Deserts

Causes of Tropical aridity

• Persistent atmospheric subsidence(e.g. Sahara, Kalahari, Australia)

• Localized subsidence in rain-shadow areas• Absence of cyclonic disturbances (e.g. Sonora)-

away from the influence of the relief/orographic rainfall

• Colder winds/ocean currents

“Hot desert”:

saguaro - ocotillo

community, Arizona

Forms of adaptation to stresses in arid environments

Four strategies:1. minimize heat intake or maximize heat

outflows;2. maximize food reserves in times of

plenty;3. maximize water inflows; and4. minimize water outflows

Evasion tactics to minimize exposure to heat and

drought1. Organism dormant for substantial part of life-

cycle:e.g. ephemeral plants, some reptiles, most insects persist through extended droughts as seeds, eggs, or larvae (only the reproductive forms remain).

2. Nocturnal or crepuscular foraging (hottest parts of day spent in burrows or shade).

Desert ephemeral flora

• Large seedbanks (esp. in sites protected from wind, e.g. around base of bushes)

• Long seed viability in dry soils;• Rapid germination if rainfall sufficient [e.g. only

rains >25mm in Arizona produce germination]

Refuging tactics: chuckwalla lizard

How much cooler is it at a depth

of 20 cm?

Refuging behaviour: camels in shade

Reducing heat load

• Low surface-area/volume ratio;

• Reflective skin/bark(colour changes in lizards)

• Vertical shoot-body architecture

Fouquieria/Idria columnaris (the ‘cirio’ of central Baja)

Reducing heat load:

a joshua tree

(Yucca brevifolia)

in the Mojave desert

Sonoran desert in bloom

Maximise food reserves in times of plenty

• camel’s hump;• berber sheep

(fat reserves in tailvary from 2-10 kg);

• pack rats/gerbils hoard seeds;

• succulents store water.

Maximise water inflows• extensive lateral (cacti) or vertical

(mesquite) roots;• rapid root growth after rains • beetles in Namib desert stand on hind

legs to catch fog droplets on raised abdomen;

• mice in Arizona often feed on low-protein herbage with high water content

• camels can drink 100 L of water in 10 min!

• practice opportunistic migration to water and food sources (desert locusts, nomadic pastoralists)

Shrub/tree root patterns, Arizona

Plant spacing determined by moisture availability and rooting niche

40

Water conservation:expandable storage organs and palisade tissue in succulents

Tissue protection:thorns and spines

Minimize water loss1. Transpiration reduced in desert plants

2. Water loss in desert fauna reduced by dry faeces, low urine production, low dilution of uric acid, adaptive hyperthermia (camel’s body temperature can vary by 6°C when animal is dehydrated).

3. Tolerate desiccation: camel can withstand water loss = 25% of body weight

Human Impacts on Deserts•Extraction of resources e.g. copper in the

Atacama Desert•Hunting e.g. Oryx and Cheetah in the Namib Desert•Tourists damaging dunes e.g. dune boarding and desert bashing in the Arabian Desert•Overgrazing killing what little vegetation there is and over cultivation e.g. Southern Sahara•Deforestation: There is limited vegetation in deserts, but often it is removed for fuel or to make space for crops•Toyotarisation: 4X4 vehicles being driven across the desert disturbing animals, killing vegetation and creating dust storms

http://www.youtube.com/watch?v=z1IBnHIOFxs-

desert in bloom

Human Causes of Desertification

•Overgrazing•Overcultivation•Deforestation•Overpopulation•Fertiliser and Pesticide Use•Unsustainable Water Use (aquifer depletion, unsustainable irrigation)•Toyotarisation

Physical Causes of Desertification

•Rising Temperatures/Falling Rainfall

•Flash floods

•Wind

Desertification

PROBLEMS CAUSED:•Dust Storms•Reduced Crop Yields•Conflict•Famine

SUGGESSTED SOLUTIONS:•Crop Rotation and Fallow Periods•Shelter Belts•Reforestation and Afforestation•Sustainable Irrigation/Grazing Quotas