Saving Darwin's Oryx

Introduction

You have been named to head the international recovery team to save Darwinís Oryx. The population has been undergoing a steady decline (Lambda = 0.90 approximately). Data has been gathered to produce the following life table (Table 1):

Table 1. Current life table of Darwinís oryx.

The recovery team has developed a number of options for increasing the growth rate of Darwinís Oryx. The six options each affect a different part of the Darwinís Oryx life table. The options are described below and costs are detailed in the tables below. Table 2 shows the current Leslie matrix for Darwinís Oryx with the parameters circled that can be changed.

Your Task

Your job is to select the combination of options that will increase the population growth rate to a safe level (Lambda = 1.05) at the lowest cost possible.

You are to do this in three steps:

1. Review the six alternatives. Hypothesize what the relative affect will be of each option on the population growth rate.

2. Using Demography, test the various options and compare the actual results to your hypotheses from step 1.

3. Using Demography, and keeping track of costs, find the combination of options that will produce the acceptable growth rate at the lowest possible cost.

You are to write this assignment up noting your progress along the way. This is not meant to be a final report, but rather a lab book where you keep your notes as you proceed.

Six Conservation Options

1. Extending the length of sexual maturity. (m9 = 0 changes to m9 = 1.5)

What is the action?: In the wild under normal conditions, sexual maturity for females ceases past age 7. Research has shown that it is possible to extend sexual maturity to age 9, allowing each surviving female to produce offspring one more time.

What is the biology involved?: Under the careful administration of sexual hormones, females retain sexual receptiveness. Successful breeding at this age requires the introduction of much greater numbers of sperm than would normally be introduced, under a very narrow timing window.

How is it accomplished?: This requires moving all females at age 8 from the wild into captivity, regularly treating them with reproductive hormones, and artificially inseminating them.

What are the costs?: Costs per female are great, but since very few females are involved, the overall cost is moderate.

2. Decreasing the age of sexual maturity. (m3 = 0 changes to either m3 = 0.5, 1.0, or 1.5)

What is the action?: In the wild under normal conditions, sexual maturity for females is reached at age 5. Research on similar species has shown that sexual maturity can potentially be reached at an earlier age, allowing females to begin producing offspring earlier.

What is the biology involved?: Age at maturity in these species is regulated by two factors, individual growth rate and density of older breeding females. Increased food supply allows animals to grow more quickly, which enables them to physiologically reach sexual maturity quicker. Removing adult females stops the production and release of a hormone that inhibits their reaching sexual maturity.

How is it accomplished?: This action is dependent on acquiring additional habitat, segregating juvenile females from adult females, and providing supplemental feeding to juveniles. This can be done for a portion or all of the juvenile females.

What are the costs?: Costs for this action are high due to the need to acquire additional habitat. There are also costs involved in segregating juveniles from adults and supplemental feeding. Feeding costs are high due to the large percentage of the population within the juvenile ages.

3. Increasing the frequency of reproduction. (m6 = 0 changes to m6 = 1.0, 1.5, or 2.0)

What is the action?: Using animals in captivity, females have been able to produce offspring every year instead of once every two years. This has only shown to work at the peak of their sexual maturity at age 6.

What is the biology involved?: Females give birth to two fawns once every two years. The fawns remain with the mothers, nursing until the age of 15 months. The act of nursing prevents the motherís sexual cycling to resume. It has been shown with animals in captivity that if the young can be removed from the females at 6 months, the females can become sexually receptive after one year instead of 2.

How is it accomplished?: Fawns must be separated from their mothers at approximately 6 months of age. The fawns must be raised by humans for approximately 6 months to ensure nourishment equivalent to that provided by the mother. This can be done for either half of the population or the entire population.

What are the costs? Costs are high because of the large amount of human intervention. It is extremely difficult to separate fawns from mothers and often required sedating the mother. Human intervention to feed the young is also expensive requiring skilled biologists.

4. Increasing the survival of juveniles. (p1 = 0.30 changes to p1 = 0.35 or 0.40)

What is the action? Survival of juveniles can be increased slightly based on research on similar species.

What is the biology involved? Lions kills a significant number of juveniles each year. If the number of lions was reduced, the number of kills would also be reduced. Unfortunately there are many other source of mortality so reducing the lions does not remove all juvenile mortality.

How is it accomplished? Allow a hunt for a limited number of lions each year, reducing the lion population.

What are the costs? Costs are fairly minimal. Hunters can pay for the opportunity to hunt a lion, covering most of the costs, other than the costs of enforcing the limited hunt.

5. Increasing the survival of adults. (p3-p6 = 0.87 changes to p3-p6 = 0.90 or 0.93 )

What is the action? Survival of adults can be improved based on tests with other species.

What is the biology involved? Poachers kill a large number of adults for their value in foreign markets. Hooves, tails, and bones are used as components of medicines. Heads are valued as trophies. Reducing the levels of poaching would increase adult survival. Older adults are not desireable to the poachers.

How is it accomplished? Poaching can be reduced through the use of armed game wardens protecting the herds at all times.

What are the costs? Costs are moderate. Hiring, training, and paying a sufficient number of game wardens is significant, although locals can be used, requiring substantially less pay.

6. Increasing the survival of the oldest adults. (p9 = 0.6 changes to p9 = 0.99)

What is the action? Under ideal conditions, survival of elderly adults can be significantly increased.

What is the biology involved? Elderly adults usually die of disease or starvation during the winter months. Protecting the elderly from severe conditions can significantly increase their survival.

How is it accomplished? The proposal is to capture all adults age 9 or over and move them into captivity at zoological parks where they can have food, vitamins, medicines, and where they can be cared for by staff veterinarians.

What are the costs? Costs per animal are large but due to the small number of animals in this age class, overall costs are moderate.

 

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Darwinís Oryx Fact Sheet. Options to Improve Population Growth Rate.

All cost in millions of US dollars.

Elderly Fecundity m9	Base Value	Option 1
Possible Values	0.0	1.5
Additional Costs	$0.0	$0.1

Juvenile Fecundity m3	Base Value	Option 1	Option 2	Option 3
Possible Values	0.0	0.5	1.0	1.5
Additional Costs	$0.0	$3.0	$7.0	$11.0

Adult Fecundity m6	Base Value	Option 1	Option 2	Option 3
Possible Values	0.0	1.0	1.5	2.0
Additional Costs	$0.0	$5.0	$8.0	$12.0

Juvenile Survival p1	Base Value	Option 1	Option 2
Possible Values	0.30	0.35	0.40
Additional Costs	$0.0	$0.2	$0.5

Adult Survival p3-p6	Base Value	Option 1	Option 2
Possible Values	0.87	0.90	0.93
Additional Costs	$0.0	$2.0	$5.0

Elderly Survival p9	Base Value	Option 1
Possible Values	0.60	0.99
Additional Costs	$0.0	$3.0

 

Leslie Matrix for Darwin's Oryx

Current values are shown. Highlighted cells can be changed by various policies.

0	0	0	0	2.0	0	2.0	0	0	0
0.3	0	0	0	0	0	0	0	0	0
0	0.7	0	0	0	0	0	0	0	0
0	0	0.87	0	0	0	0	0	0	0
0	0	0	0.87	0	0	0	0	0	0
0	0	0	0	0.87	0	0	0	0	0
0	0	0	0	0	0.87	0	0	0	0
0	0	0	0	0	0	0.6	0	0	0
0	0	0	0	0	0	0	0.6	0	0
0	0	0	0	0	0	0	0	0.6	0