graph, you will notice that there is a relatively slow increase during the first few years. Then, there is a tremendous increase between 1930 and Why do you think this might be? 8. What effect might 2000 reindeer have on the island and its vegetation? 9. Consider all the factors an organism requires to live. What might have happened to the island to cause the change in population between 1938 and 1950? 10. If some of the 8 reindeer that were still alive in 1950 were males and some females, what do you predict would happen to the population in the next few years? Why? 11. What is carrying capacity? What evidence do you see that the carrying capacity for reindeer on this island was exceeded? 12. An unchecked population is one that does not have predators, hunters, or disease pulling out members. What does this study tell you about unchecked population growth? What difference might hunters or predators have made?

5 CA TUDY #2: TH KAIBAB DR Title: 1. During 1906 and 1907, what methods did the Forest ervice use to protect the Kaibab deer? 2. Were these methods successful? Use the data from your graph to support your answer. 3. Why did the population of deer start to increase in 1905? 4. Why did the population of deer decline drastically in 1925, even though their predators were eliminated? 5. Why do you think the deer population size in 1900 was 4,000 when it is estimated that the plateau had a carrying capacity of 30,000?

6 6. Based on these lessons, suggest what YU would have done in the following years to manage deer herds: a. 1915: b. 1923: 7. What happens to populations as they reach or exceed the carrying capacity of their ecosystem? CA TUDY #3: TH M F IL RYAL Title: N U M B R F M N U M B R F W L V

7 1. Describe what happened to the moose and wolf population between 1971 and Define density-dependent and density-independent limiting factors. Give a density-dependent factor and a density-independent factor that could influence the moose population. 3. When the moose (prey population) increases, what happens to the wolf (predator) population? 4. What causes the moose population to decrease? When this happens, what happens to the wolf population? 5. Why is this interaction between predator and prey called a cycle? 6. Is predation a density-dependent or density-independent factor? 7. Why is this cycle so easily studied on Isle Royale? 8. What do you think would have happened to moose on the island had wolves NT been introduced? 9. Are predators good for a prey population? Would the moose on the island be better off, worse off, or about the same without the wolves? Defend your answer.

Regents Biology LAB. NATURAL CONTROLS OF POPULATIONS

Regents Biology LAB. NATURAL CONTROLS OF POPULATIONS Period Date LAB. NATURAL CONTROLS OF POPULATIONS The size of populations in a community are regulated in many ways. Some of the controls acting on populations are: the amount of food and water available