Tree Kangaroos live in the lowland and mountainous rainforests of Papua, New Guinea, Indonesia, and parts of Queensland, Australia. Uniquely adapted to these cloud forests, their sharp claws help with climbing, and a long tail keeps them balanced. Meanwhile, their thick, chestnut-colored fur insulates against the damp and provides camouflage.

They prefer to eat mature leaves and fresh-picked fruit, but will also collect fruit that has fallen to the ground. They also enjoy grains, flowers, sap, eggs, young birds, and even bark. A truly unique species, they were probably introduced to New Guinea by humans and adapted to their new, arboreal environment over time. Pretty cool, right?

Considered critically endangered, the major threats facing Tree Kangaroos are hunting and habitat loss.

Foxes build their dens in forests, rock formations, hollow logs and trees, and brush piles. Lined with grass and leaves, these dens provide a safe shelter for blind pups, especially in the first 5 weeks of their lives. They also use trees to escape their predators, grabbing on with their forepaws and scrambling up with their claws and hind feet.

Foxes help control forest populations of rodents like voles and rabbits, and help disperse seeds, further enhancing plant biodiversity around their habitats.

Fox species around the world are threatened by factors such as disease, predation and habitat loss — and the Darwin's fox is considered critically endangered. 

Gopher tortoises depend on forests for their very survival. As dry-land turtles, they live in the well-drained sandy soils of longleaf-pine forests, and dry oak sand hills. And as extraordinary diggers, they carve extensive burrows beneath the forest floor, providing habitat and refuge for hundreds of other species, thus earning their designation as a keystone species.

Primarily herbivorous, they like to eat grasses, mushrooms, saw palmetto berries, and prickly pear cactus pads, fruits and flowers, along with blackberries, blueberries, gopher apples and other low-growing fruits.

As their habitat is lost to urban development, gopher tortoises are struggling to survive. According to the US Fish and Wildlife Service, they are state-listed as threatened in Louisiana, Mississippi, Georgia, and Florida; state-listed as endangered in South Carolina; and protected as a non-game species in Alabama.

The largest tree-climbing mammals in the world,  orangutans sleep in nests of trees and vegetation. Ripe fruit is their favorite thing to eat, but you'll also catch them dining on young leaves, bark, flowers, honey, insects, vines, and the inner shoots of plants.

Thanks to their diet, they play a vital role in protecting biodiversity by eating fruits whole, then expelling the seeds in their droppings. In one study, scientists found 828 seeds in a single fecal sample! And because their droppings create a nice, moist environment for the digested seeds, successful germination is much more likely. Clearly, they’ve earned their title “man of the forest.”

Orangutans are considered critically endangered, and are particularly threatened by deforestation, illegal hunting, and the illegal pet trade. 

Spruce grouse live in the coniferous forests of the northern U.S. and Canada. Unusually nonchalant about human activities, their dark feathers provide an effective camouflage as they navigate their woodland homes. In fact, their plumage and demeanor combine to make them the bane of hunters and birders alike — so if you happen to see one, consider yourself lucky!

Pine, spruce, and other coniferous needles make up the bulk of their diet, but they also dine on fresh green shoots, leaves, berries, flowers, insects, snails, and fungi, helping to keep their populations in check. 

While spruce grouse populations are dwindling in the Northeast and elsewhere, they are currently considered "least concern" and are protected only in certain states.

North American river otters make their dens in abandoned burrows near water’s edge and can thrive in a range of ecosystems, including rivers, lakes, swamps, and estuaries. Their homes are comprised of hollow logs, log jams, piles of driftwood, and boulders. Although most of their lives are spent in the water, these playful Mustelids depend on healthy forests just as much as their terrestrial counterparts.

With razor sharp teeth and claws, they play an integral role in the maintenance of river ecosystems. As predators, they help control populations of fish, crayfish, crabs, frogs, birds and some reptiles, which make up the bulk of their diet. Thanks to a fast metabolism, they need to eat often!

North American river otter populations are considered stable, leading the species to be classified as "least concern." However, the related Giant River Otter, found in Peru, is endangered.

Gray tree frogs depend on water to breed, but live in adjacent mixed and deciduous forests. As a nocturnal species, they rest in tree holes, under bark, in rotten logs, under leaves, and under tree roots during the day. But once the sun goes down, the world is their oyster as they deftly traverse the forest in search of food, using their specially adapted toe pads to climb vertically and move horizontally as needed.

These frogs play an important ecosystem role, consuming enough algae as tadpoles to alter the biology of the ponds they depend on. And once they reach adulthood, they feast on pesky bugs like mosquitoes, gnats, plant lice, mites, snails, slugs, and flies. They also provide a vital food source for larger frogs, some birds, and small mammals.

While Eastern Gray tree frogs aren't currently an endangered species, many frog and toad species are steadily declining — often due to pollution and habitat loss.