Desert Ecology Presented by the McDowell Sonoran Field Institute - - PowerPoint PPT Presentation

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Desert Ecology

Presentedbythe

McDowellSonoranFieldInstitute

aprogramofthe

McDowellSonoranConservancy

PeoplePreservingNature

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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

• 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

• Interaction of

earth’s rotation, cold ocean currents and falling dry air at 30° N/S creates stable zones of warm, dry air over

• ceans west of

continents

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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

• f 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
• nly ~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

• f 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

• nly 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 Dealing with temperatures General strategy lack of water

endure evade escape or expire

tolerate dehydration store it fast conserve it well acquire it fast use it fast tolerate hyperthermia dissipate heat fast reduce heat input

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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

• ut 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

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

Because of mobility most desert animals evade heat

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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 Dealing with temperatures General strategy lack of water

endure evade escape or expire

tolerate dehydration store it fast conserve it well acquire it fast use it fast tolerate hyperthermia dissipate heat fast reduce heat input

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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

Pleasenotethatalthoughtheinformationpresentedisbelievedtobecorrect,is derivedfromreputablesources,andhasbeenreviewedbyArizonaGame&Fish staffandotherexperts,MSCdoesnotguaranteeitsaccuracy.