Rats and Population

Martha Hoffman

As I discussed in my first essay “Calamine Lotion Won’t Cut It,” humans are unable to find a balance with their surrounding environment.  We (like invasive species which I talked about in my third essay), significantly alter the ecosystems that we inhabit.  When we have begun to run out of resources we have always been able to find a technological solution to keep us alive and thriving.  However, as the human population grows beyond what is considered to be the carrying capacity for the Earth, the difficulties we’ve created (both in terms of food/energy shortage and in terms of climate change) seem unavoidable.  A consequence that tends to be un-discussed by environmentalists is the psychological effects of our overcrowding.  As space becomes harder to come by, and humans are frequently forced to live in more densely populated areas, crime and other problems increase.           

            Malthus examined the geometric rate at which populations grow (particularly unchecked human populations) and compared it to the arithmetic rate at which available resources increase (Malthus, 1965).  His main hypothesis stemming from this assessment was that humans would, eventually, outstrip their available food and energy supplies to the point where there would be a population crash.  While this may be an inevitable occurrence at some point, we have thus far managed to avoid it by way of various “tech fixes.”

            The population growth rate has taken a turn for the worst.  For most of human existence the population of the earth was around 10 million.  In 8000BC, when the agricultural revolution occurred, the population began to expand.  As Veena Raleigh outlines, the population doubled between the year 1 and the year 1600 (taking 1600 years) (1999).  More recently, however, the population doubled (from around 3 billion to about 6 billion) from 1960 to the present.  Another way of looking at the growth of human numbers is presented in “Human Impacts on Planet Earth,” by Charles Southwick (1996).  He looks at how quickly the population currently grows by one billion people and compares it to the population growth over the past 3 million years, when it initially took until about 1800 to reach one billion. 

             The human population is currently facing three problems: shortage of oil, shortage of water, and global warming (caused by us).   Although the fuel shortage is the most solvable of these troubles, it is the one that is receiving the most global attention.  While it may be difficult economically to shift away from our dependence on oil, there are other possible energy resources for us to tap into.  The water shortage, on the other hand, is less easily fixed.  We cannot stop needing water, and as the world population continues to increase, water sources are becoming scarcer.  Global warming, largely caused by our enormous carbon dioxide emissions, is a threat that is tied into our use of fuel.  However, even if we decrease carbon dioxide emissions substantially, the global carbon cycle will take a very long time to remove it from the atmosphere. 

            There are many creatures that effect major changes in their ecosystems.  For instance, the early earth contained very low levels of oxygen.  Without plants and other organisms performing photosynthesis and oxygen producing processes, the atmosphere would never have had enough oxygen to support animal life (Walker, 1984).  So, while creatures eventually reach equilibrium with their environment, they begin by shifting the balance (whether it is chemically or simply in terms of changing the food chain).  A similar thing is occurring with humans as we overpopulate the earth.  We are changing the environment in ways that will make it more difficult for ourselves and other organisms to survive both by heating up the planet, and by depleting valuable resources.

            An additional way in which we are making things more uncomfortable for ourselves was looked at by John B. Calhoun in the 1960’s.  He studied the effects of overcrowding on rats.  He took a group of rats and put them in a cage that was substantially smaller than the amount of space which rats generally have, but he gave them unlimited food and unlimited water (1962).  He found that aggressive behavior and mortality increased, and observed abnormal mating and nurturing habits.  Calhoun called this the “behavioral sink,” and explained that this occurs with all species, including humans.  According to him, this is why crime rates increase in cities.  These behaviors, in fact, act as a natural way of limiting population growth.

            Our environmental footprint has grown too large (Conner, 2006).  Between the shortage of water, global warming, and densely populated areas of the earth, there are many convincing reasons to work on decreasing population.  However, if nothing is done, it is easy to imagine that there will be an external factor that will force a drop in the human population. 


Calhoun, John B. 1962.  “Population Density and Social Pathology.”  Scientific American 206:139-148.

Conner, Steve. “Overpopulation is Main Threat to Planet.” Financial Times 7 January. 2006.

Malthus, T.R. First Essay on Population. A.M. Kelly, New York. 1965.

Walker, James C.G. “How Life Affects the Atmosphere.” BioScience 34.8 (1984): 486-491.

Raleigh, Veena Soni.  “Trends in World Populaion: How Will the Millenium Compare With the Past?” Human Reproduction Update 5.5 (1999): 500-505.

Southwick, Charles H., Ch. 15 from "Global Ecology in Human Perspective" Oxford Univ. Press, 1996, pp. 159-182.


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last updated 5/13/06