Environmental Engineering Reference
In-Depth Information
Extinction is a natural process, although in many cases it has been accelerated by humans. Species that are
en-
demic
to an area, or occur in only one location on the planet, are more susceptible to these changes, as are spe-
cies that are considered
specialists,
filling a narrow niche in their ecosystems.
When extinctions happen gradually over time, the rate at which they occur is considered to be the
background
extinction rate.
When extinction occurs relatively quickly and on a large scale, affecting many species, it is
considered a
mass extinction event.
So far, there have been five known mass extinction events and each has
killed between 50 percent and 95 percent of all species on Earth. Because of the brisk rate at which species are
presently being lost, some say that the sixth mass extinction event is underway, this time at the hands of hu-
mans.
Natural Ecosystem Changes
Climate Shift
Over geologic time, the Earth's climate naturally shifts as the planet changes and evolves. In the past one billi-
on years, the Earth has experienced six glacial periods. Currently, the Earth is in a period of glacial retreat
called the Holocene epoch, which started about 14,000 years ago. Within this epoch, though, is a brief time
period that is an exception to this retreat. The 1500s to the 1800s were a time of cooling, and because it was the
coldest climate since the Holocene began, it has been termed the "Little Ice Age." From the mid-1800s to
present times, there has been a period of general warming. Some of this is natural, but many people think it is a
result of human activities.
Natural climate shifts can be caused by many changes on Earth. The following are events that can alter the
Earth's climate:
• Volcanoes block solar radiation from reaching the Earth's surface, creating a cooling effect.
• Continental drift alters oceanic currents and atmospheric winds, both of which impact the distribution of
heat on the planet and the altering of water evaporation into the atmosphere.
• Earth's tilt can change, which affects seasons and the amount of sunlight reaching various parts of the
planet.
• Comets and meteorites can have a catastrophic impact on Earth, sending debris and clouds into the atmo-
sphere and blocking solar radiation.
• Geomagnetic reversals may cause localized climate shifts and are currently being researched.
Human activities that can alter the climate include the extensive combustion of fossil fuels and deforestation.
For more information on human-induced climate change, see Chapter 7.
Species Movement
As climates have shifted and changed during the coming and going of the ice ages, species have either adjusted
or died out. Plant species, which can't physically move, must adjust where they grow or face extinction. Plants
can spread by a variety of methods: seeds can blow in the wind, attach to animals and drop a great distance
away from the original plant, or pass through the digestive systems of animals and be spread as the animals mi-
grate. Plants may spread slowly along the edge of their ranges to extend their territory. Since most ice ages take
long periods of time to form and retreat, plants can adjust their territory as the climate shifts.
