Overshoot and Collapse Explained
When I told a friend that I was planning on starting a blog titled Shifting the Human Extinction Trajectory, he said it was a terrible name and that it would be better to give the blog an upbeat title so as not to discourage prospective readers. As I noted in my Introductory Blog, I anticipate focusing primarily on the positive potentials for significantly reducing the monumental suffering that the current trajectory toward catastrophic systems collapse would cause. Nevertheless, I will not sugar-coat the seriousness of our current situation. Three relatively simple concepts provide a framework for articles that will follow:
— Carrying capacity is the maximum population size of a biological species that can be sustained by a specific environment, given the food, habitat, water, and other resources available. For the human species, the environment in question is the entire planet Earth.
— Population Overshoot and Collapse (O&C) occurs when the population of a species significantly exceeds the carrying capacity of its environment. An O&C graph of population over time tends to look like a bell-shaped curve with an upside (overshoot) and a downside (collapse).
Species overshoot and collapse is rare in healthy ecosystems. More typically a species population fluctuates above and below the carrying capacity as a result of environmental fluctuations and interactions with other species as shown in the figure below:
I’ve been collecting examples of overshoot and collapse at the species level, and every one that I’ve found can be traced to human activity. At the beginning of the twentieth-century reindeer were introduced to the two largest of the Alaskan Pribiloff Islands:
Scheffer’s paper makes it clear that the factors affecting the two different overshoot and collapse curves are complex. Nevertheless, I interpret the evidence in the paper showing that the higher the population overshoot, the greater the reduction in carrying capacity after the crash.
Aldo Leopold’s classic overshoot-and-collapse curve, attributed mainly to killing pumas, wolves and coyotes who preyed on the deer population on the Kaibab Plateau in Arizona, suggests a post-crash carrying capacity that is one-third the original capacity (10,000 vs 30,000). Sixty years later the population is still around 10,000.
Overexploitation, human harvesting of renewable resources to the point of collapse, leads to reduced carrying capacity for life on Earth at best, and large-scale species extinction at worst. A classic example of overexploitation is the cod fishery in the North Atlantic ocean, and the graph looks very similar to an O&C curve:
When good news in the environmental arena is hard to come by, I’ll close this article on a somewhat upbeat note. The northern cod fishery has made a comeback and may return to historical sustainable levels by 2030 (Rose and Rowe, 2015). However, the devil is in the details. The figure below shows that while the Iceland and northeast Arctic fisheries are thriving, all the cod fisheries in the southern areas of the North Atlantic are doing poorly.
Coming soon: When Will the Human Overshoot Curve Peak?
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