Deserts are defined as areas that receive very little precipitation. The threshold for a desert is less than 250 mm of precipitation per year. With such little rainfall, deserts support relatively few plant species compared to other biomes like rainforests or grasslands. However, the plant diversity in deserts is often underestimated. When counting cryptic diversity, there may be thousands of plant species thriving in desert environments around the world.
Definitions of Deserts
Deserts are characterized by their aridity or lack of moisture. There are several factors that contribute to creating desert climates:
- Low precipitation – Deserts receive less than 250 mm of rainfall per year
- High temperatures – The lack of cloud cover and vegetation means more solar radiation reaches the surface, heating the air
- Rain shadows – Deserts are often located on the leeward side of mountains which blocks moisture
- Distance from oceans – Inland locations far from moisture sources tend to be more arid
- Global circulation patterns – Large-scale atmospheric circulations distribute moisture unevenly
Based on average precipitation, deserts are defined as regions that receive less than 250 mm (about 10 inches) of rainfall per year. The surrounding biomes receive more precipitation, such as grasslands (250-500 mm) or forests (>500 mm).
Desert Biome Characteristics
The desert biome is characterized by a lack of moisture and high temperatures:
- Low precipitation and humidity
- Extreme diurnal temperature ranges (hot days, cold nights)
- Lots of sunlight and solar radiation
- Sparse vegetation and limited biodiversity
- High rates of evaporation leading to saline soils
- Specialized adaptations in animals and plants to store water and survive heat and aridity
The world’s major deserts include the Sahara Desert (Africa), Arabian Desert (Middle East), Gobi Desert (Asia), Great Victoria Desert (Australia), Chihuahuan Desert (North America), and Atacama Desert (South America).
Desert Plant Adaptations
Despite the harsh conditions, many plants have adapted to survive in arid desert environments. Some key plant adaptations include:
- Drought tolerance – Avoiding water loss through reduced leaf surface area, water storage in fleshy tissues, hairy or waxy surfaces, and CAM photosynthesis.
- Heat tolerance – Through reflective leaf hairs and surfaces, evaporative cooling, and altering chemical processes.
- Saline tolerance – Excreting salt or accumulating compatible solutes to draw water into cells.
- Reproduction strategies – Rapid flowering after rain events, seed dormancy, short life cycles, and radiation from underground food storage organs.
These adaptations allow plants to grow, flower, and reproduce despite the desert conditions. Many desert plants only appear after rare rainfall then quickly complete their lifecycle before dry conditions return.
Number of Desert Plant Species
Early estimates suggested deserts supported around 2,000-3,000 plant species globally. However, as botanists explore deserts more thoroughly, it is clear there are many more species adapted to desert environments. Recent checklists suggest there are at least:
- 4,400 plant species in the Sahara Desert of Northern Africa and the Arabian Desert in the Middle East.
- Over 6,000 species in the deserts of Mexico.
- Approximately 4,000 species in the deserts of the Southwestern United States including the Sonoran, Mojave, and Chihuahuan deserts.
- Over 3,500 species in Australia’s deserts.
- Around 1,000 species in the extreme Atacama Desert of Chile.
These numbers continue to grow as botanists discover more plant diversity in deserts around the world. Rare rainfall events bring sprouting and flowering in unusual species. New plant species are still described from desert environments each year.
Underground Desert Diversity
One reason desert plant diversity was underestimated is much of the diversity occurs underground. Many desert species exist as bulbs, corms, rhizomes, rootstocks or seeds waiting for rare rainfall. For example:
- The Mojave Desert has over 600 underground perennials waiting for occasional rains before emerging.
- The Colorado Desert has a “forgotten flora” of 500 underground perennials.
- The Arabian Desert has over 140 geophytes that persist as underground storage organs.
This underground desert diversity is hard to survey. Botanists are still cataloging and describing new geophytes and annuals that emerge opportunistically following El Niño events or wetter rainy seasons.
Factors Influencing Desert Plant Diversity
Several factors influence the diversity and distribution of plants in a desert region including:
- Climate – Amount of rainfall, temperatures, evaporation rates.
- Geography – Latitude, proximity to moisture sources, rain shadows, soil types.
- Geology – Surface minerals, rocks, and glaeology that affect soil chemistry.
- Evolutionary history – Past diversification and migration from adjacent regions.
- Disturbance – Fire, grazing, human activities promoting diversity.
Higher plant diversity is found in deserts with more precipitation, topographic complexity, soil heterogeneity, and biogeographic connections to speciose regions.
Plant Diversity Between Deserts
Some deserts support more plant species than others based on climate, geography, and evolutionary history. For example:
- The Sonoran Desert has around 2,500 species due to 180-350 mm of rain, proximity to tropical areas, and complex geology.
- The Mojave Desert has around 1,200 species as it is smaller, drier, and more isolated than the Sonoran Desert.
- The Chihuahuan Desert has around 1,500 species with more summer rain from the tropics.
- The Great Basin Desert has just over 1,000 species due to high aridity and long distance from speciose regions.
In extreme deserts like the Atacama and Sahara with less than 50 mm of rainfall, plant diversity is only a few hundred hardy species.
Cryptic Diversity in Deserts
Molecular studies are revealing hidden “cryptic” diversity undetectable by physical appearance and increasing known desert plant species. Cryptic genetic diversity occurs when seemingly identical plants are actually multiple species. Botanists thought certain widespread species had adapted across deserts, but genetic analysis shows they are actually different species cryptically inhabiting the same niche in different locations, including:
- White bursage (Ambrosia dumosa) – 5 potential cryptic species across warm North American deserts
- Brittlebush (Encelia farinosa) – up to 6 cryptic species just in Sonoran Desert
- Senecio flavus – potential cryptic species across the Atacama and Monte Deserts
There are likely thousands more cryptic species hidden genetically within what we thought were habitat-generalist desert plants. Molecular studies will help uncover this diversity.
Narrowly Endemic Desert Species
While some desert plants have vast geographic distributions, many are narrowly endemic and restricted to small habitats. Studies in Australia’s arid zones have documented:
- Over 530 endemic plant species just in the Carnarvon Basin
- 280 endemics of Western Australia’s Pilbara region
- Over 100 endemic plants from the MacDonnell Ranges
- 2,000 total vascular plant endemics across all Australia’s deserts
These narrowly distributed endemics contribute greatly to global desert plant diversity. Protecting small unique habitats helps conserve these rare species.
Annuals and Ephemerals
Winter and spring annuals that grow and flower quickly after rainfall contribute significant diversity to desert flora including:
- In North American Deserts: phacelia, sand verbena, desert parsley, desert dandelion, popcorn flower, sun cups.
- In Australian Deserts: copperburrs, fiddleneck, flannel bush, plumed emubush.
- In Atacama Desert: Aa, Chaetanthera, Lepidophyllum, Nolana, Cristaria species.
Mass blooming events of annuals and short-lived perennials after El Niño rains bring exceptional floral diversity to desert landscapes.
Invasive Weeds
Invasive alien plants are problematic in many deserts where they displace native vegetation, increase fire risk, and reduce diversity including species such as:
- Buffelgrass in Sonoran and Mojave deserts
- Tamarisk in American Southwest deserts
- Prickly acacia in Australia’s arid zones
- Atriplex species invading Australia, Africa and Americas
Control of invasive plants protects native diversity and endemic species restricted to isolated habitats. Careful land management can suppress invasive plants in desert environments.
Conclusion
Deserts support a diverse array of vascular plants. Early estimates of around 2,000-3,000 species worldwide were far too low. Current checklists suggest deserts likely support over 15,000 plant species globally. However, cryptic diversity, molecular studies, and continuing discovery mean the true diversity is likely far higher.
Desert plants exhibit incredible adaptations to thrive in extreme aridity and temperatures. Their diversity is an important part of global biodiversity and endemic to these specialized ecosystems.
Understanding desert plant diversity has implications for conservation. Desert ecosystems may suffer severely under climate change, especially from shrub encroachment. Many narrowly endemic species could become extinct before they are even discovered. Recording and protecting desert plant diversity before it disappears must be a priority for conservationists.
