Sunday, July 20, 2008

Population Growth Curves

Living organisms exhibit exponential patterns of growth given ample supplies of food and living space.

Here's a simple curve for bacteria:
Bacterial population over time

Wikipedia:Bacterial Growth

Here's an oscillating curve based on the number of pelts from the Hudson Bay Trading Company:
Hare and lynx populations over time

Courtesy of Duke University's Math Dept.

Here's the human population curve:
Human population over time

Courtesy of the United Nations and the University of California at San Diego's Jim Moore (no relation)

Here's another teaching link from the University of Michigan.

There has likely been no sustainable human population in human history. This is due to climate change, predators, catastrophes, and disease. In other words, up until the last 250-300 years, the environment has controlled our numbers. This will not change over the long haul. Scientists don't know the carrying capacity of the Earth, but we are now seeing signs that the oceans are stressed on a global level. Fisheries (cod and salmon) are collapsing and unprecedented jellyfish blooms are two indicators that fish populations are being depleted. With a warming planet, whether it's due to excess carbon dioxide or increased solar activity, likely fresh water shortages will occur first if agricultural irrigation practices are not optimized to conserve water. Currently, they are not. However, people die in less than a week without water. A person can live for three weeks without food - longer with suboptimal rations. Now the water shortages to come can be lessened with the solar distillation of seawater or vapor compression distillation. (If your water is polluted with light oil or petroleum distillates that have a lower boiling point than water you are screwed.) Such water won't be cheap, but a thirsty man will pay for it. But all of this is beside the point. We are approaching a point in time where the human race will face a population limit due to the lack of either fresh water or food. If we act wisely, we might be able to create a steady state population sort of like the stationary phase that bacteria have. If we don't act wisely, we will likely have a huge population crash due to famine and disease. The Black Death killed over a third of the human populations it affected. (Then again, from the planet's perspective humanity could be seen as an infectious disease that is out of control akin to a bacterial infection. I wonder what antihuman mechanisms Nature will use to eradicate this infection.) Which would you choose, bacteria or lynx? The only difference between the curves is the length of the stationary phase.


Looking at the population per square mile of places like Singapore and Hong Kong I think population can grow a lot more. The UN suggests 9 billion by 2050 ( ). There'll be little natural environment (e.g. rainforest) left by then (that will be the tradeoff), but lots of people, crops and fodder. Plague or a nuclear exchange may well cause a tragic adjustment to estimates.

Our track record on islands is poor. The trees are cut down, the soil erodes into the sea. The planet is our island now. There is no place to go unless we make it habitable and those places are much less forgiving than planet Earth. Sooner or later, we'll have to leave this world, but we'll need something to populate the arks with besides just people.

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