Population Size & Reproduction

Environmental factors that may influence the reproductive process (e.g., nutrition, weather, crowding, predation, disease) often have more severe impacts on reproductive success when wildlife populations are HIGH rather than low. Why?

animal animal photography bear big
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In large populations there is more competition for both quantity and quality of food and cover, which leads us into a discussion of density-dependent reproduction.

Population Density is the number of animals per unit of area (e.g., 21 deer/square mile). Since environmental factors influencing the reproduction process often become more severe as populations increase, reproductive rates are often density-dependent.

Reproduction rates DECLINE with INCREASING density, with the greatest total reproduction occurring at INTERMEDIATE levels of density. Surprising right?!

In order to keep tabs on our populations, measuring reproduction can be a helpful tool. Data on reproductive success of wildlife are widely used as a basis for management decisions. Wildlife managers consider any progressive decline in reproduction to be serious.

Reproductive rates are used as indices of trends; total reproduction is often estimated as a basis for calculating allowable harvest for any wildlife species.

animals back light beaks close up
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3 categories of data are gathered for calculating allowable hunter harvest:

  1. Breeding Rates: breeding rate data indicate the proportion of the female population that is participating in reproduction
    • How would you collect this type of information?
      • Aerial surveys of pregnant females, examine harvested female reproductive tracts, evidence of lactation, etc
  2. Litter and Clutch Sizes: indicates number of offspring per female being produced.
    • How would you collect this type of information?
      • Direct counts, placental scars, embryo counts, etc
  3. Age Ratios: used to indicate reproductive rates (often expressed as ratios).
    • How would you collect this type of information?
      • Aerial surveys, road surveys, nighttime spotlight counts, etc

Deer – fawn: doe ratio

Elk/Moose – calf : cow ratio

Mt. Sheep – lamb : ewe ratio

Mt. Goats – kid : nanny ratio

If you’re managing wildlife for harvest, wether that is for a large government agency or your own farm, its important to make sure what you’re harvesting is being replaced! Keeping these principles in mind will be a great way to keep a sharp eye on your herd and sustain a healthy population for years to come!

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