« February 2008 | Main | December 2010 »

March 22, 2008

Oestrus Island

"A struggle for existence inevitably follows from the high rate at which all organic beings tend to increase... It is the doctrine of Malthus applied with manifold force to the whole animal and vegetable kingdoms" -- Charles Darwin, Chapter 3, The Origin of Species
This week's paper is more about ecology and sustainability than evolution per se. In recognition of Easter, a holiday originally honoring Oestre (the goddess of spring, who also lent her name to oestrus), and still retaining much of its original association with fecundity, I will discuss "The simple economics of Easter Island: A Ricardo-Malthus model of renewable resource use" written by J.A. Brander and M.S. Taylor and published in 1998 (Am. Econ. Rev. 88:119). The other logical choice for the holiday would be a discussion of mammals that lay eggs, such as the platypus a missing link that makes a brief appearance in a great recent post on Pharyngula, which discusses some of the evidence that we are descended from egg-laying ancestors.

Although this week's paper focuses on Easter Island, it also discusses many of the same societies (from Chaco Canyon to Rwanda) in Jared Diamond's 2005 book "Collapse." The book includes much that is not in the paper, but the paper has the advantage of being shorter and of supporting specific points with specific citations, in contrast to the diffuse "Further Reading" approach used in Collapse.

The authors attempt a quantitative explanation of human population growth, resource depletion, and population decline on Easter Island from initial settlement around 400 AD to the arrival of slave-taking smallpox-infected outsiders around 1860. They recognized the need to test their hypotheses using comparisons with other Pacific islands. They developed a mathematical model of the interactions between humans and their environment that resembles a standard predator-prey model, with humans playing the role of "predators" and natural resources being the "prey." That is, they assumed that human population growth increases with the availability of natural resources (particularly forests, which include trees that can be used to make canoes to catch fish), whose ability to recover from harvest is mathematically similar to the population growth of a prey species.

This relatively simple model matched archaeological and historical estimates of population growth and decline on Easter Island reasonably well. The human population grew enough that resources were harvested faster than nature could replenish them, leading to faster degradation of the resource base and a population collapse. The same model, with appropriate changes in parameters, also apparently worked fairly well in correctly predicting sustainability on other islands, where populations leveled off rather than crashing. One major difference is that the wine palm that grows on Easter Island can take 60 years or more to produce fruit, whereas the coconut palms that grow elsewhere in Polynesia take as little as 7 years. This was modeled as faster "reproduction" of the resource base on other islands, but you could think of it as the difference between planting a tree for your own use versus planting a tree that your grandchildren might use, assuming that your family controls the land and manages to hold onto it long enough.

One might ask "what were they thinking when they cut down the last canoe tree?", but the authors point out that the destructive trends on Easter Island would not have been obvious. Population growth never exceeded 1% per year (less than the current world average) and loss of forest cover never exceeded 5% per human lifetime (also less than the current rate of deforestation). If carbon dioxide in the atmosphere were increasing 5% in a lifetime, rather than 50%, would we be worrying about it?

The paper has lots of interesting discussion (with citations) on other societies, from Mesopotamia to Rwanda, that suffered breakdowns linked partly to environmental degradation. Could we face similar collapses on a larger scale, today? The relationship between population growth and resource supply is more complicated today than assumed in their model. Global trade and migration might help us survive problems that only affect a small area, such as the 50-year drought that led to the collapse of Chaco Canyon. But are current world grain stocks, equal to only two months of consumption, sufficient reserves to buffer the possible effects of drought, war, or crop disease epidemics? The paper also discusses social factors that affect willingness to make the changes needed to prevent collapse. In particular, "institutional change is more likely to occur when the individuals that must make the change are confident that they will be among the beneficiaries." This principle seems to be broadly applicable, even to challenges like healthcare reform.

This paper would make a great centerpiece for a seminar class, with supplemental readings from later papers that have cited it, including those with alternative points of view.