The concept of overshoot is an important one if one wishes to understand the changes that started 300 years ago at the beginning of the industrial revolution and are now entering a turning point that will take us to the next stage of overshoot.
So what is Overshoot?
Overshoot is a simple concept that is described easily by visuals. See the graph on the right, that is what overshoot looks like, except that the variable is normally population not consumption, and it has two sides one of an expanding and the other contracting but both are the same process. Overshoot is just when consumption goes above the long-term carrying capacity of the ecosystem and it normally keeps on climbing but at the expense of a diminishing carrying capacity. This is okay until the point that carrying capacity can no longer support the population and in response population drops until it reaches carrying capacity that is now only a fraction of what it was before. The upside may look rosy but from the perspective of an individual in the population but once the decline happens, which is inevitable once overshoot has appeared, the previous growth phase is a hindrance that prolongs the length of time until sustainability is reached and reduces the end carrying capacity.
What happens next depends on the exact nature of the carrying capacity in question, the species place in its ecosystem and the ecosystem itself. In the case of the snowshoe hare and lynx, their populations continually cycle up and down. This is characteristic of simpler ecosystems with a small amount of controls on population. In the case of the snowshoe hare there is only the crude mechanism of predator-prey to keep their populations in check and as the graph shows this is not enough. But the north Atlantic Cod decline (due to overfishing), which has permanently reduced the long-term carrying capacity of fishing in that region, is permanent. This was caused by a shift in the Cods ecosystem, the predator prey part, that has lowered the reproductive ability of Cod. The shift was driven by the low cod numbers which allowed their prey to have a population boom, the prey eat cod eggs and young (this is quite common) and now they stop the Cod population from regaining its former levels. The north Atlantic cod fisheries will now never regain their previous level of consumption without another major ecosystem shift, one which would probably do massive harm somewhere else.
The difference in these cases is a function of the different complexity levels and its effects in their respective ecosystems. In a complex ecosystem, overshoot is less common since more controls are in place and step in more frequently when one or more fail. But as the north Atlantic cod shows these complexities can also enable shifts in an ecosystem to occur when overshoot does happen, such as when allowed by the use of fossil fuels and advanced technology. The world we live in, as befits an entire planet, is incredibly complex and the shifts that will occur will dramatically change humanities position, power and abilities as the multitude of ecosystems we live in change drastically in response to our decline.
We are at or close to the peak in population or consumption levels and it is unlikely that we can hold of the plunge much longer. The way down will be a long, slow and ragged process until we reach our carrying capacity dictated by the planets biosphere and subject to the changes induced by climate change.