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 rattlesnakes, making it the state with the greatest variety of these reptiles. Only four species of rattlesnakes 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 almost nine million square kilometers (3.5 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 less than 300 square kilometers (116 square miles).

Species with ranges that cover most of 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 montana var 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 throughout continental Europe to Siberia 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.

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. The natural range of the giant panda is limited to the location of certain bamboo species, mostly the mountains in western China. The bamboo is limited to these locations largely because of deforestation caused by humans.

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 (Vulpes 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 (Vulpes vulpes).) 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 habitat range has been cut off. In fact, loss of habitat is the leading threat to endangered species today.

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. 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 accompanies 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 intentionally introduce species to new ranges. People transport plants and animals to use for food, decoration, pest control, or as pets. One of the most famous examples of an introduced species is the Burmese python (Python bivittatus) in the Everglades of the U.S. state of Florida. 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 (Alligator mississippiensis) 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 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

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 wooly mammoths (Mammuthus primigenius)—no longer exist in today’s warmer climate.

Earth’s climate has changed many times over the course of our planet’s existence. 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 (Farfantepenaeus aztecus), grouper, and southern flounder (Paralichthys lethostigma) are expanding their range from the Carolinas to the Chesapeake Bay. Unfortunately, creatures that have traditionally lived in the bay, including Atlantic striped bass (Morone saxatilis), 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 U.S. 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.