Page 81 - Environment: The Science Behind the Stories

nearly driven to extinction by human hunting; livestock and over time. Populations generally grow when resources are
alien plants that destroyed and displaced the vegetation it fed abundant and natural enemies are few. Populations can decline
on; and rats, cats, dogs, pigs, and mongooses that preyed on in response to loss of resources, negative impacts from other
its eggs and young. Ne¯ne¯s disappeared from all islands except species, or natural disasters that kill large numbers of individ-
the island of Hawai‘i, and in recent decades biologists and uals. Researchers estimate that the ne¯ne¯ population surpassed
wildlife managers have labored to breed it in captivity and 25,000 birds before Europeans reached the Hawaiian Islands.
reintroduce it to protected areas on other islands. These efforts By the 1950s, after two centuries of impacts from hunting,
have met with success, and today ne¯ne¯s live in at least seven agriculture, non-native mammals, and invasive plants, the
populations on four islands. population was down to just 30 individuals. Since then, inten-
In contrast, the human species faces few threats from sive conservation efforts have turned this decline around, and
other animals, is a consummate generalist, and has spread into now over 2000 ne¯ne¯s live on the Hawaiian Islands.
nearly every corner of the planet. As a result, it is difficult to The passenger pigeon, now extinct, illustrates the extremes
define a distinct human population on anything less than the of population size (Figure 3.13). Not long ago it was the most
global scale. In the ecological sense of the word, all 7 billion abundant bird in North America; flocks of passenger pigeons
of us comprise one population. literally darkened the skies. In the early 1800s, ornithologist
Alexander Wilson watched a flock of 2 billion birds 390 km
(240 mi) long that took 5 hours to fly over and sounded like a
Populations show characteristics tornado. Passenger pigeons nested in gigantic colonies in the
that help predict their dynamics forests of the upper Midwest and southern Canada. Once set-
tlers arrived and began cutting the forests, however, the birds
All populations—from humans to ne¯ne¯s—exhibit characteris- made easy targets for market hunters, who gunned down thou-
tics that help population ecologists predict the future dynam- sands at a time and shipped them to market by the wagonload.
ics of the population. Attributes such as density, distribution, By the end of the 19th century, the passenger pigeon popula-
sex ratio, age structure, and birth and death rates all help the tion had declined to such a low number that the birds could
ecologist understand how a population may grow or decline. not form the large colonies they apparently needed in order to
The ability to predict growth or decline is useful in monitor- breed. In 1914, the last passenger pigeon on Earth died in the
ing and managing threatened and endangered species (see The Cincinnati Zoo, bringing the continent’s most numerous bird
Science behind The STory, pp. 82–83). It is also useful in studying species to extinction within just a few decades.
human populations (Chapter 8). Understanding human popula-
tion dynamics is a central element of environmental science and Population density The flocks and breeding colonies of
is one of the prime challenges for our society today.
passenger pigeons showed high population density, another
attribute that ecologists assess to understand populations.
Population size Expressed as the number of individ- Population density describes the number of individuals in
ual organisms present at a given time, population size may a population per unit area. High population density makes it
increase, decrease, undergo cyclical change, or remain stable easier for organisms to group together and find mates, but it can

(a) Passenger pigeon (b) 19th-century lithograph of pigeon hunting in Iowa
Figure 3.13 The passenger pigeon was once North America’s most numerous bird. Its flocks literally
darkened the skies when millions of birds passed overhead. However, hunting and deforestation drove the
80 species to extinction within just a few decades.

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