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| Arctic Tundra: Musk Oxen |
| Musk oxen are hoofed mammals found in the arctic reaches of North America, Greenland, and Siberia. |
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| Carbon Cycle |
| Carbon cycles through Earth’s environment in several forms. |
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| Desert: Cactus |
| This cactus survives in a desert environment outside of these solar-heated greenhouses. |
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| Gray Wolf |
| The gray, or timber, wolf (Canis lupus) is the largest wild member of the dog family. |
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| Grizzly Bear |
| Grizzly bears and Alaskan brown bears are thought to be members of the same species. |
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| Mono Lake |
| These tufa towers at Mono Lake, California, formed in the salty waters of the lake. |
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| Nitrogen Cycle |
| Nitrogen circulates through Earth’s environment in several forms. |
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| Oxygen Cycle |
| Oxygen cycles through Earth’s environment in several forms. |
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| Rivers: Alaska |
| Salmon spend their adult lives in the ocean and then return to the streams where they were born to lay their own eggs. |
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| Savanna: African Elephants |
| African elephants are the largest land mammal in the world, weighing several tons. They are intelligent, social animals. Elephants need large amounts of food, water, and space to roam. Elephant populations declined in the late 1980s due to poaching. |
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| Seashore: Green Sea Turtle |
| The green sea turtle is one of seven species of sea turtles, all of which are on the U.S. Endangered and Threatened Wildlife and Plants List. |
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| Studying Lake Superior |
| The study of lake environments is called limnology. Here you see a plankton net being lowered into Lake Superior to collect samples of zooplankton and phytoplankton. |
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| Swallowtail Butterfly |
| Swallowtail butterflies are found everywhere around the world except in the Arctic. |
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| Swamp: Bromeliad |
| A U.S. Fish and Wildlife Service biologist examines a plant called a bromeliad in Okefenokee National Wildlife Refuge in Georgia. |
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| Water Cycle |
| Water is always moving through Earth's environment. |
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| Wetland: Florida |
| This wetland is part of the Loxahatchee National Wildlife Refuge in Florida. |
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