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A snake slithers up the side of a saguaro cactus. Danger is all around it. Sharp spines stick out of the cactus. One poke could hurt the snake. So it makes its way upward slowly.

Above the snake, a small, gray bird sits on one of the cactus's arms. The bird doesn't see the snake. Its eyes are on the desert below.

The snake moves closer to its prey. Suddenly, it lunges. It opens its mouth to grab the bird. The bird rears back, flaps its wings, and flies off. It's a costly mistake for the snake. Missing a meal here can be the difference between life and death. Food is hard to come by.

The snake slithers down. The bird will return later. Both animals need the saguaro cactus. They aren't the only ones. A saguaro can live for about 150 years. Throughout its life, animals will come upon this amazing plant. They will use it for food, shelter, and water.

Birds will nest in the cactus. Bats will drink from its flowers. Javelinas will eat its fruit. In the parched and scorching desert, the saguaro is the center of life.

The saguaro doesn't start out this way. It's a slow grower. In its first 10 years, it may grow only to about the size of your thumb. That might not seem like much, but most plants can't even grow that tall in the desert.

That's because a desert is an ecosystemthat gets very little rain. Only about 25 centimeters (10 inches) of rain falls each year. Since water is scarce, plants and animals have to make every drop count. The saguaro has adaptations,or behaviors and body parts that help it survive in this tough environment.

Its skin is one of these adaptations. The blazing sun kills most plants in the desert. It dries them out. It doesn't dry out the saguaro, though. The saguaro's dark green skin is thick and waxy. It traps water inside so that the cactus doesn't dry up.

The saguaro also relies on its roots for survival. The roots soak up water to keep it alive. They also hold the cactus in the ground. They grow out far from the plant. Some roots spread as far from the plant as it is tall.

These roots don't go very deep. They are often no more than 8 centimeters (3 inches) under the soil. Having shallow roots is perfect for desert life. During a heavy rain, a saguaro's roots suck up water as soon as it soaks into the ground. Then the roots send the water up into the stem.

Think of the stem as a big storage tank for water. It holds stored water. The saguaro changes as its "tank" gets full. Its skin is pleated, or folded. As the roots take in more and more water, the pleats expand like an accordion. The stem begins to swell.

The stem can store water for months. When there is little rain, the saguaro taps into this tank. As its water supply runs lower and lower, the saguaro gets skinnier and skinnier.

To survive in the desert takes more than just water. The saguaro needs food, too. Like all plants, saguaros make their own food by using sunlight. This is called photosynthesis.

For a cactus, photosynthesis starts at night. That's when tiny pores, or holes, in the plant's stem open. Carbon dioxide, a gas in air, flows into the pores. The cactus stores the gas.

At dawn, the sun rises. The cactus takes in lots of sunlight. Light gives the cactus the energy to make food. The cactus then mixes the stored carbon dioxide with water. That makes sugar, or plant food.

With enough food and water, a saguaro is able to grow flowers. During the spring, its large, white flowers open at night. Bats swoop over the saguaros. They poke their heads into the flowers to sip the sweet nectar.

Bats aren't the only animals that rely on the cactus flowers for food. After the bats finish, white-winged doves fly toward the flowers. They have to work fast to slurp the nectar. The flowers close each day.

In late June, the flowers dry into brown stalks. Then fat fruit grow from the base of the stalks. They sit on top of the tall saguaro like a spiky crown. In the sizzling sun, the fruit split open. Insects and birds feed on the sweet, oozing pulp.

Some of the fruit fall to the ground. A band of hog-like javelinas attack the fruit. Their tough jaws and sharp tusks rip through the spikes and into the soft, ripe, fruity flesh. The javelinas grunt softly and bump into each other as they snack.

After the javelinas leave, other animals stop by to eat the leftovers. Ground squirrels and kangaroo rats eat what remains of the fruit. Mice and rabbits gobble their soft, black seeds.

Not all desert animals eat the saguaro. Some animals make their home in it. For example, a tall saguaro is a good place for a mother Gila woodpecker.

With her long, sharp beak, she breaks into the saguaro's tough skin. Tap, tap, tap. Soon she has drilled a hole deep into it. The hole makes for a cool nesting place to lay eggs.

The woodpecker family only nests in the hole for one season. When the chicks are ready to fly, the family moves on. The nest is not abandoned for long, however. As soon as one kind of bird moves out, another moves in.

Elf owls might be the next to move in. Unlike woodpeckers, elf owls may stay in the same hole for years. From the safety of their nests, these tiny creatures look across the desert each night, looking for prey.

Over time, branches grow like arms out from the saguaro's sides. Red-tailed hawks nest where the arms meet the stem. Lots of other animals now live in or visit the saguaro. It is more like a crowded village than a plant.

Mule deer and other animals eat the tender plants that grow in its shade. A bobcat might sit on top. Up here, it can keep an eye out for predators like coyotes.

Animals come and go. For 150 years or more, the saguaro provides an important habitat where animals find food and shelter. Yet, in the end, old age and disease weaken the stem of the great plant.

Then one night, raindrops splash to the desert floor. The rain soaks the cracked and dusty ground. Suddenly, a jagged bolt of lightning strikes the saguaro. It topples to the ground with a sickening crunch.

The next day, the saguaro becomes a cool, shady home for animals that live close to the desert floor. Scorpions creep into the fallen plant. Rattlesnakes slither around and through it. A black-tailed jackrabbit sips at a long root that still holds water.

Soon the saguaro begins to rot. One day, it will disappear. Other saguaros will grow there. Life in the desert continues.

Article by Barbara Seeber.