Desert Ecology Presented � by � the McDowell � Sonoran � Field � Institute a � program � of � the McDowell � Sonoran � Conservancy People � Preserving � Nature 1
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 2
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 3
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 4
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. 5
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 6
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. 7
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 8
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. 9
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) 10
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 11
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 12
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 13
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 14
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 tolerate dehydration endure tolerate hyperthermia store it fast dissipate heat fast conserve it well reduce heat input acquire it fast evade use it fast escape or expire 15
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. 16
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. 17
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 18
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 19
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 20
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 21
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 22
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 23
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 24
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