Many thanks to Eric Rimmer for sending me this lecture by William Rees (see Word document below), and thanks to Al Bartlett for the following clear statement by William Rees on ‘carrying capacity’ and ‘productive capacity.’
Are Humans Unsustainable by Nature (William Rees) (Word doc., 157 KB)
I believe there is some confusion both in Lawrence’s note and in Hopfenberg’s presentation between the terms ‘carrying capacity’ and ‘productive capacity’ (or simple ‘productivity’).
Carrying capacity is unambiguously defined in ecology as a POPULATION number. It is the maximum population of a given organism that a particular ecosystem can sustain without the ecosystem being overexploited by that organism. Carrying capacity is highly variable because the ecosystem (the species’ environment) is constantly changing with climate, season, etc., i.e., its ‘productivity’ is changing. For this reason, even many ecologists do not see it as a particularly useful term.
By contrast, productive capacity refers to the rate of biomass fixation or accumulation at a specific trophic level in the ecosystems. For example, gross primary production refers to the grams of carbon fixed (as biomass) by green plants per square meter per day in a particular ecosystem. Net primary production is what’s left over for consumer organisms after respiration by plants.
The terms are related as follows: The carrying capacity of the system for a particular consumer species will increase if net primary production (productive capacity) increases. For example:
People can increase the human carrying capacity of their supportive ecosystems by eliminating competition (habitat takeover) and by increasing the productive capacity of those systems using technological inputs–fertilizer, irrigation, etc. In short, human carrying capacity is a variable dependent on a variety of non-renewable inputs.
Humans can also increase their local populations through trade without increasing either local productivity or carrying capacity. In effect, globalization only seems to increase local carrying capacity. Most high-income and densely populated countries have long exceeded their domestic carrying capacities–their populations depend largely on ‘surplus’ productive capacity imported from elsewhere.
Regrettably, trade obscures the fact that local populations exceed local carrying capacities and thus helps to perpetuate the perpetual growth myth among wealthy overpopulated nations. As Lawrence observes, civilization’s whole shaky house of cards will come tumbling down with human carrying capacity if we are unable to maintain the constant throughput of resources necessary to sustain productive capacity.
And I don’t think we will.
Current World Population
Net Growth During Your Visit