1
Lecture 6:
What is carrying capacity?
Introduction.
Today we have the modest task of finishing the second half of the Cohen
text and coming to some conclusion about what the carrying capacity of the earth is.
The way
I’m going to get there is by thinking about the different concepts of carrying capacity in the
Cohen text, seeing what the problems with these concepts have proven to be and what these
problems tell us.
I want to start with several mathematical models of carrying capacity, look
at one of these modelsthe Law of the Minimumin some detail, and then do a long
summingup of the question and basically of the past two weeks.
Estimates of human carrying capacity
The text warns us that arriving at a consensus about the maximum size of the
population that the Earth can support is not easy.
Figure 11.1
gives another illustration of
this.
The point is not only the huge spreadfrom one billion to one trillion is a good spread
but also that the spread does not seem to be decreasing as time goes on.
Estimates of carrying
capacity range from just over a billion to 40 or fifty billion.
The estimates do center around the
range 8 to 16 billion; meanwhile, as we were saying last week, estimates of population are
projecting stabilization in the 9 to 10 billion range, with estimates edging downward as time
goes on.
Estimates of human carrying capacity
:
methodology
.
The text presents eight former
estimates of carrying capacity.
The point here is not to memorize each of the eight or the
numbers they each came up with, but to look at them from the point of view of methodology,
of how they came up with the numbers they did and what these methods tell us about how to
calculate the carrying capacity of the earth.
Algebraic methods of calculating the carrying capacity of the Earth
Ravenstein.
Writing in 1893, Ravenstein divided the surface of the earth into three types of covers
fertile, steppe or grassland and desert, calculated the maximum population density of each
type of land and the amount the area of each type on the earth’s surface and decided that the
carrying capacity of the earth was a bit under 6 billion.
What was wrong with that?
Why
aren’t the Ravenstein calculations the last word?
Well, his method was to assume that the population density of Europe in 1893about
80 people to the square kilometer or 0.8 people per hectareis the carrying capacity of fertile
land, or that Europe had then achieved its maximum population. Moreover, since fertile land
was supposedly responsible for almost all food productionsteppes were supposed to
support 4 people to the square kilometer, or onetwentieth the density of fertile land, and
deserts onequarter of thatsupposing that European fertile land of 1893 represented the
maximum meant that maximum world population could be was approximated by
multiplying the thencurrent European population density by the amount of fertile land in the
world.
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 Summer '08
 Staff
 World population

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