Natural ecosystems can be described as self-sustained ecological units, low in human involvement or disruption, living in balance, and having a high proportion of native biodiversity. Unnatural ecosystems, including urban, suburban and agricultural areas, are significantly altered and maintained by human activity. Natural ecosystems offer opportunities for scientific study and for enjoying the complexity and beauty of nature. While no ecosystem is completely untouched by human activity, some nature reserves and parks are protected as natural ecosystems.
Muir Woods National Monument
In the cool mists of Muir Woods National Monument in northern California, ancient coastal redwood trees tower over the forest understory of California bay laurel and bigleaf maples. Northern spotted owls and pileated woodpeckers nest in tree cavities, while coho salmon navigate the streams. Mountain lions stalk black-tailed mule deer, and Pacific giant salamanders prey on banana slugs, which transform organic matter to humus-rich soil. The oldest redwood in this fog-dependent old-growth coastal redwood forest sprouted more than 1,200 years ago.
Great Barrier Reef
The most diverse of aquatic ecosystems, coral reefs grow in warm, shallow ocean waters, built by animal polyp colonies in a symbiotic relationship with algae. The world's largest coral reef system, Australia's Great Barrier Reef, is a richly diverse and complex natural ecosystem comprising some 2,500 reefs and more than 900 islands. Covering 134,363 square miles (348,000 square kilometers), the Great Barrier Reef is home to more than 250 bird species, 600 types of hard and soft coral, more than 1,500 fish species, 4,000 mollusk species, as well as sponges, sea urchins, octopuses, sea stars and jellyfish, many of which are prey for sharks, rays and whales.
Jatun Sacha Biological Reserve
Considered to be the most complex ecosystems on earth with more than half of the world's plant and animal species, tropical rainforests grow in latitudes within 28 degrees of the equator, where temperatures are high and rain is frequent and abundant. In Ecuador's Jatun Sacha Biological Reserve, pumas and jaguars hunt prey such as saddleback tamarins, and many species of colorful frogs, snakes, butterflies and orchids live among more than 1,000 species of large trees. Less than half of the original tropical rainforests remain today.
Cuyahoga Valley National Park
Wetlands -- land areas that have been saturated by water long enough to support aquatic plants -- help control floods, filter pollutants, and provide habitats for diverse aquatic species and other creatures. In the marshes of Ohio's Cuyahoga Valley National Park, beaver, muskrats and river otters swim among cattails and sedges, while great blue herons, bald eagles and northern water snakes search for fish or frogs. Invasive plant species threaten this ecosystem.
Yugyd Va National Park
Taiga, or boreal forests, cover much of northern North America and Eurasia in regions of brief, humid summers and long, cold winters. At Yugyd Va National Park in the Ural Mountains of the Komi Republic of Russia, lynx, wolves, wolverines, sables, white foxes and martens hunt wood grouse and white hares under Siberian spruce, while golden eagles and black kites fly above. Reindeer feed on lichen, while moose and marsh otters forage in wetland areas. In summer, inhabitants battle large mosquitoes and blackflies.
Gates of the Arctic National Park and Preserve
Arctic tundras lie north of the taiga, while alpine tundras lie above the timberline of mountains around the world. With eight months of winter, little precipitation and poor soil, the tundra of Alaska's roadless Gates of the Arctic National Park and Preserve supports only cold-tolerant life: mosses, lichen and liverworts; caribou, brown bears, lemmings and ptarmigans. Some animals, such as arctic terns, fly south or migrate to southern forests in winter. Increasing global temperatures are rapidly altering tundra ecosystems.
Death Valley National Park
Hot deserts -- the driest places on Earth -- support a great variety of life, disproving the impression of desert as a barren place. Death Valley National Park in southern California hosts more than 1,000 plant and 440 animal species. Joshua trees, mesquite and cottontop barrel, silver cholla and prickly pear cacti dot the landscape. Desert cottontails, black-tailed jackrabbits and desert bighorn sheep browse, while kit foxes chase southern grasshopper mice. Some 36 reptile species, including desert tortoises, chuckwallas and coachwhip snakes, thrive here, and birdwatchers have recorded more than 350 bird species, from year-round roadrunners to warblers and hummingbirds.
Olympic National Park
Twelve to 14 feet of rain per year and moderate temperatures help sustain the old-growth temperate rainforests at Olympic National Park, Washington. Huge Sitka spruce, Douglas fir, western hemlock and epiphytic ferns and mosses cover this lush ecosystem. Northern spotted owls, mink, wolverines, red foxes, mountain beaver, marbled murrelets, rubber boas and many amphibians move among the shadows and streams. Fishers, once overhunted to extinction, have been reintroduced.
Tallgrass Prairie National Preserve
Less than 4 percent of the native tallgrass prairie ecosystem of central North America remains today. Some of it is included in the Tallgrass Prairie National Preserve in Kansas, maintained by the National Park Service and the Nature Conservancy. Hundreds of plant species bloom among the prairie grasses. Roots of this "sea of grass" sink deep into the fertile soil, enabling the plants to survive fire and drought. Wildlife ranging from coyotes, bobcats and badgers, to prairie voles and raccoons move about under the big bluestem and Yellow Indiangrass. Whitetail deer, bison and eastern cottontail rabbits are common, and 79 bird species are permanent or summer residents.
Hydrothermal vents escaped human discovery until the 1970s. They lie at sunlight-deprived ocean depths in spots where water heated by proximity to hot magma seeps from cracks in the ocean's floor. Unlike other life on Earth, the strange shrimp, crabs, giant tubeworms and other species that flourish in the extremely hot water depend on hydrogen sulfide and chemosynthesis -- not photosynthesis -- for the primary energy source in their food chain. Some sea vent bacteria have evolved little since life began on Earth.
- The Living Wilderness; Wilderness as a Land Laboratory; Aldo Leopold
- US Forest Service: The Report of the Ecological Society of America Committee on the Scientific Basis for Ecosystem Management
- Conservation Biology; A Conceptual Framework for Quantifying Naturalness; Jay E. Anderson
- National Park Service: Muir Woods National Monument
- National Park Service: Cuyahoga Valley National Park
- Yugyd va National Park: The Park's Nature
- Yugyd va National Park: The Park's Nature
- National Park Service: Gates of the Arctic National Park and Preserve
- National Park Service: Death Valley National Park
- National Park Service: Olympic National Park
- National Park Service: Tallgrass Prairie National Preserve
- National Oceanic and Atmospheric Administration: Ocean Facts; A hydrothermal vent forms when seawater meets hot magma; January 13, 2013
- The Franklin Institute: Resources for Science Learning: Neighborhoods: Ecosystems, Biomes, and Habitats
- Clearing and Present Danger? Fog That Nourishes California Redwoods Is Declining: Scientific American: Michael Tennesen
- The Ecology and Evolution Department at Stonybrook University: Coral Reefs
- Comstock/Stockbyte/Getty Images