Encyclopedic Entry

This map shows that the species range of crocodilians is mostly limited to tropical, humid climates.

Photograph by Prakash Yk, MyShot

From Fish Farms to Invasive Species
People brought the Pacific oyster to Europes North Sea in the 1960s as a commercial shellfish. This oyster requires a very specific, warm-water temperature to reproduce. The cold North Sea was not the right temperature, so people believed the species was safely confined to aquaculture farms. Unfortunately, water temperatures have changed due to global warming. In the 1990s, the Pacific oyster started reproducing successfully in the North Sea, and it has begun displacing some of the native oyster species.

A species range is the area where a particular species can be found during its lifetime. Species range includes areas where individuals or communities may migrate or hibernate.

Every living species on the planet has its own unique geographic range. Rattlesnakes, for example, live only in the Western Hemisphere, in North and South America. The U.S. state of Arizona is part of the range of 13 species of rattlers, making it the state with the greatest variety of these reptiles. Only four species of rattlers have a range east of the Mississippi River.

Some species have a wide range, while others live in a very limited area. For example, the range of the leopard (Panthera pardus) encompasses more than 20 million square kilometers (7.7 million square miles) across Africa, the Middle East, and Asia. Another type of wild cat, the rare Iriomote cat (Prionailurus iriomotensis), lives only on Japan’s Iriomote Island. Its range is only about 292 square kilometers (113 square miles).

Species with ranges that cover most of the Earth are said to have a cosmopolitan distribution. Blue whales (Balaenoptera musculus) have a very cosmopolitan distribution—they are found in every ocean on the planet. Human beings (Homo sapiens) also have cosmopolitan distribution, inhabiting every continent except Antarctica.

The Antarctic midge (Belgica antarctica), on the other hand, is found only in Antarctica—it is endemic, or native, to that continent. Species with a limited range, like the Antarctic midge or Iriomote cat, have an endemic distribution.

Species with two or more ranges that do not connect with each other have a disjunct distribution. Mountain ranges, deserts, or oceans sometimes separate the ranges of these species. The kudzu plant (Pueraria lobata) has a disjunct distribution in the southern islands of Japan and the southeast Asian mainland, as well as the United States. The Eurasian pygmy shrew (Sorex minutus) has a disjunct distribution in Europe and the island of Ireland.

Factors Contributing to Species Range

Several factors determine species range. Climate is one important factor. For example, polar bears (Ursus maritimus) travel on sea ice, so the limit of their range is determined by the amount of sea ice that forms in the winter. Many species of cacti and other succulent plants are adapted to live in very hot, dry climates. They cannot survive in areas with lots of rainfall or long periods of cold.

 

Food sources also affect species range. Living things can only survive in regions where they can find food. The giant panda (Ailuropoda melanoleuca) obtains almost all of its nutrients from various species of bamboo, especially dragon’s head bamboo (Fargesia dracocephala). The natural range of the giant panda is limited to the natural range of the dragon’s head bamboo, mostly the Qinling and Minshan mountains in western China.

When a food source disappears or alters its range, species that rely on it must find another food source, extend their range, or risk extinction. The range of the Arctic fox (Alopex lagopus) is the cold, northern latitudes. It feeds mostly on small rodents such as lemmings. The Arctic fox is uniquely adapted to the Arctic and cannot change its range if lemmings become more difficult to hunt. (Lemmings are not rare or endangered. They only become more difficult to hunt when the Arctic fox must compete for the prey with other animals, such as the red fox.) However, Arctic foxes have other food sources in their range: seals, fish, and even carrion, or dead animals.


Like food, water is a critical component in a species range. Some creatures live in riparian habitats—areas on the banks of rivers or streams. Animals such as river otters depend on the river’s ecosystem for survival. When people dam rivers to make reservoirs or produce electricity, the wildlife downstream often cannot survive. Their range has been cut off. In fact, loss of habitat is the leading threat to endangered species today.

Access to water can also determine a species range for animals that do not have a freshwater habitat. Many species of African elephants migrate more than 60 kilometers (100 miles) to find watering holes and streams in the dry season. The search for fresh water determines the limits of their range.

Landscape features can also determine species range. The mountain goat (Oreamnos americanus) got its name because it lives in mountainous areas. Its large range extends throughout western North America: the Rocky Mountains, the Cascade Mountains, and the Chugach Mountains.

Changes in Ranges

Species range can change over time. Many species have different summer and winter ranges. Canada geese (Branta canadensis) spend summers in Canada and the northern United States, but migrate to the southern U.S. and northern Mexico during winter.


Some species also have different ranges for breeding. Many species of Pacific salmon have a freshwater range and a saltwater range. They hatch and spend their early lives in freshwater rivers and streams. On reaching adulthood, they migrate to the ocean. Some salmon stay within a few hundred kilometers of their home stream, while others, like the Chinook salmon (Oncorhynchus tshawytscha), can travel as far as 4,023 kilometers (2,500 miles). When it is time to reproduce, salmon return to their freshwater range. The eggs hatch in the fresh water, and the cycle begins again.

Humans have changed the range of many species by transporting them. These are “introduced species.” Introduction can happen accidentally, when a living thing “hitches a ride” with unsuspecting human travelers. This has been happening for thousands of years. The disjunct distribution of the Eurasian pygmy shrew, for instance, is probably a result of introduction. Scientific research shows that the Irish population of Eurasian pygmy shrews appeared about the same time that Europeans sailed to Ireland and established settlements there.

People continue to accidentally introduce species to new ranges today. The natural range of the zebra mussel (Dreissena polymorpha) is central Asia, in lakes and the Black and Caspian seas. In the 20th century, these animals were accidentally transported beyond Asia when they attached themselves to large cargo ships. They eventually reached the Great Lakes of North America, where they established a new range.

Zebra mussels, like many introduced species, are a major threat to native species of the area. For example, the brown tree snake (Boiga irregularis), native to Australia and nearby islands, was accidentally transported to Guam through air or ship cargo. Few local animals could defend themselves against this new predator, and the brown tree snake caused the extinction of many native birds and lizards on the island. Because some of the animals it killed were pollinators, many native plant species also declined.


People also introduce species to new ranges on purpose. People transport plants and animals to use for food, decoration, pest control, or pets. One of the most famous examples of an introduced species is the Burmese python (Python molurus bivittatus) in the Florida Everglades. People kept the snakes, whose native range is the jungles of southeast Asia, as pets. The care and feeding of Burmese pythons is intense, and some pet owners who could not support the reptiles simply released the snakes into the wetlands of the Everglades. The pythons thrive in the Everglades, which has a hot, humid climate similar to southeast Asian jungles. Pythons compete with native species like the American alligator for food and resources.


Plants can also be introduced to new ranges, and threaten endemic species. The purple loosestrife (Lythrum salicaria), with its pretty lavender-colored flowers, hardly seems threatening. But this plant has done extensive damage to North American wetlands. People brought the flower from Europe in the 1800s for decoration and medicinal purposes. The plant grows rapidly along river banks and other freshwater wetlands. It produces many seeds, and is pollinated by many insect species. As a result, it can spread quickly, reduce water flow, and crowd out native plants such as cattails.

In some of the wetlands of the Chesapeake Bay, in the U.S. state of Maryland, purple loosestrife has displaced more than half of the native plant species. It provides poor food, shelter, and nesting sites for local wildlife. Its dense, snarled root system can clog drainage and irrigation ditches.

Effects of Climate

The Earth’s changing climate affects species range. Ranges can move, shrink, or grow as a result of climate changes. Sometimes, changes in climate can even cause species to go extinct. For instance, many animals that were adapted to Ice Age conditions—such as mastodons, mammoths, and saber-toothed cats—no longer exist in today’s warmer climate.

Earth’s climate has changed many times over the course of our planet’s lifespan. These changes happen as a result of natural events and cycles. Today, human activities are contributing to climate change. This global warming has an effect on the ranges of many organisms.

European bee-eaters (Merops apiaster), for instance, are brightly colored birds native to the Mediterranean coast of Europe and northern Africa. In the 20th century, bee-eaters began to be spotted in central Europe. Today, their range includes nesting sites in Germany and the Czech Republic—countries that once would have been too cold for these warm-weather birds.

In aquatic environments, climate change favors warm-water species. On the Atlantic coast of the U.S., brown shrimp, grouper, and southern flounder are expanding their range from the Carolinas to the Chesapeake Bay. Unfortunately, creatures that have traditionally lived in the bay, including rockfish, sturgeon, and clams, are threatened by warming temperatures and increased competition for resources.

Species that are considered pests or that spread diseases can wreak havoc on local populations when their ranges expand. For example, many species of the spruce budworm are destructive to evergreen trees in western North America. The insect’s traditional range includes the forests of the U.S. state of Washington and the Canadian province of British Columbia. Warming temperatures are allowing this caterpillar to eat its way northward, all the way to the U.S. state of Alaska, for the first time in history. The expanded range of the spruce budworm threatens the American and Canadian timber industries.

Many species of mosquito are expanding their range as the climate grows warmer. Mosquitoes carry a variety of diseases that can be deadly to people: malaria, encephalitis, West Nile virus, and yellow fever. Many communities and health-care organizations are unprepared for the increased number of cases brought by mosquitoes’ expanded range.

Vocabulary

Term Part of Speech Definition Encyclopedic Entry

antler

Noun

horn-like bony outgrowth on deer and related animals.

aquatic

Adjective

having to do with water.

breed

Verb

to produce offspring.

cargo

Noun

goods carried by a ship, plane, or other vehicle.

Carolinas

Noun

the U.S. states of North Carolina and South Carolina.

carrion

Noun

flesh of a dead animal.

caterpillar

Noun

larva of a butterfly or moth.

cattail

Noun

aquatic plant.

Chesapeake Bay

Noun

large, shallow estuary of the Susquehanna and other rivers that flow through the U.S. states of Maryland, Virginia, West Virginia, Delaware, Pennsylvania, and New York and the capital of Washington, D.C., before emptying in the Atlantic Ocean.

climate

Noun

all weather conditions for a given location over a period of time.

Encyclopedic Entry: climate

climate change

Noun

gradual changes in all the interconnected weather elements on our planet.

Encyclopedic Entry: climate change

continent

Noun

one of the seven main land masses on Earth.

Encyclopedic Entry: continent

cosmopolitan distribution

Noun

range of a species found across most parts of the Earth.

critical

Adjective

very important.

dam

Noun

structure built across a river or other waterway to control the flow of water.

dense

Adjective

having parts or molecules that are packed closely together.

disease

Noun

a harmful condition of a body part or organ.

disjunct distribution

Noun

range of a species found in two or more geographic areas that are not connected to each other.

downstream

Noun

in the direction of a flow, toward its end.

dry season

Noun

time of year with little precipitation.

ecosystem

Noun

community and interactions of living and nonliving things in an area.

Encyclopedic Entry: ecosystem

electricity

Noun

set of physical phenomena associated with the presence and flow of electric charge.

encephalitis

Noun

sometimes fatal disease, often transmitted by mosquitoes, affecting the brain.

endangered species

Noun

organism threatened with extinction.

Encyclopedic Entry: endangered species

endemic

Adjective

native to a specific geographic space.

endemic distribution

Noun

range of a species found in a very limited geographic area.

establish

Verb

to form or officially organize.

Everglades

Noun

vast swampy region flowing south of Lake Okeechobee in Florida.

evergreen

Noun

tree that does not lose its leaves.

extend

Verb

to enlarge or continue.

extinct

Noun

no longer existing.

extinction

Noun

process of complete disappearance of a species from Earth.

flounder

Noun

popular food fish with a flat body.

forest

Noun

ecosystem filled with trees and underbrush.

freshwater

Noun

water that is not salty.

global warming

Noun

increase in the average temperature of the Earth's air and oceans.

Encyclopedic Entry: global warming

Great Lakes

Noun

largest freshwater bodies in the world, located in the United States and Canada. Lake Huron, Lake Ontario, Lake Michigan, Lake Erie, and Lake Superior make up the Great Lakes.

grouper

Noun

bottom-dwelling marine fish native to non-polar waters.

habitat

Noun

environment where an organism lives throughout the year or for shorter periods of time.

Encyclopedic Entry: habitat

havoc

Noun

violent destruction.

hibernate

Verb

to reduce activity almost to sleeping in order to conserve food and energy, usually in winter.

Ice Age

Noun

last glacial period, which peaked about 20,000 years ago.

inhabit

Verb

to live in a specific place.

insect

Noun

type of animal that breathes air and has a body divided into three segments, with six legs and usually wings.

interdependent

Adjective

two or more individuals or communities that rely on each other for survival.

introduced species

Noun

a species that does not naturally occur in an area. Also called alien, exotic, or non-native species.

Iriomote cat

Noun

small wild cat native to Iriomote Island, Japan.

irrigation ditch

Noun

channel dug between a source of water and crops. Also called an irrigation canal.

jungle

Noun

tropical ecosystem filled with trees and underbrush.

landscape

Noun

the geographic features of a region.

Encyclopedic Entry: landscape

latitude

Noun

distance north or south of the Equator, measured in degrees.

Encyclopedic Entry: latitude

malaria

Noun

infectious disease caused by a parasite carried by mosquitoes.

mammoth

Noun

one of many extinct species of large animals related to elephants, with long, curved tusks. The last mammoths became extinct about 5,000 years ago.

mastodon

Noun

one of many extinct species of large animals related to elephants. The last mastodons became extinct about 11,000 years ago.

midge

Noun

tiny insect related to the fly.

migrate

Verb

to move from one place or activity to another.

mosquito

Noun

insect capable of piercing the skin and sucking the blood of animals.

mussel

Noun

aquatic animal with two shells that can open and close for food or defense.

nesting site

Noun

place where birds build nests and raise their young.

nutrient

Noun

substance an organism needs for energy, growth, and life.

Encyclopedic Entry: nutrient

ocean

Noun

large body of salt water that covers most of the Earth.

Encyclopedic Entry: ocean

pest

Noun

harmful or annoying person or thing.

pest control

Noun

process of removing unwanted species (pests) from a specific geographic area.

pollinator

Noun

animal, object, or force such as wind that transfers pollen from one plant to another, allowing seeds to develop.

predator

Noun

animal that hunts other animals for food.

prey

Noun

animal that is hunted and eaten by other animals.

python

Noun

non-venomous snake native to tropical climates.

rattlesnake

Noun

venomous reptile, native to North America, with hollow joints at the end of its tail that can be rattled to warn predators.

reptile

Noun

animal that breathes air and usually has scales.

reservoir

Noun

natural or man-made lake where water is stored.

Encyclopedic Entry: reservoir

resource

Noun

available supply of materials, goods, or services. Resources can be natural or human.

riparian

Adjective

having to do with a river or stream.

river

Noun

large stream of flowing fresh water.

Encyclopedic Entry: river

river otter

Noun

mammal, related to the weasel, that lives near streams and rivers.

rockfish

Noun

marine fish that lives near rocky shores.

rodent

Noun

order of mammals often characterized by long teeth for gnawing and nibbling.

saber-toothed cat

Noun

one of many extinct species of large, wild cats with long, sharp teeth.

sea ice

Noun

frozen ocean water.

settlement

Noun

community or village.

shelter

Noun

structure that protects people or other organisms from weather and other dangers.

shrew

Noun

type of small mammal resembling a mouse with a long nose.

species

Noun

group of similar organisms that can reproduce with each other.

species range

Noun

native, geographic area in which an organism can be found. Range also refers to the geographic distribution of a particular species.

Encyclopedic Entry: species range

spruce budworm

Noun

moth larva that is a destructive pest to evergreen trees.

sturgeon

Noun

type of marine or freshwater large, long, bony fish.

succulent

Noun

type of plant that has thick leaves and stems for storing water.

thrive

Verb

to develop and be successful.

timber

Noun

wood in an unfinished form, either trees or logs.

unique

Adjective

one of a kind.

watering hole

Noun

small pond or spring where animals travel to drink.

Western Hemisphere

Noun

area of the Earth west of the prime meridian and east of the International Date Line.

West Nile virus

Noun

infectious disease spread by mosquitoes, with symptoms ranging from mild flu to possible death.

wetland

Noun

area of land covered by shallow water or saturated by water.

Encyclopedic Entry: wetland

wildlife

Noun

organisms living in a natural environment.

wreak

Verb

to inflict or bring about something painful.

yellow fever

Noun

infectious disease spread by mosquitoes, primarily affecting the liver.

Credits

Media Credits

The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited.

Writer

Kim Rutledge
Melissa McDaniel
Diane Boudreau
Tara Ramroop
Santani Teng
Erin Sprout
Hilary Costa
Hilary Hall
Jeff Hunt

Illustrator

Tim Gunther
Mary Crooks, National Geographic Society

Editor

Kara West
Jeannie Evers

Educator Reviewer

Nancy Wynne

Producer

Caryl-Sue, National Geographic Society

Sources

Dunn, Margery G. (Editor). (1989, 1993). "Exploring Your World: The Adventure of Geography." Washington, D.C.: National Geographic Society.

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