The Oxford dictionary defines an ecosystem as "a biological community of interacting organisms and their physical environment." In other words, it's a network of plants, animals and microbes, which work together as a unit in a particular area, and the abiotic factors of that area, such as location, climate and soil. Though there is no definitive boundary of an ecosystem as such, following its nutrient cycle and energy flow can be helpful if you are to get well-versed with the concept.
As for a desert, firstly, it's NOT a barren land devoid of life, as it is usually believed to be. In fact, there is much more to the desert ecosystem, beyond the cliched trio of sand dunes, cacti, and camels (which is what most people think). While their numbers may not be as large as in case of rainforests, or grasslands for that matter, plants and animals do exist in deserts. The myths about deserts, which exist in plenty, can be attributed to our lack of knowledge about this ecosystem, and to clear these myths, it is important to start from the basics.
In geography, a desert is defined as a region, which receives an annual precipitation of less than 10 inches on an average, and where the amount of water lost to evapotranspiration is more than the amount of water gained (by precipitation).
Other than low precipitation, deserts are also characterized by scarce vegetation and extreme temperature oscillating between 115 °F, or more during the daytime and 32 °F, or less at night. Out here, climate is predominantly the deciding factor when it comes to existence of lifeforms.
Two types of desert
Basically, there are two types of deserts: (i) hot deserts, like the Sahara and Mojave; and (ii) cold deserts, like the Gobi and Antarctica. One of the prominent differences between the two is the form of precipitation, i.e., rainfall in hot deserts and snowfall in cold deserts. Irrespective of whether it is a hot, or cold desert, the characteristic traits of both are nearly the same.
Even hot deserts record a significant drop in temperature at night, and such fluctuations only make things worse from the survival point of view.
In the harsh environment of deserts, the key to survival is adaptation, and that is made obvious by several plants and animals, which have adapted to the seemingly unsuitable conditions over the years. At the same time, the species surviving here have also mastered the art of interdependence, and that is made obvious by the nutrient cycle and energy flow. Though a desert may seem like a barren land devoid of lifeforms, a closer look and you realize that it is quite diverse in itself.
As with any given ecosystem, the plants play the role of producers in deserts. They capture sunlight (solar energy) and convert it into chemical energy by resorting to photosynthesis. This energy is eventually transferred to primary consumers (i.e., the herbivores and omnivores), when they feed on plants, and eventually to the secondary consumers (i.e., the carnivores), when they feed on plant-eating animals. In this manner, the energy transfers from one member of the food chain to another. When either of these animals, or plants die, the decomposers (i.e., bacteria, fungi, and different types of worms) come into the picture. As these microbes work on dead animals, the energy is returned to the environment through soil (nutrients) and air (carbon dioxide).
The desert cottontail -- a primary consumer in the desert food chain, feeds on cacti -- the producer, and derives energy from it. The red-tailed hawk -- the secondary consumer in the food chain, feeds on the cottontail, and derives energy from it. Interlinked food chains like these form a desert food web. Approximately 90 percent of the energy is lost in course of being transferred from one trophic level to the next; this is called progressive loss of energy. It is because of this that the number of carnivores in any given ecosystem is less than the number of prey.
Members of the Desert Food Chain
The interdependence of species in deserts is not restricted to nutrient cycle and energy transfer. The species even share a symbiotic relationship, wherein plants provide animals with food and shade, and animals return the favor by helping them pollinate and spread.
Then there are the cases of commensalism, wherein only one party benefits from the relationship without causing harm to the other. The relationship between the cactus wren species and cholla cactus, wherein the wren builds its nest in the cholla species to keep its young ones safe from the predators, would be an apt example of the same.
Animals and their Adaptations
Biodiversity of the deserts is as unique as other biomes of the world. Animals found here include the Gila monster, chuckwalla, desert tortoise, horned lizard, rattlesnakes, bobcat, meerkat, spotted hyena, pronghorn, kangaroo rats, camels, scorpions, coyotes, gemsbok, sand cats, jerboa, and so on. Other than these, the deserts are also home to birds, like the hawks, vultures, roadrunner, cactus wren, ostriches, etc. You might not see these animals as you see zebras and wildebeests in Africa, but they do exist, and each of these species play a crucial role in the desert ecosystem food chain.
The desert food chain
In the desert food chain, rodents, insects, and the few reptiles which feed on plants are the primary consumers. Then come the secondary consumers, mainly comprising large reptiles and mammalian species which feed on primary consumers. At the top of the desert food chain are the apex predators in the form of birds, like the red-tailed hawks, and mammals, like the coyote. The number of predators in a typical food chain is always small to make up for energy lost at every stage.
Xerocoles is the scientific term which refers to animals that have adapted themselves to the deserts. Most of these animals are either nocturnal (active during the night), or crepuscular (active at twilight) in nature, known to spend the entire day in their burrows and step out only after sunset. Even those few animals that are diurnal are active in the morning and evening, and reduce activity at midday. This is mainly done to avoid the extreme heat during the daytime. Other adaptations, which help these animals survive, include physical adaptions, like light color and long ears and behavioral adaptations, like spending the hot part of the day in the shade, or making minimal contact with the ground.
Water being a scarce resource in deserts, these animals have also modified themselves to make the most of the available water. While the herbivorous species absorb water from the plants that they feed on, the carnivores derive it from the meat and blood of their prey. Some species also store the water in their fatty tissues. Certain species are also assisted by some peculiar adaptations to reduce water loss, like the absence of sweat glands in kangaroo rats, wax-like coating in frogs, low metabolic rate in rodents, etc.
That the desert vegetation only comprises cacti is undoubtedly the biggest misconception about deserts. Most of the people fail to understand that several different species of plants -- i.e., different varieties of grasses, shrubs, and trees, also grow in deserts. In fact, the Arabian desert -- the second largest desert in the world -- is completely devoid of native cacti species.
Within cacti itself, there are numerous species which grow in different deserts of the world. Interestingly, not all cacti species are restricted to deserts.
Plant survival adaptations
Though scarce, the vegetation in deserts is highly diverse with some unique species resorting to unique adaptations to survive. All the plants found in deserts have modified themselves to sustain in the typically harsh environment prevailing there. While some of these species store water in their specialized tissues, others have small leaves with hair-like structures, which reduce evaporation.
To reduce transpiration, some of these trees have few, or at times no leaves at all. Some desert plants have an extensive root system -- well in excess of 100 ft. at times -- especially designed to access water from deep inside the ground, and some species only thrive for a short period after the rains.
The list of desert plants is considerably lengthy and includes cacti, like the barrel cactus -- the most abundantly found cacti species of the North American deserts, hedgehog cactus, pancake prickly pear cactus, Saguaro cactus, etc., as well as species like the brittlebush, saltbush, creosote bush, desert ironwood, grevilleas, joshua tree, Mojave aster, soaptree yucca, and jumping cholla.
All the deserts of the world put together may not boast of biodiversity at par with the rainforest, or grasslands, but they do boast of species with exceptional adaptation skills and the ability to live together as a unit. If it was not for these attributes, the deserts would have been reduced to vast barren stretches of land devoid of life.