The size at which a theoretical population would stabilize, indicated as "K" on the graph in the previous section, is referred to as the carrying capacity. The carrying capacity is the theoretical equilibrium population size at which a particular population in a particular environment will stabilize when its supply of resources remains constant. It can also be thought of as the maximum sustainable population size; the maximum size that can be supported indefinitely into the future without degrading the environment for future generations.
At carrying capacity, births balance deaths, hence G = 0. (Remember, G = (b - d) * N, so when b = d, the term goes to zero.)
You can see that as the population size (N) begins to approach K (the carrying capacity), resources or other forces begin to limit growth. These limits could be imposed by decreases in b or by increases in d.
Logistic growth is idealized, of course. K is not likely to be constant (for example, year-to-year changes in weather affect food production; the richer a life we desire, the lower K for humans is likely to be, etc.). However, the model is useful conceptually. Initially when the population is small and resources are abundant, growth is rapid. Then as resources decrease, growth decreases and finally stops.
Most organisms can do little to change the carrying capacity of their environments, but humans have. The historical pattern of human population growth shows the effect of breakthroughs that essentially allowed increases in K. As examples, the switch from hunter/gatherer societies to agricultural societies with increasingly intensive agriculture, technological advances that allowed us to inhabit new places (irrigation, heating) and use new resources to increase food production, all increased the effective carrying capacity for humans. Our growth has been enabled by development, which has allowed us to use an increasing proportion of Earth's resources for human needs. (As pointed out in the Vitousek et al. article from your assigned reading, humans now use abut 19% of the net primary production of Earth (terrestrial + aquatic) or about 30-40% of terrestrial net primary production for meeting human needs; to eat directly, to feed our animals, clear land for urban and other uses, etc. We also use ~ 1/2 of the accessible fresh water.)
Does the carrying concept then apply at all to humans, and if so is there a set carrying capacity number (and have we already passed it?)? Relevant questions include:
(1) Can we keep developing the new technologies and abilities to use new resources that enable increases in K?
(2) At what price will these developments come, relative to our quality of life?
Some argue that the concept does apply, at least in the general sense that resources are finite for us and we are fundamentally constrained as are all other organisms. It is unlikely, however, that we can define a specific K (that is, an actual number) for humans. (People seem to enjoy trying though!) It is logical that K depends on what lifestyles we want; that is, we must think of human carrying capacity in cultural terms, as in a "cultural carrying capacity."
Others, of course, argue that the concept has no relevance to the human condition, because of our infinite capacity to develop solutions, as previously described.
Others argue that the concept does apply to humans, but not just because of resource limitations; they argue that we have an ethical burden to limit our population. This perspective argues that if we destroy the planet by living as though K doesn't apply to us, we may take the rest of life with us.
You might find it interesting to make a list (alone or with friends) of the pros and cons of having more people on Earth. That is, what things would be better if we had more people, and what things would be worse? After doing so, go through the lists, and evaluate the kinds of value judgments that went into making your lists. Do patterns emerge?
Feel free to submit your thoughts on sustainablility of human population growth to the BI301 Blackboard Discussion Group!
Final note: it is often claimed that the only reasons that people are starving on Earth at present are that our system of food distribution is inadequate for the task and that there is so much extreme poverty, not that we can't produce or aren't producing enough food. We'll talk more about what kinds of food choices we make and their environmental impacts and impacts on human nutrition around the world when we discuss prospects for sustainable agriculture. Obviously, the answer depends on what kind of diet we have in mind! For example, according to Brown University's Hunger Report Update back in 1991, we could feed 5.9 billion people (fewer than we now have on Earth....) well on a completely vegetarian diet on harvest levels similar to those in 1989 (note that while total harvests have been slightly higher than those in 1989 since then, the increases haven't been great, as we'll discuss later...)
If you want to add 15% meat or animal products (typical of a South American diet), we could only feed 3.9 billion people, or less than 75% of the current world's population).
If the diet included 25% of the calories from animal products (as is typical of the wealthiest nations) then we could feed only 2.8 billion people, or less than half of the world's present population, again based on those late 1980's harvest levels.
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Page maintained by Patricia Muir at Oregon State University; page last updated Oct. 2011.