©2015 mcdowell sonoran conservancy 1 · 3 a “natural history presentation” by the steward...

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Desert Ecology… Environment, Challenges, Implications

and Adaptation

©2015 McDowell Sonoran Conservancy

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A “Natural History Presentation”

by the Steward Education Program

McDowell Sonoran Conservancy


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Jack McEnroe Master Steward Citizen Scientist



We are the volunteers who help protect and promote the McDowell

Sonoran Preserve 5 ©2015 McDowell Sonoran Conservancy

How It Was Established.


What Is A Desert?


�Aridity�, not just

limited rainfall

What Is A Desert?


•  Very low humidity – primary cause of aridity

•  High ratio of evaporation and transpiration (water loss) to rainfall (water gain). Ratio > 5 or 6 / 1

•  Lack of rainfall •  Lack of surface or bio-

available water

What Is A Desert?


•  Often extreme variability of water availability

– Strong monsoonal storms, floods – Seasonality of Rainfall – Average rainfall is very low – Climatic variation may produce extended dry periods and drought

What Is A Desert?


Where Are Deserts Located


Where Are

Deserts Located?


Where Are Deserts Located?

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Earth�s Deserts ©2015 McDowell Sonoran Conservancy

Antarctica


Major Causes of Deserts


•  The 30° Latitude Effect •  The Rain Shadow Effect •  Isolation from Water •  Cold ocean currents (continent

western edges) •  Desertification

Major Causes of Deserts


30° Latitude Effect

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•  Atmospheric circulation driven by sun’s rays and Earth’s rotation

•  General, predictable patterns

•  Air at equator warmed by sun

•  Warm air, less dense, rises causing low pressure at Earth’s surface



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•  As warm air rises near equator, it cools

•  Water condenses and rains

•  Rising air replaced at surface by winds moving toward equator due to lower pressure

•  Earth’s rotation causes winds to move easterly angled toward equator



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•  High in the atmosphere the rising air moves toward the poles

•  At ≈ 30° north & south of equator, air cools and descends

•  Descending, dry air warms but, due to lower moisture content, precipitation is infrequent


•  Winds forced upwards over mountains •  Air rises, cools, saturates, water vapor

condenses… rain falls on western slopes

Rain Shadow Effect


•  Dryer air passes over the mountains, descends, and warms

•  Leeward, increasingly dry air yields little moisture…creating aridity

Rain Shadow Effect


•  Sea water evaporates into atmosphere •  Wind carries moist air over land •  Moisture precipitates on land •  Less precipitation as distance from sea increases

Isolation from Water


•  Effect of isolation accentuated if desert is: –  Located on leeward side of large mountains –  Located ≈ 30° north or south of equator –  Higher elevation than hot deserts

Isolation from Water


Near Cold Ocean Currents

•  Occurs along western coast of continents where coastal waters are cold (ex., California Current)

•  Sea evaporation humidifies air

•  Moist air blown toward coast

•  Moist, warm air passes over cold coastal waters


Near Cold Ocean Currents

•  Air cools, water condenses, rains/fog at sea/shore

•  Air, dryer than initially, passes over land becomes warmer.

•  Rewarmed air holds less moisture, probability of rain decreases


Deserts form/expand due to denuded vegetation and soil erosion from overgrazing, development, and marginal land cultivation

Desertification


Desertification is currently the single greatest cause of desert formation.

Desertification


Deserts of North America


The Sonoran Desert


How Did Sonoran Desert Form?

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•  Southern portion of Sonoran Desert in Baja California & Mexico –30° Latitude Desert

•  Northern Sonoran Desert is Rain Shadow Effect



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•  Tectonic activity ≈ 9 Ma uplifted mountain ranges in western Mexico & southern California and Colorado Plateau

•  Interior of Sonoran Desert isolated from water



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•  Climatic changes that produced the Ice Ages caused the Sonoran Desert to expand and contract many times

•  Current area of the Sonoran Desert has only existed ≈ 9,000 years



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•  Current biota have only been stable ≈ 4,500 years

•  Sonoran Desert continues to change due to climatic and human impacts


Why Is Sonoran Desert Unique?

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•  Only North American desert not land-locked

•  Infrequent, brief freezes • Subtropical plants adapt to less water but not to freezing

• Tropical origin of many species, including giant columnar cactus, found only in the Sonoran Desert


Why Is Sonoran Desert Unique?

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• Winter Storms • Northern Pacific • Westerly winds • Widespread, steady rains

• Summer Storms • Subtropics • Southerly winds •  Localized thunderstorms

• Episodic Rain – Tropical storms and El Nino


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Desert Environmental Challenges:

Aridity, Low Humidity, Temperature


Desert Ecology

•  Study of the Relationships between organisms (plants and animals) and their environment

©2015 McDowell Sonoran Conservancy 38

Desert Environmental Challenge: Aridity

•  Principle cause -continued influx of warm, dry air

•  Consistently low humidity


•  Humidity - amount of moisture in air •  Dew point - temperature to which air needs to be

cooled to become saturated •  Arid regions air temperature may be 50-60° above

dew point •  Consistently low humidity has many environmental

effects

Desert Environmental Challenge: Low Humidity


•  Inhibits clouds & precipitation

•  Rapid evaporation & runoff

•  lack of available surface water

•  Limited water retention •  Shallow water absorption



•  Plants & animals - rapid tissue dehydration

•  Plants wilt due to rapid moisture loss through transpiration

•  Organic material decomposes slowly

•  Rapid wind dispersal results in nitrogen-poor soil



•  Large daily temperature swings (30 - 60°F)

•  Large seasonal temperature changes (70 – 100°F)

Desert Environmental Challenge: Temperature Extremes


•  Intense sunlight - maximum air temperatures ≈ 120° F with ≈160° F surface temperatures

•  Lethal to most plants and animals without physical/behavioral adaptations

Desert Environmental Challenge: Temperature Extremes


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Desert Environmental Challenges: Implications


•  Variety of water & thermal management strategies required

•  No one successful strategy for survival

•  Multiple survival strategies encourage ecological diversity

•  Survival requires basic adaptations

Implications for Life in the Sonoran Desert


Animals & plants adaption strategies •  Endure •  Evade •  Escape



•  Plants cannot move – show greatest physical adaptations

•  Animals can move -

also show behavioral adaptations



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Desert Environmental Challenges: Adaptation


Array of physical/behavioral adaptations to desert conditions includes:

Summary of Basic Adaptations

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High Temperature

General Strategy

Lack Of Water

Tolerate Hypothermia Dissipate Heat Fast Reduce Heat Input

Tolerate Dehydration Store It Fast

Conserve It Well Acquire It Fast

Use It Fast

Escape Or Expire

Evade

Endure


•  Succulence – Ability to store water for extended periods of time

•  Mucilaginous tissues hold water

Plant Adaptation: Endure Lack of Water


•  Leaf tissue temperatures above ≈ 115° F may be lethal

•  Shallow root systems absorb water rapidly

Plant Adaptation: Endure High Temperatures


•  Stored water defended by spines, bitter taste, & toxicity

•  Special forms of photosynthesis

Plant Adaptation: Endure Lack of Water


•  Small leaves with high surface area to volume ratio for heat dissipation

•  Closed stomata when temperature is high and humidity is low



•  Physical adaptations to control tissue temperature – Light colors to reflect heat – Vertical leaf orientation to

minimize area exposed to sunlight

– Self-shading via dense spines, paired leaves, dense leaf hairs



•  Evasion focuses on: – drought tolerance – water acquisition

•  Shed leaves, roots, and branches during drought

•  After rain, some plants grow new leaves and roots to absorb water and replace tissue

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Plant Adaptation: Evasion


Plant Adaptation:

Evasion •  Evading plants rely

on heavier rains and deeper soil moisture

•  Evading plants - deeper root systems


•  Annuals die as environment dries out

•  Produce drought & temperature-resistant seeds

•  Perennials - dormant when insufficient water is available

•  Annual & perennial life cycles are compressed when water is available

Plant Adaptation: Escape


Animal Adaptations • Water - primary body coolant • More water needed for cooling, less available • Primary challenge - balance water use for evaporative cooling with retaining enough tissue water to maintain metabolism


•  Mobility allows most desert animals to evade heat

•  Nocturnal or crepuscular (dawn and dusk) activity

•  Exploit cooler microhabitats and shade

Animal Adaptations: Evade Heat


•  Knowledge and use of local water sources

•  Seasonal migrations or range adjustments

•  Burrowing below surface soil layer

Animal Adaptations: Evade Heat


• Animals endure heat - regulating body heat gain & loss – Shed thicker winter

coats – Dilate blood vessels

near bare skin (nasal passages, tongue, ears)

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Animal Adaptations: Endure Heat


– Adjust body temperature up in daytime (hyperthermia) to reduce water use for evaporative cooling

– Evaporative cooling (panting, sweating in larger animals)

Animal Adaptations: Endure Heat


Animal Adaptations:

Endure Heat •  Some larger animals

simply endure heat without major behavioral changes: – More stable thermal

mass – Slower metabolism – Reduced surface area

to volume ratio


•  Adaptations to lack of water - conservation to endure it

•  Arthropods & some small vertebrates - fatty or waxy surface coatings or layers

•  Some animals - concentrated, even crystalline urine and dry feces

Animal Adaptations: Endure Water Shortage


•  When dehydrated, smaller animals, especially reptiles, may estivate or enter torpor with reduced metabolism

•  Some animals have greatly increased dehydration tolerance



•  Exploit alternative water sources

•  Water is obtained as a metabolic by-product of food



•  Dry seeds can absorb water from humidity in the air — some animals store seeds in cooler burrows made more humid by respiration


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Animal photo


•  Most larger animals require periodic access to free water which limits their range



Animal Adaptations: Escape

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•  Migrating Birds •  Insects that spend life

in different forms •  Amphibians (toads &

frogs) spend months underground to avoid extreme dry conditions with insufficient surface water


Animal Adaptations: Endure Scottsdale Style


•  A Natural History of the Sonoran Desert, eds. Phillips and Comus, (Arizona – Sonora Desert Museum Press, 2000)

•  Desert Biology, ed. Brown (Academic Press, 1974) •  Desert Ecology, Sowell (University of Utah Press, 2001) •  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

Selected References & Sources

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Note that although the information presented is believed to be correct, is derived from reputable sources, and has been reviewed by staff and other experts, MSC

does not guarantee its accuracy.


For more information Visit the


website: www.McDowellSonoran.Org


Thanks For Listening!


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END


©2015 mcdowell sonoran conservancy 1 · 3 a “natural history presentation” by the steward...

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