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What's a "harvestable surplus"?

posted Apr 11, 2016, 12:22 PM by Scott Slocum   [ updated Apr 12, 2016, 8:00 AM ]
A couple of theories of wildlife population dynamics are summarized here in order to reveal the "Miracle of Compensatory Mortality" and the "harvestable surplus." 

Readers can claim extra credit for remembering that these are not the only theories in population dynamics (or biology or ecology), that they don't explain everything that happens in natural or artificial systems, and that there's always more to know than what the lobbyists and the conspiracy theorists are choosing to say. 

General Population Dynamics.
A basic reality of life for wild animals is that each year, some will be born and some (including some of the newborns) will die

This is the reality even for the toughest, "top-of-the-food-chain," apex predators that some might suggest have no "natural enemies," no "predators," in short nothing but "noble sportsmen" to regulate their numbers. Their young, their weak and old are vulnerable to smaller predators. They're vulnerable to injury in their high-risk predatory operations, and in defending their territories. They're exposed to limited or unpredictable food supplies, diseases, and the dangers of crossing roads. And they're subject to violent crime.

A basic theory of wildlife population dynamics is that the size (and other characteristics) of a population of a given species in a given environment will tend to stabilize somewhere near the environmental carrying capacity (e.g. somewhere near the point at which the animals have enough food and cover to be successful on an ongoing basis). 

This is the rule even for the toughest, "top-of-the-food-chain," apex predators that some "fear" might at any time "eat through" their natural prey base and overrun border settlements, starving, preying instead on children at bus stops. In other words, when they have trouble finding food, they starve. A few, gaunt, dispersing animals will appear around people's bird feeders, but most will maintain their valuable territories in the wild until their fortunes improve.

On the birth side of the above population-stabilizing equation, wild animals tend to reproduce in excess. Not all of the young survive, but some do, to replace individuals from previous generations who don't. 

Game Management.
Following from the general population dynamics described above, a basic theory of recreational hunting & fishing (i.e. "game management" which some might overextend and call "wildlife management") is that wildlife populations will tend to compensate for animals that are taken by hunting & fishing. Although there are at least two compensatory functions--increased birth rate or decreased mortality from other sources--the latter makes for much better media coverage, so it's the only one we hear most about.

First, the "good news" is that, even though the particular causes of mortality might vary from year to year, the overall levels of mortality in a stable population will tend to stabilize. So, the theory goes, if recreational hunting & fishing cause modest levels of mortality (below the natural, overall level of mortality), then nature will tend to "compensate" with lower levels of mortality from other causes. Thus the theory of "compensatory mortality," that animals that are taken through hunting & fishing are spared worse deaths from other causes. 

And then the "bad news"--without such a pretty interpretation--that the compensatory reaction can also be through increased birth rate. In other words, although the hunting & fishing mortality will add to the overall level mortality, the animals that are taken will be replaced by their orphaned young. Right, that doesn't make for such great media coverage. It's more along the lines of the recovery of human populations following natural and man-made disasters. If one takes a high-level view (say from an aircraft window) the recovery of a destroyed human city might be spun as a testimony to the courage and determination of human civilization. At ground level, however, it is cruel.

Many, many corollaries spring from the theory of compensatory mortality--as if it's not just a theory, and as if they're not just corollaries. The faith-based approach to "game management" is to spin as many hunting & fishing practices possible into "Revelations of the Miracle of Compensatory Mortality" including (but not limited to) the following:
  • We should manage wolves the same way we do other wild animals. (Not true.)
    • Ignoring the differences between wild-living wolves and those prone to human/wildlife conflicts.
      • Assuming instead that recreational hunters will tend to deal with conflict situations. 
    • Ignoring the importance of intact, functioning social units to individual success.
      • Focusing instead on the flexibility of wolves to adapt to high-mortality environments.
    • Ignoring the dependence of predators on prey, and the consequent self-regulating population dynamics of predators.
      • Preferring instead to warn of predators "eating through" their natural prey and overrunning human civilization.
    • Ignoring the negative aspects of wolf flexibility to high-mortality environments.
      • Pretending instead that there are no disadvantages to increased reproductive rates by younger wolves.
        • Tends to aggravate problems with livestock depredation.
        • Tends to compensate for temporary local exterminations (and thus require annual local exterminations).
      • Focusing instead on the more easily-visualized results of annual local extermination of wolves near livestock operations.
  • Every animal in nature dies a violent death: but their deaths are easier through hunting & fishing. (Not necessarily true.)
    • Ignoring quick kills by predators' bites to the necks of their prey.
      • Focusing instead on cases in which predators don't deliver that quick bite to the neck.
    • Ignoring merciful predation on injured or diseased prey that are killed before they suffer long from the injury or disease.
      • Pretending instead that hunting & fishing are the equivalents of predation.
    • Ignoring the resistance of healthy prey animals to predation.
      • Pretending instead that hunting & fishing are the equivalents of predation.
  • Trapping helps to control disease. (Not necessarily true.)
    • Ignoring the probabilities of healthy vs. diseased animal capture.
      • Focusing instead on the probability that some diseased animals will die from trapping rather than from disease.
    • Assuming that lowering the population density will lower the rate of disease transmission.
      • Ignoring the complications of disease transmission in social animals.
      • Focusing instead on simpler epizootic models for solitary animals.
    • Ignoring potential disease transmission through bait piles to untrapped scavengers.
      • Focusing instead on how "every part of the animal is used by the trapper."


CooleyWielgusEtAl_Ecology_2009_RejectedCompensatoryMortalityHypothesis_snapshot
Snapshot of the abstract:
Cooley, Hilary S., Robert B. Wielgus, Gary M. Koehler, Hugh S. Robinson, and Benjamin T. Maletzke. 2009. 
Ecology 90 (10): 2913–21.