EC6. Carrying Capacity

In an influential essay entitled, “Essay on the Principle of Population,” Thomas Malthus noted that the growth of population and what he thought should be the inevitable tidal wave of people that would eventually inhabit the Earth, hadn’t occurred and that somehow the human population must be kept in check by other factors such as famine and war. 

Charles Darwin on his voyage around the world on the HMS Beagle, was greatly influenced by Malthus’ work and noted that in nature, populations also do not overrun an environment but are kept in check in a struggle for existence. 

In Chapter 2, we discussed negative feedback mechanisms that contribute to the stability of a system. All populations of organisms are limited by a combination of environmental and biological factors such as sunlight, physical space, temperature, shelter, water, food supply, predation, and disease. If a system is in dynamic equilibrium, its populations of plants and herbivores will fluctuate slightly over time, as populations of predators, parasites, and disease causing organisms rise and fall in response to food availability. These Limiting Factors determine the Carrying Capacity of an ecosystem. The carrying capacity of an ecosystem will also fluctuate slightly with changes in weather patterns.

In the past, large predators such as wolves, bears, and mountain lions have controlled the populations of deer, elk, caribou, and moose by eating fawns and sick individuals. With the extermination of large predators from most areas of the country, populations of herbivores have often grown to the point that they damage the vegetation and soil, thereby reducing the carrying capacity of the area. Removal of predators from natural systems has been a major positive feedback mechanism contributing to instability in our North American ecosystems.

All of our public parks, forests and range lands have wildlife biologists who conduct research and develop management plans to maintain diversity and stability in the ecosystems. Their management plans focus on restoring species of plants and animals that have been lost from the system and controlling populations of herbivores.

In a recent success story, wolves have been reintroduced to Yellowstone National Park. Over the last few decades, coyotes have taken over the role played by wolves before their extinction in the lower 48 states. Coyotes expanded their range rapidly and are currently found in every state. They have even become pests, killing family pets and have reportedly attacked small children where human population density has clashed with the habitat of the animal.

However, a study of the effects of returning the wolves to Yellowstone has found that the wolves have quickly reestablished themselves as the primary predator and the coyotes have been “put in their place” by the return of wolves, seeing their range shrink and population dwindle to their proper levels.

This successful reintroduction of wolves bodes well for the reestablishment of a dynamic equilibrium to ecosystems that existed before the advent of large numbers of humans in North America. 

I. Habitat Fragmentation and Carrying Capacity

In a landmark study in 1966, E.O. Wilson and Dan Simberloff wrapped a small Mangrove island in Florida in plastic and proceeded to eliminate every insect on the island. The purpose of this study was to see how long it took for the island to regain its former insect population levels. It took only a year before the same number and variety of insect species inhabited the small island. 

In a similar study, scientists saved tracts of the Amazon Rainforest as it was being clear cut to determine the area that was needed to avoid a collapse of the rainforest ecosystem. The answer was a large area. Why was this? The answer can be found by looking at how many species an area supports prior to deforestation.

If we look at a map of species density, we see that the areas around the equator support the most species. Conversely, the further north and south we go and the further offshore, the less the number of species we find. 

Tropical rainforests support a large number of species. If the area of the forest shrinks then the number of species that can survive is limited as well. As humans change the ecosystem by converting forest to farmland, the area available to the tropical species shrinks and many of them become extinct. Many scientists suggest that only a fraction of the species in tropical forest ecosystems have been identified, particularly smaller ones such as insects and microfauna. 

Numbers of Species

	Estimated Numbers of Species	Described species as % total of and level of accuracy
Viruses	4000	1.00 VP
Bacteria	4000	0.40 VP
Protozoa and algae	80000	13.33 VP
Vertebrates	52000	94.55 G
Insects and myriapods	963000	12.04 M
Arachnids	75000	10.00 M
Molluscs	70000	35.00 M
Crustaceans	40000	26.67 M
Nematodes	25000	6.25 P
Fungi	72000	4.80 M
Plants	270000	84.38 G

Note: Estimates of described species are invariably incomplete because new species are being added all the time. The generally accepted working totals are about 1.75 million for all described species and 13.6 million for all species, both described and unknown. For latest data, see the source: UNEP-WCMC.

II. Habitat Change Close to Home

The first settlers to North America saw a very different vista than that which we see today. It was said that a squirrel could travel tree to tree, from the east coast to the west coast, without ever touching the ground. Since that time we have changed our ecosystems significantly. We have made farms, built cities and created ribbons of highways crisscrossing the country. The original, vast ecosystems of the United States have been fragmented into smaller units, and we have “designed” our environments to reflect our economic needs. This has resulted in a number of unintended consequences. 

III. Invasive Species

IV. Unwanted Interaction with Animals

V. Loss of Biodiversity

While we have made good progress in preserving vast tracts of national forest, we are less vigilant about protecting the ecosystem around us. Many small towns have seen explosive growth in population and the once open fields and forests that characterized the area have given way to subdivisions and shopping centers. Many towns can’t afford to buy open space to preserve it because of the skyrocketing costs of land. 

What is your city or town doing to preserve open space or combat the invasive species found in your area?

VI. Food Resources and Carrying Capacity

If climates warm and precipitation patterns change this will probably affect food production in various areas of the world. Foods that a local culture formerly depended upon may be unable to grow in the region’s changing climate. For example, climate models could predict less precipitation falling in the Midwest, the nation’s main grain growing region. Currently, our country ships surplus grain to nations without adequate food resources. If the United State’s crop production declines, we may be unable to ship food to people in other countries.

Farmers and herdsmen have been managing plant and animal populations for more than 10,000 years. Farmers, who are trying to maximize production, whether it is wheat, chickens, or catfish, must consider all the factors that might limit the population to be harvested. In the case of wool production, the rancher may focus on the carrying capacity of the rangeland and the rainfall fluctuations that affect the location of drinking water and the growth of grass. For an intensive farm system to produce pigs, the farmer uses sophisticated technology for feeding, breeding, and medical intervention to prevent disease and maximize the productivity of the herd.

For every domesticated plant or animal, there is a fascinating history of cultural evolution and the application of mathematics and science. Humans have been very ingenious in removing the limits to population growth for the species they want to harvest.

VII. Over Use of Natural Resources

Evidence from archaeology suggests that a number of ancient cultures may have overused the land resulting in a decline in the culture itself. For example, archaeological evidence reveals that Easter Island, a small island off the coast of western South America, was once forested with several species of trees including a giant palm that today grows only on the coast of Chile. The island also once supported enormous bird rookeries. 

About 500 years ago Polynesian explorers landed in canoes and established a civilization that is famous for creating hundreds of gigantic heads sculpted in stone. When Admiral Jacob Roggreveen of the Dutch West India Company discovered the island on Easter Sunday in 1722, the islanders were using simple reed floats as their only boats and appeared to have little knowledge of the culture that produced the large stone statues. The trees and birds had also become extinct, almost certainly as a result of over-harvesting by the island’s early human population. The population of the island reached a peak of over 10,000, far exceeding the carrying capacity of the island’s ecosystem. Resources became scarce and the lush forests were destroyed. In this regard, Easter Island has become a metaphor for ecological disaster. 

See Staying current for this chapter.