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grander scale than have all other species. Some of these human-induced changes – such as the destruction of the
world’s tropical rain forests to create farms or grazing land for cattle – have led to altered climate patterns. In
turn, altered climate patterns have changed the way animals and plants are distributed in different ecosystems.
Scientists study the long-term consequences of human actions on the environment, while environmental-
ists-professionals in various fields, as well as concerned citizens-advocate ways to lessen the impact of human
activity on the natural world.
The science of ecology attempts to explain why plants and animals live where they do and why their popu-
lations are the sizes they are. Understanding the distribution and population size of organisms helps scientists
evaluate the health of the environment.
In 1840 German chemist, Justus von Liebig first proposed that populations could not grow indefinitely, a
basic principle now known as the Law of the Minimum. Biotic and abiotic factors, singly or in combination,
ultimately limit the size that any population may attain. This size limit, known as a population’s carrying capac-
ity, occurs when needed resources, such as food, breeding sites, and water, are in short supply. For example, the
amount of nutrients in soil influences the amount of wheat that grows on a farm. If just one soil nutrient, such
as nitrogen, is missing or below optimal levels, fewer healthy wheat plants will grow.
Either population size or distribution may also be affected, directly or indirectly, by the way species in an
ecosystem interact with one another. In an experiment performed in the late 1960s in the rocky tidal zone along
the Pacific Coast of the United States, American ecologist Robert Paine studied an area that contained 15 spe-
cies of invertebrates, including starfish, mussels, limpets, barnacles, and chitons. Paine found that in this eco-
system one species of starfish preyed heavily on a species of mussel, preventing that mussel population from
multiplying and monopolizing space in the tidal zone. When Paine removed the starfish from the area, he found
that the mussel population quickly increased in size, crowding out most other organisms from rock surfaces.
The number of invertebrate species in the ecosystem soon dropped to eight species. Paine concluded that the
loss of just one species, the starfish, indirectly led to the loss of an additional six species and a transformation of
the ecosystem.
Typically, the species that coexist in ecosystems have evolved together for many generations. These popu-
lations have established balanced interactions with each other that enable all populations in the area to remain
relatively stable. Occasionally, however, natural or human-made disruptions occur that have unforeseen conse-
quences to populations in an ecosystem. For example, 17th-century sailors routinely introduced goats to iso-
lated oceanic islands, intending for the goats to roam freely and serve as a source of meat when the sailors re-
turned to the islands during future voyages. As non-native species free from all natural predators, the goats
thrived and, in the process, overgrazed many of the islands. With a change in plant composition, many of the
native animal species on the islands were driven to extinction. A simple action, the introduction of goats to an
island, yielded many changes in the island ecosystem, demonstrating that all members of a community are
closely interconnected.
To better understand the impact of natural and human disruptions on the Earth, in 1991, the National Aero-
nautics and Space Administration (NASA) began to use artificial satellites to study global change. NASA’s un-
dertaking, called Earth Science Enterprise, and is a part of an international effort linking numerous satellites
into a single Earth Observing System (EOS). EOS collects information about the interactions occurring in the
atmosphere, on land, and in the oceans, and these data help scientists and lawmakers make sound environ-
mental policy decisions.
The problems facing the environment are vast and diverse. Global warming, the depletion of the ozone
layer in the atmosphere, and destruction of the world’s rain forests are just some of the problems that many sci-
entists believe will reach critical proportions in the coming decades. All of these problems will be directly af-
fected by the size of the human population.
Human population growth is at the root of virtually all of the world’s environmental problems. Although
the growth rate of the world’s population has slowed slightly since the 1990s, the world’s population increases
by about 77 million human beings each year. As the number of people increases, crowding generates pollution,
destroys more habitats, and uses up additional natural resources.
The Population Division of the United Nations (UN) predicts that the world’s population will increase from
6.23 billion people in 2000 to 9.3 billion people in 2050. The UN estimates that the population will stabilize at
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