Unraveling the Mystery of hunting declining populations of small game

Years ago, I was running a study of prairie-chickens in Kansas.  A conservation-minded rancher, on whose property I was working, was upset about new livestock grazing requirements associated with an easement he had with a government agency.  He asked me as we sat over coffee one morning, “how do they get off telling me how I should manage my land to protect chickens when they continue to sell hunting licenses to kill them?”  To him, the agency in question appeared greedy, intending to keep selling hunting licenses while asking him to foot the bill for population recovery.  In my career as a wildlife biologist, I have heard many people – landowners, farmers, hunters, and anti-hunters alike – similarly question the collective intelligence of wildlife managers when it comes to allowing hunting seasons for game populations that those same managers have identified as vulnerable and declining.  What gives?

The reasoning behind allowing hunting seasons on declining populations may have a little to do with conservation funding (OK, maybe more than a little but I’ll get to that); it has a lot to do with accepted theories in population biology.  On the surface, it seems reasonable that killing any members of a population will have a negative effect on that population. However, theoretical and real data rarely support this perfectly negative relationship.  Harvest strategies of small game populations have been strongly influenced by the early work of Dr. Paul Errington, a student of Aldo Leopold and a giant of wildlife management in his own right.  In a series of experiments on a variety of small game populations, Errington showed that increases in hunting mortality, the proportion of a population killed by hunters, during the fall were offset by reductions in natural overwinter mortality.  Imagine a population of your favorite species of game bird in which a large hatch results in more birds than its winter habitat can support.  A proportion of the population is going to die from exposure, starvation, disease, or predation. Errington termed this proportion the “doomed surplus”, and said that if this proportion is going to die anyway, harvest rates up to that proportion will be compensatory to natural mortality.  Here’s a visual to illustrate what this translates to for the annual survival of the population:

​
Figure 1.  Effects of hunting mortality on annual survival rates under the completely additive and partially compensatory mortality hypotheses.
 
If harvest is additive to natural mortality – that is, it results in more dead birds than would die naturally without harvest – then annual survival for a population would decline linearly with harvest rate, as in the left panel of Figure 1.  However, what we often see for game birds is a non-linear relationship where losses from harvest offset natural mortality up to some threshold rate, after which harvest then becomes additive (right panel; Figure 1). If this graph was from an actual population, then we could say that harvesting up to 30% of a fall population would not negatively affect the overwinter survival or standing population size the following Spring.  This pattern typically holds for stable or increasing populations of small game and waterfowl, but the verdict is out for declining populations. Some researchers have suggested that harvest mortality may be additive for some declining populations of game birds; others disagree.  Timing of the harvest season may be more important.  The later in the annual cycle that the harvest occurs, the more additive hunting mortality is to natural mortality.  That’s because a greater proportion of the population has already died from those natural causes before harvest occurs.  Spring harvests of overabundant snow geese are designed specifically to take advantage of temporal phenomenon; birds that were going to die over the winter from natural causes have already left the population and all Spring harvest is additive to overwinter mortality.

Two additional issues to consider when setting harvest is a species’ life-history strategy and population sensitivities.  Many species of upland game birds, have relatively ‘fast’ life-histories; that is they mature quickly, have high reproductive effort, and can expect relatively short lives.  Generally, studies of fast species have demonstrated that population growth rates are affected much more by changes in vital rates linked to reproductive success and recruitment; vital rates like nest success and chick survival have much higher influence on rates of population growth than do annual survival rates of adult birds.  Life-history theory suggests that many small game populations are just not that ‘sensitive’ to changes in adult survival.  Figure 2 below demonstrates how sensitive a hypothetical population is to changes in two vital rates, adult survival and nest success.

---

Figure 2. Hypothetical example of population response to relative changes in two vital rates: nest success and adult survival. Population growth increases at a much higher rate with an increase in nest success than it does with a similar increase in adult survival.

In this hypothetical example, improving nest success from 20% to 50% results in a 40% increase in the rate of population growth, whereas a similar increase in adult survival only increases the population growth rate by 17%.  Or if we think about this relationship in reverse, a reduction in adult survival from 50% to 20%, perhaps through harvest, results in a lower population impact relative to a similar decline in nest success that might result from a poor nest cover year. This means that typical environmental conditions during the nesting and offspring-rearing periods, conditions like nesting cover or Spring precipitation, will have a larger relative effect on population stability than will typical annual variation in harvest rates.

Another thing that wildlife biologists know that most of us don’t is the harvest rates of populations they manage. Estimating harvest rates for game birds requires banding a sample of the population and using voluntary band returns from hunters to estimate harvest rates.  These days, harvest rates of many game bird species is fairly low.  For example, reported harvest mortality ranged from 0 to 2% for nine populations of lesser and greater prairie-chickens across Nebraska, Kansas, and Oklahoma studied intensively during the past 15 years.  My conservation-minded landowner friend might be surprised to learn that not a single bird of my radio-marked sample was killed by a hunter during that study despite open seasons every year.  Of course, harvest rates of some game bird populations is higher.  For example, harvest rates of pheasants can be more than 60%, but harvest of wild pheasants is limited to roosters, and higher proportional harvest on males don’t mean much for a promiscuous species where a few males can mate with all the females in a population.  In fact, under ideal habitat and winter conditions, up to 90% of pheasant roosters can be harvested without harming the population.

When all this information is taken together, wildlife managers generally conclude that 1) harvesting up to 20% of a population in the Fall has relatively little impact on long-term small game populations, and that 2) population or habitat management that addresses reproductive success and recruitment of young into the population will have a much greater impact to population growth and stability than more restrictive harvests.  Unfortunately, management actions aimed at improving nest success or chick survival at the landscape scales necessary for population recovery are often costly to implement. Several state wildlife agencies provide financial incentives to private landowners who adopt habitat management or livestock grazing practices that provide adequate nesting and brood habitat for upland game birds.  From where does this money come?  Nearly all of a state’s money for wildlife management comes from revenue generated by the sale of hunting licenses and federal monies from taxes on hunting equipment allocated back to state based on the number of hunting licenses it sells.  Overall, eliminating hunting seasons reduces the ability of agencies to leverage hunter-generated funds to manage or recover those non-hunted species.  So from a wildlife agency perspective, the relatively small benefits to annual survival resulting from the elimination of hunting opportunity are far outweighed by improvements to reproductive success resulting from habitat improvement paid for by the sale of hunting licenses. More importantly, agencies know from experience that re-establishing hunting seasons after they have been eliminated is often politically difficult to do.   - L.B. McNew