Joshua Tree National Park Ecology
Looking around Joshua Tree National Park, the barren landscape may at first seem lifeless, but a closer look reveals a thriving ecosystem of plants and animals perfectly adapted to the harsh desert environment.
Over fifty species of mammals, forty species of reptiles, and seven hundred species of plants have been identified in the park, including the unique tree the park is named after. Joshua Tree National Park is located in Southern California, along the boundary of two distinct deserts defined by elevation and precipitation patterns: the Mojave and Colorado Deserts.
The Mojave Desert
Located between the Great Basin Desert to the north and the Sonoran Desert to the south, the Mojave Desert is the smallest of North America’s four deserts. The western half of Joshua Tree National Park is located on the southern tip of the Mojave ranging from 2,000-5,000 feet in elevation. On average, the Mojave Desert receives about 3-5 inches of rain annually, most of which occurs during the winter. Because of low annual precipitation competition for survival is fierce resulting in some plants and animals evolving defense mechanisms such as deadly toxins and spikes in order to survive. The Mojave Desert has a slightly cooler climate than the Colorado Desert and is also characterized by the thousands of Joshua Trees (Yucca brevifolia) native to this region.
The Colorado Desert
The eastern half of Joshua Tree National Park is located on the westernmost edge of the Colorado Desert, which is part of the Sonoran Desert. This region is characterized by low elevations (< 2,000 feet), hotter temperatures, less rainfall, and wide-open spaces between mountain ranges. Compared to the Mojave Desert, the Colorado Desert may at first seem barren. It is the most arid region in North America, but actually supports a greater diversity of plants and animals. For example, the kangaroo rat goes its entire life without drinking one drop of water. One of the most noticeable differences between the Mojave and the Colorado Deserts is that the Joshua Tree is absent here, while other plants, such as the ocotillo, smoketree, and numerous holla varieties thrive, especially at Cholla Cactus Garden and the Ocotillo Patch in the southeastern part of the park. The Cholla Cactus Garden occurs just off the road and is filled with a large amount of Teddy bear Cholla. Although the holla cactus appears fuzzy-looking and has a cute name, they have a tendency to viciously jump and latch onto nearby animals (including humans), which is why they are also called Jumping Cholla. Most animals shy away from the Teddy bear Cholla, but desert woodrats build their nests within this species to keep predators away. About a mile and a half from the Cholla Cactus Garden is the Ocotillo Patch. The ocotillo is a spindly plant with thorny vertical branches and can grow up to thirty feet tall. Frequently mistaken for a type of cactus, the ocotillo is a unique species that belongs to a family of Mexican trees. Most of the year, the ocotillos appear greyish green to brown, but precipitation ignites small, bright green leaves that fall off as soon as the arid conditions return. Ocotillos can grow their leaves up to eight times a year, depending on rainfall, and during spring red flame-like flowers appear on the tips of their branches.
The Pinyon Pine and Juniper Zone
The Mojave and Colorado Desert are the dominant ecosystems in the park, but a third, high-elevation ecosystem hosts pinyon pine and juniper trees. This woodland forest provides abundant shelter, shade, and food resources within the highest mountain regions of the park.
Weather and Climate
Daily high and low temperatures can vary as much as 50 degrees Fahrenheit within Joshua Tree National Park due to its aridity and proximity to nearby mountains. This part of southeastern California is in a rain shadow desert, a place where the topography controls climate. As moist air moves inland from the Pacific Ocean, mountains block its path, forcing the air to rise. As the air rises, it cools. Cool air cannot hold as much moisture as warm air can, so precipitation falls on the western flanks of the mountains before traveling eastward. By the time the air gets to the eastern side of the mountains, there is very little moisture left. When it does rain in the desert, most rain evaporates before hitting the ground (virga) or runs off before the dry soil can soak it up, which can also lead to dangerous flash floods. Although all species have adapted to the limited water resources, most of them cannot survive without any water. Severe drought challenges Joshua Trees, the desert tortoise, desert bighorn sheep, and other species to shift ranges to higher elevations in the park with greater rainfall.
Plants
In an ecosystem food chain, primary producers are the first link in the chain because they obtain their energy from the sun. In addition to sunlight, plants need water to survive long enough to reproduce. Although the desert lacks abundant water, the plants in Joshua Tree National Park have adapted to survive by hoarding water for long periods of time and producing seeds quickly during periods of rainfall.
WATER STORAGE
Cacti store water so well that dehydrated animals target their reserves whenever possible. Cacti have, in turn, developed spines to keep animals away. Their pointy spines also help shade the stem and lessen wind stress by breaking the wind into pieces. Cacti root systems are large but shallow so they can soak up as much water as they can as soon as it rains. Large barrel cactus can survive for over a year without rain.
DROUGHT TOLERANCE
Desert trees and shrubs have also adapted to the desert climate with additional adaptations. The brittlebush is a common shrub found in Joshua Tree National Park with fuzzy gray leaves that act as a moisture-retaining blanket, protecting the plant from extreme temperatures. The leaves are a lighter color to reflect sunlight and help keep the plant cool. Sage brush also tolerates long periods of drought well.
DROUGHT AVOIDANCE
Plants in desert ecosystems are patient and strategic. Wildflowers and grasses avoid droughts by not blooming if there isn’t enough precipitation during the winter season. Wildflowers produce seeds under good conditions that lie dormant for years in the harsh desert environment. Seeds will germinate and bloom during brief rainy seasons and leave their seeds once again to endure the hottest and driest times. To detect rainfall, some wildflowers produce seeds covered in resinous coating that can only be removed by large amounts of water. If wildflowers do bloom, they do it in the spring for only a few weeks. If you travel through Joshua Tree National Park in the spring, you will see bursts of color everywhere. And if you’re lucky, you may encounter a “super bloom” during years with higher than average precipitation, which results in more abundant and larger flowers than usual.
Animals
At the very top of the food chain are the apex predators of the desert ecosystem. These include mountain lions, bobcats, coyotes, and golden eagles. Coyotes often travel in packs and make their howls echo giving the impression of sounding like more coyotes than there actually are. They feed on small mammals such as cottontail rabbits, mice, and squirrels. Mountain lions are seen on rare occurrences in the park, stalking their prey from dusk to dawn. Their diet consists of deer, bighorn sheep, coyotes, birds, and small rodents. Bobcats spend their days resting and prey on smaller animals like rabbits, squirrels, snakes, and birds in low-light conditions at dawn and dusk.
Snakes and scorpions are also found in the park, especially after dark. Rattlesnakes consume rodents and small birds. They have poor eyesight but can detect body heat using infrared sensors on their heads. When a rattlesnake bites its prey the venom paralyzes the victim so that is easy to swallow.
Giant Hairy Scorpions have a diet consisting of insects, lizards, and other scorpions and are eaten by roadrunners, owls, bats, and coyotes. They live in sandy areas and are rarely seen because they are nocturnal. They are well-adapted to the desert climate because their bodies are surrounded by a waxy coating that helps them retain water. Their sting is similar to the sting of a wasp.
The most studied animal in the park is the Mojave Desert Tortoise. These well- camouflaged, slow-moving reptiles spend most of their time in burrows, but can occasionally be seen crossing park roads. They are currently listed as a threatened species, in part due to habitat loss, bacterial infections, and predation of hatchlings by an rapidly increasing raven population. When stressed, they release precious water from their bladders and are at greater risk of dehydration.
