Wildlife Mortality Concepts

There are many types of mortality that wildlife can succumb to. Mortality is merely the cause of death; some being normal and others abnormal. Today we will discuss the differences between the two and the general concepts of mortality that can damage your wildlife populations.

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Normal Mortality vs. Abnormal Mortality

Normal Mortality: Types of mortality that have been common in an animal’s evolutionary history. The species is adapted to this mortality. Most observed wildlife mortality is “normal mortality”

Abnormal Mortality: Types of mortality that have not been common in an animal’s evolutionary history. The species is not adapted to this mortality. Examples:

  • Trapping and Poisons
  • Introduction of new predators or pests
  • Diseases that are new
  • Man-constructed hazards

The control of normal types of mortality usually involves providing good habitat for each species. With good habitat, mortality will still occur, but at a normal rate.

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Before we get deeper into mortality, its important we go over a few concepts:

Concepts of Wildlife Mortality

  1. Density-Dependent Mortality: involves an increase in mortality rate as population density increases
    • E.g., ELK: overpopulation ⇒ over browsing ⇒ increased stress ⇒ greater number of animals dying from disease.
  2. Density-Independent Mortality: Mortality occurs regardless of the population density, affects but does not regulate populations
    • E.g., MOOSE: long cold winter with deep snows ⇒ old and very young in a population may die, regardless of population density
    • E.g., CARIBOU: drown crossing river during migration
  3. Compensatory Mortality: The combined effect of two or more types of mortality, in which each mortality type may vary from year-to-year in its impact on the animal – while the total mortality remains somewhat constant. A KEY CONCEPT!

Example of Compensatory Mortality idea:

(Implement coyote control in order to have more deer to hunt)

Natural System —- 5 years after coyote control was started

Predators: 35 —- Predators: 0

Weather: 20 —- Weather: 15

Malnutrition: 25 —– Malnutrition: 45**

Accidents: 5 —– Accidents: 15

Disease: 15 —– Disease: 25**

100 —- Totals # of animals dying —- 100

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TIME is a MAJOR FACTOR! Getting rid of coyotes may increase the deer population for awhile – but over time the deer population will come back to about the same level as before coyote control was started – instead of predators (coyotes) being the major source of death, the lack of predators will be compensated for by the increase in deaths due to malnutrition and disease (density-dependent mortality factors).

Next post we will go over the various types of wildlife mortality! Don’t forget, sometimes the easiest and most apparent way to raise your populations (removing predators or prohibiting hunting) doesn’t exactly get you the results you’re looking for!

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