Desert Ecology
Presented by the
McDowell Sonoran Field Institute
a program of the
McDowell Sonoran Conservancy
People Preserving Nature
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What Are Deserts?
Deserts are characterized by aridity
rather than just limited rainfall
Environments where there is a
high ratio of evaporation and
transpiration (water loss) to
rainfall (water gain)
Very low humidity
Lack of frequent or sustained
surface or other bio-available
water
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What Are Deserts? (continued)
There often is extreme variability of water availability in
deserts
Strong monsoonal storms and floods
When average rainfall is very low, normal climatic
variation can produce extended dry periods and drought
Seasonality of rainfall
There are four major causes of deserts around the world
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How Do Deserts Form?
Cause #1: The 30° Latitude Effect
Interaction of
earth’s rotation,
cold ocean currents
and falling dry air at
30° N/S creates
stable zones of
warm, dry air over
oceans west of
continents
Constant influx of warm, dry air from oceans on prevailing westerly
winds dries out surface vegetation along west coasts of all major
continents at 30° N/S, creating and perpetuating deserts there
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How Do Deserts Form?
Cause #2: The Rain Shadow Effect
Wind is forced upwards by
mountains
At higher altitudes air cools,
saturates, water vapor
condenses and falls as rain on
windward slopes
Over the mountains the air
descends and warms but now is
dry
Where prevailing winds move from moist areas over mountains,
rain shadow deserts often are produced on the leeward side.
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How Do Deserts Form?
Cause #3: Isolation from Water
Deserts may form in areas that are
a long way from any major source
of water, especially if also at 30°
N/S and/or in the rain shadow of
large mountains
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How Do Deserts Form?
Cause #4: Desertification
Deserts may form or expand as a result of desertification due
to denuded vegetation and soil erosion from overgrazing,
development, cultivation of marginal land, etc—currently this is
the single greatest cause of desert formation.
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How Did The Sonoran Desert Form?
The southern Sonoran Desert
(SD) in Mexico is a 30° latitude
desert
The northern SD (Arizona,
California) is a rain shadow desert
SD formed about 9 million years
ago as tectonic activity lifted up
mountains in western Mexico and
also uplifted the southern coast
ranges in California, cutting off the
interior from major water sources
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How Did The Sonoran Desert Form?
(continued)
The SD has expanded and
contracted many times due to the
same climate changes that
produced the Ice Ages
The most recent form of the SD is
only ~9,000 years old and the
current biota have been stable only
~4,500 years
The SD continues to change today
due to development, replacement
of native vegetation, water use,
grazing, etc.
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What’s Unique About The Sonoran Desert?
Many unique features promote the
great diversity of the SD
Only North American desert that
isn’t land-locked (there is a
coast)
Infrequent and generally brief
freezes
Tropical origin of many species
(e.g. columnar cactus, which are
only found in the SD)
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What’s Unique About The Sonoran Desert?
(continued)
The distinguishing characteristic of the SD is its two
periods of rainfall
Winter storms come from the northern Pacific on westerly
winds, producing widespread, steady rains
Summer storms come from the wet subtropics on southerly
winds and produce localized thunderstorms
There are other sources of episodic rain in the SD
In the fall, tropical storms moving up the coast may send
moisture north toward the SD
El Niño warms eastern Pacific water, creating low pressure
which shifts prevailing westerlies southward over the SD
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The Main Environmental Challenges:
Aridity and Low Humidity
Large day/night swings (30 – 60° F) and seasonal changes
(70 – 100° F)
Intense sunlight produces maximum air temperatures of ~120° F and
surface temps up to ~160° F which would be lethal to most plants and
animals without physical and/or behavioral adaptations
Rapid evaporation and rapid runoff results in a lack of available
surface water
Soil doesn’t retain water long and water absorption is shallow
Both plants and animals face rapid tissue dehydration
Slow decomposition of organic material and rapid wind dispersal
result in nitrogen-poor soil
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What are the Implications of These Challenges
for the Things That Live Here?
Plants and animals in the SD need to develop a variety of
water and thermal management strategies to deal with aridity
and temperature extremes
There is no one successful survival strategy in SD,
encouraging ecological diversity
The basic adaptations are to endure, evade, or escape the
general lack of water and highly variable temperatures
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What are the Implications of These Challenges
for the Things That Live Here?
(continued)
These adaptations can be physical and/or behavioral
Plants generally show the greatest range and degree of
physical adaptations because they can’t move
Due to mobility, animals also show behavioral adaptations
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Summary of Basic Adaptations
The array of physical and behavioral adaptations to desert conditions
includes:
Dealing with high
temperatures
General strategy
endure
tolerate hyperthermia
Dealing with
lack of water
tolerate dehydration
store it fast
dissipate heat fast
conserve it well
reduce heat input
evade
acquire it fast
use it fast
escape or expire
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Plant Adaptations - Endure
The major plant strategy for enduring
lack of water is succulence
Water bound in mucilaginous tissues
Extensive root systems near surface to
absorb water fast
Various water conservation strategies
Special forms of photosynthesis (used by
many succulents and by grasses)
Defense of stored water via spines, bitter
taste or toxicity, etc.
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Plant Adaptations - Endure
(continued)
Desert plants need to endure high
temperatures, since leaf tissue temperatures
exceeding ~115° F usually are lethal
Physical adaptations to control tissue
temperature include:
Small leaves with high surface area to
volume ratio for heat dissipation
Closed stomata when temperature is
high and humidity low
Light colors to reflect heat
Vertical leaf orientation to minimize area
exposed to sun
Self-shading via dense spines, paired
leaves, dense leaf hairs, etc.
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Plant Adaptations - Evade
Evasion focuses on drought
tolerance and water acquisition
The main strategy is to shed leaves,
roots, and branches during drought
Some plants can grow new leaves
and roots to absorb water and
replace tissue very rapidly after rain
Evading plants tend to rely on
heavier rains and deeper soil
moisture than enduring plants and
have deeper roots
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Plant Adaptations - Escape
There are limited strategies for plants to
escape lack of water and inhospitable
temperatures
Annuals die as environment dries
out but first produce drought- and
temperature-resistant seeds
Perennials undergo periods of
dormancy when insufficient water is
available
Annuals and perennials exhibit
compressed life cycles when water
is available
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Animal Adaptations
Primary challenge for animals is balancing water use with
water conservation
The problem is that water is the primary body coolant
In the desert more water is needed for cooling but less
water is available
Need to balance use of water for evaporative cooling
with retaining enough tissue water to maintain
metabolism
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Animal Adaptations - Evade
Because of mobility most desert animals evade heat
Nocturnal or crepuscular
(dawn and dusk) activity
Exploitation of cooler
microhabitats and shade
Knowledge and use of local
water sources
Seasonal migrations or range
adjustments
Burrowing below the hot
surface soil layer
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Animal Adaptations - Endure
Animals also endure heat by regulating
body heat gain and loss
Shed thicker winter coats
Dilate blood vessels near bare skin
(nasal passages, tongue, ears, etc)
Evaporative cooling
(e.g. panting, sweating in larger
animals)
Adjust body temperature up in
daytime (hyperthermy) to reduce
water use for evaporative cooling
Some larger animals can simply endure heat without major behavioral
changes due to more stable thermal mass, slower metabolism, and
reduced surface area to volume ratio
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Animal Adaptations – Endure
(continued)
Adaptations to lack of water mostly
involve conservation to endure it
Arthropods and some small
vertebrates have fatty or waxy
surface coatings or layers
Some animals produce
concentrated, even crystalline urine
and dry feces
Smaller animals, especially reptiles,
may estivate or enter torpor with
reduced metabolism when
dehydrated
Some animals have greatly
increased tolerance for dehydration
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Animal Adaptations – Endure
(continued)
Some animals also exploit alternative
water sources
Water is obtained as a metabolic
by-product of food
Some foods like dry seeds can
absorb water from humidity in the
air—some animals store seeds in
cooler burrows made more humid
by respiration
Note that most larger animals require
periodic access to free water (e.g.
water holes) which limits their range
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Summary of Basic Adaptations
The array of physical and behavioral adaptations to desert conditions
includes:
Dealing with high
temperatures
General strategy
endure
tolerate hyperthermia
Dealing with
lack of water
tolerate dehydration
store it fast
dissipate heat fast
conserve it well
reduce heat input
evade
acquire it fast
use it fast
escape or expire
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Selected References and Sources
A Natural History of the Sonoran Desert, eds. Phillips and Comus,
(Arizona – Sonora Desert Museum Press, 2000)
Desert Biology, ed. Brown (Academic Press, 1974)
Goode’s World Atlas, ed. Espenshade (Rand McNally & Company, 1995)
House in the Sun, Olin (Southwest Parks and Monuments Association, 1977)
Reference Handbook on the Deserts of North America, ed. Bender
(Greenwood Press, 1982)
www.desertusa.com/du_plantsurv.html
http://wc.pima.edu/BFiero/tucsonecology/adaptations/adaptations_home.htm
Please note that although the information presented is believed to be correct, is
derived from reputable sources, and has been reviewed by Arizona Game & Fish
staff and other experts, MSC does not guarantee its accuracy.
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