December 2, 2011

Compensatory harvest mortality in deer?

Two fawns find food and water during November in southeast Nebraska.
Photo by Larkin Powell.
The December 2 issue of the Lincoln Journal Star carried an insightful Letter to the Editor, regarding deer harvest.  I'm currently covering 'harvest management' in my Wildlife Management Techniques course.  The concept of Compensatory Harvest Mortality is often over-simplified or applied in a misleading manner.  Because it is hard to explain the concept to wildlife students, it was nice to see a glimmer of appreciation for the concept from a member of the public.

Compensatory Harvest Mortality, in short, is the concept that hunting (harvest of wild animals) does not always result in a net reduction of the population (we call situations in which harvest does result in a net reduction, Additive Harvest Mortality: harvest 'adds' to other forms of mortality).  Think of a situation in which you can make an annual count of a wildlife population immediately before harvest occurs (this rarely actually happens in the US, but happens in many other countries through 'game counts' on private land).  OK, so you have 100 deer or 100 quail on a piece of property.  Now, harvest happens (in the fall, normally), and 20 animal are removed, leaving 80.  Those eighty animals have to survive through the winter, spring, and summer until the next count, and they also will be mating and having offspring.

If a species has completely Additive Harvest Mortality, those 80 animals survive and reproduce at the same rates that the 100 original animals would have.  Survival and reproduction do not adjust to the new population density.  The 80 animals will have fewer offspring than 100 animals would have had, and the 20 animals removed will add to the number of animals that die from other causes.  If the probability of surviving a year for that species is 75%, then 75 would have survived without harvest.  But, only 60 survive from the 80 that were left after harvest.

But, in a Compensatory situation, the 80 animals that are alive after the harvest will now have higher productivity (more offspring/female) or higher survival, because of the lower density.  More food per animal equals better nutrition equals higher survival or bigger clutches/litters.  It is possible that the net result, measured in the following fall, could (and stress: could) be that the effect of the harvest has been erased by the ability of the remaining individuals to reproduce or survive at higher rates when they face less competition.

Obviously, Compensatory harvest mortality is the optimum situation for harvest biologists (people who set regulations for hunting) who want to provide opportunities for many people to hunt and take game animals.  The problem is that we rarely understand wildlife populations in enough depth to document the effects of density on reproduction and survival.  We have such data for quail (although there is still disagreement) and deer; recently my research lab provided indirect evidence for prairie-chickens in southeast Nebraska and there is some suggestion from the Nebraska's annual wing counts that prairie-chickens produce more young/adult when the breeding population is smaller.

With that, here is the excerpt of the letter from a Lincoln resident in the Lincoln Journal Star:

"This letter is in response to "Lengthen deer season" (letter, Nov. 23).  The writer stated that the "deer population has gotten way out of hand" and that "Nebraska Game and Parks should have a lengthier season."

"I disagree with this for the following reasons.  Studies show that the more deer are hunted, the more they compensate by having more offspring.  After a deer season, when a high number of animals are killed, two things occur.  First, there is more food for the remaining deer, which helps trigger increased reproductive rates.  Second, nature's reaction to the loss of large numbers of deer during a short period of time is for a greater number of does to ovulate and mate at an earlier age.  This increased reproductivity in heavily hunted areas can result in abnormally high deer populations year after year.

"Studies also show that in nonhunted, unmanipulated wild areas that have an optimum number of predators, deer populations tend to stay at or just below the maximum number the habitat can safely support.  Since all game animals in Nebraska are managed to promote hunting, to lengthen the deer season will only result in more deer, not fewer."

A proud youngster with his first buck. 
If bucks are removed without harvesting
does, the result can be higher populations
as the does are left with many resources
and fewer competitors for food.  The necessity
to harvest does has been reinforced by
recent regulations for 'earn a buck' (harvest
a doe before you can harvest a buck) in
many hunting districts in Nebraska. 
Photo by Larkin Powell.
So, a couple comments:

First, the writer nailed the concept of compensatory harvest mortality in the first 'reason' stated: increased reproductive rates are triggered by lower densities.  Nicely worded.

And, second, the writer did a nice job of summarizing how this can happen--higher pregnancy rates (percent of does that are pregnant) and more young does that are pregnant (thereby increasing their lifetime contribution of fawns to the population).

However, the writer overstates the impact of the 'hunting effect'.  Hunting does not result in "abnormally high deer populations year after year".  And, a longer deer season would not result in"more deer" than a shorter season.  If deer had the capacity to have 8 or 10 or 15 fawns at a time, then this type of response might be possible.  But, deer can have 1 (common), 2 (sometimes) or 3 (rarely) fawns.   While they can respond with higher reproduction, they are limited by their species' reproductive potential--it only compensates for increased harvest to a point. 

It is very possible to harvest more deer than the population can compensate for.  In fact, wildlife biologists count on this fact.  This is exactly why Nebraska Game and Parks Commission has increased the number of permits, especially for does, in the past few years.  And, it is why they have lengthened the season--an October 'antlerless' (does and fawns only) season was added to the normal November season to help reduce deer numbers.  To take the example to the extreme, if a longer season and cheaper licenses resulted in reducing a population of 100 does to 10 does, there is no way that the 10 does could reproduce at rates high enough to make the population higher than 100, as the letter-write infers.

Managing wildlife populations always involves stakeholders.  This is an example of how important it is to keep educating the public about the reasons for management decisions.  Kudos to the author of the letter,  however, for explaining the concept of Compensatory Harvest Management.  I may use the letter in my class next year, rather than my long-winded explanation...!


  1. Nicely explained Larkin (please excuse if this comes up many times I am in the dark as to how to do this)