Learning from ecology: revisiting human carrying capacity
The renewed debate around the natural capital constraints on the economy demands that we revisit the ecological concept of carrying capacity. Does it make sense to talk of the human carrying capacity of Earth? For purposes of game and range management, carrying capacity is usually defined as the maximum population of a given species that can be supported indefinitely in a specified habitat without permanently impairing the productivity of that habitat. However, because of our seeming ability to increase human carrying capacity by eliminating competing species, by importing locally scarce resources, and through technology, this definition does not seems applicable to humans. Indeed, trade and technology are often cited as reasons for rejecting the concept of human carrying capacity out of hand.
This is an ironic error — shrinking carrying capacity may soon become the single most important issue confronting humanity. The reason for this becomes clearer if we define carrying capacity not as a maximum population but rather, following William Catton, as the maximum "load" that can safely and persistently be imposed on the ecosphere by people. Human load is a function not only of population but also of per capita consumption and the latter is increasing even more rapidly than the former due (ironically) to expanding trade and technology. This led Catton to observe that "...the world is being required to accommodate not just more people, but effectively 'larger' people...." As a result, load pressure relative to carrying capacity is rising much faster than is implied by mere population increases.
These trends underscore the fact that despite our technological, economic and cultural accomplishments, human beings remain ecological beings. Like all other species we depend for both basic needs and the production of artifacts on energy and material resources extracted from nature. All this energy and matter is eventually returned to the ecosphere as waste. A full understanding of the human ecological "niche" must therefore include full consideration of the flows of available energy and matter into the economy and the return flows of degraded energy and material (wastes) back to the ecosystem.
Analysis of this biophysical "throughput" shows that humankind, through the industrial economy, has become the dominant consumer in most of the Earth's major ecosystems. By 1986, humankind — one species among millions — was already "appropriating," directly and indirectly, 40 percent of the net product of terrestrial photosynthesis and recent studies suggest that the human "take" from rich coastal marine environments is approaching 30 percent (which may be beyond the sustainable yield — despite increasing effort, the world's fisheries catch has declined since 1989). What are the implications of such dominance for ecosystems integrity? Can it be safely extended? (Remember the North Atlantic groundfish stocks!) Meanwhile, such trends as ozone depletion and greenhouse gas accumulation show that critical global waste sinks are also filled to overflowing. All such data indicate that even today's levels of appropriation are unsustainable. The human "load" has grown to the point where total consumption already exceeds sustainable natural income.
Achieving ecological sustainability clearly requires that economic assessments of the human condition be based on, or at least informed by, ecological and biophysical analyses. The fundamental ecological question for ecological economics is whether remaining species populations, ecosystems and related biophysical processes (i.e., critical self-producing "natural capital" stocks), and the waste assimilation capacity of the ecosphere are adequate to sustain the anticipated load of the human economy into the next century while simultaneously maintaining the general life-support functions of the ecosphere. This critical question is at the heart of ecological carrying capacity but is virtually ignored by mainstream approaches.
-pp. 48-51; Our Ecological Footprint: Reducing Human Impact on the Earth; Mathis Wackernagel & William Rees