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Chapter 4: Variations in the Physical Environment

Robert E. RicklefsThe Economy of Nature, Fifth Edition

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Background

Variations in the physical environment underlie the diversity of biological systems.

We seek understanding of the physical environment and the principal determinants of this variation.

Climate is perhaps the most important element of environmental variation.

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Background, Cont’d

The physical environment varies widely over the earth’s surface.

Conditions of temperature, light, substrate, moisture, and other factors shape: distributions of organisms adaptations of organism

Earth has many distinctive climatic zones: within these zones, topography and soils

further differentiate the environment

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Focus on Climate - Spatial Variation

Climate has predictable and unpredictable components of spatial variation: predictable:

large-scale (global) patterns primarily related to latitudinal distribution of solar energy

regional patterns primarily related to shapes and positions of ocean basins, continents, and mountain ranges

unpredictable - extent and location of stochastic perturbations

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Focus on Climate - Temporal Variation

Climate has predictable and unpredictable components of temporal variation: predictable:

seasonal variationdiurnal variation

unpredictable:large-scale events (El Niño, cyclonic storms)small-scale events (variable weather patterns)

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Earth as a Solar-powered Machine

Earth’s surface and adjacent atmosphere are a giant heat-transforming machine: solar energy is absorbed differentially over planet this energy is redistributed by winds and ocean

currents, and is ultimately returned to space there are interrelated consequences:

latitudinal variation in temperature and precipitationgeneral patterns of circulation of winds and oceans

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Global Patterns in Temperature and Precipitation

From the equator poleward, we encounter dual global trends of: decreasing temperature decreasing precipitation

Why? At higher latitudes: solar beam is spread over a greater area solar beam travels a longer path through

the atmosphere

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Temporal Variation in Climate with Latitude

Temporal patterns are predictable (diurnal, lunar, and seasonal cycles).

Earth’s rotational axis is tilted 23.5o relative to its path around the sun, leading to: seasonal variation in latitude of most intense

solar heating of earth’s surface general increase in seasonal variation from

equator poleward, especially in N hemisphere

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Hadley Cells

Hadley cells constitute the principal patterns of atmospheric circulation: warm, moist air rising in the tropics spreads

to the north and south as this air cools, it eventually sinks at about

30o N or S latitude, then returns to tropics at surface

this pattern drives secondary temperate cells (30-60o N and S of equator), which, in turn, drive polar cells (60-90o N and S of equator)

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The Intertropical Convergence

Surface currents of air in tropical Hadley cells converge near the equator.

Warm, moist air rising in equatorial regions cools and loses much of its moisture content as precipitation there.

As cool, dry air descends and warms near 30o N and S latitude, its capacity to hold moisture increases, resulting in prevalence of arid climates at these latitudes.

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Surface Winds

Surface flow of air in Hadley cells is deflected by earth’s rotation to the right in N hemisphere to the left in S hemisphere

Net effect of deflections on surface flows: air flows toward the west in tropical cells air flows toward the east in temperate cells air flows again toward the west in polar cells

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Rain Shadows

Moisture content of air masses is recharged when they flow over bodies of water: rain falls more plentifully in S hemisphere (81%

of surface there is water, versus 61% in N hemisphere)

Air masses forced over mountains cool and lose moisture as precipitation.

Air on lee side of mountains is warmer and drier (causing rain shadow effect).

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Proximity to bodies of water determines regional climate.

Areas downwind of large mountain ranges are typically more arid (rain shadow effect).

Continental interiors are also typically arid: distant from source of moisture recharge air masses reaching these areas are likely to

have previously lost moistureCoastal areas have less variable maritime

climates than continental interiors.

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Ocean currents redistribute heat and moisture.

Ocean surface currents propelled by winds.Deeper currents established by gradients

of temperature and salinity.Ocean currents constrained by basin

configuration, resulting in: clockwise circulation in N hemisphere counterclockwise circulation in S hemisphere

Warm tropical waters carry heat poleward.

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Western coasts have Cold currents.

Oceanic water circulation: cold polar water forced equatorward from the

poles along west coasts of major continents this water acts as a barrier to warm, moist air net result is coastal deserts, especially on west

coasts of South America and Africa

Equatorward flows are deflected to W in both hemispheres, causing upwelling of cold, nutrient-laden water in these regions.

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Seasonal Variation in Climate

Seasonal progression of sun’s zenith causes familiar patterns of temperature.

Intertropical convergence also migrates seasonally: region of high precipitation shifts N or S with

intertropical convergence regions of arid conditions (30o N and S of

intertropical convergence) shift accordingly

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Seasonality of Rainfall in Tropics

Latitudes between about 20oS and 20oN experience greatest seasonality of precipitation.

Some examples: Mérida (20oN) - has a single summer rainy

season, alternating with a long dry season Rio de Janiero (20oS) - pattern similar to that of

Mérida, but displaced 6 months Bogotá (0o) - two rainy seasons, spring/fall,

separated by drier periods

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Similar Patterns Outside Tropics

At 30oN in Chihuahuan Desert: at northward limit of intertropical

convergence, summer rainfall, winter drought

At 35oN in San Diego: beyond northward limit of intertropical

convergence, summer drought, winter rainfall (Mediterranean-type climate)

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Seasonal Cycles in Temperate Lakes 1

The four seasons of a small temperate lake - each season has its own characteristic temperature profile: winter: coldest water (0oC) at surface, just

beneath ice layer, increasing to 4oC near bottom

spring: ice melts; as surface warms, denser water sinks, resulting in uniform 4oC profile, with little resistance to wind-driven spring overturn

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Seasonal Cycles in Temperate Lakes 2

summer: continued warming of surface results in thermal stratification, a stable situation and resistant to overturn; strata established:epilimnion - warm, less dense surface waterthermocline - zone of rapid temperature changehypolimnion - cool, denser bottom water (may

become oxygen-depleted)

fall: water cooling at surface sinks, destroying stratification, once again permitting wind-driven fall overturn

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Climate Sustains Irregular Fluctuations

El Niño is an annual event which can assume extreme proportions, with implications for worldwide climate.

Background: annual El Niño events involve a warm oceanic

countercurrent flowing southward toward Peru reversal of high/low pressure areas in central

Pacific Ocean (Southern Oscillation) accentuate this effect leading to El Niño “event” (ENSO)

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El Niño brings severe weather.

Severe El Niño events occur irregularly, about once every 10-12 years.

Consequences of severe El Niños: drought in tropical South America, Africa,

and Australia increased precipitation outside of tropics disruption of fisheries and seabird

populations

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Far-Reaching Effects of El Niño

A severe El Niño leads to cascading effects in both terrestrial and aquatic systems:

restructuring of Great Salt Lake ecosystemdramatic consequences for Galapagos ecosystems

• deterioration of cold-water fish stocks leads to crash of populations of seabirds and sea lions

• abundant rainfall leads to increased terrestrial production

La Niña events represent return to strong trade winds (reversal of El Niño effects).

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Topographic and Geologic Features

Topography can modify environment on local scale: steep slopes typically drain well, leading

to xeric conditions bottomlands moist and may support

riparian forests, even in arid lands in N hemisphere, south-facing slopes

are warmer and drier than north-facing slopes

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Gradients in Mountains

Adiabatic cooling of air masses crossing mountain barriers leads to: temperature decrease of 6o-10oC for each

1,000 m increase in elevation precipitation typically increases

Some consequences: in tropics, snow line is reached at 5,000 m in temperate zone, +1,000 m of altitude

corresponds to +800 km of latitude

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More on Mountain Climates

Decrease in temperature as air masses are forced over mountains is the result of adiabatic cooling (air expands, performs work, and therefore cools).

As air cools, its capacity to hold moisture declines, forcing moisture out as rain/snow.

Descending air rewarms, resulting in warm and dry air at base of lee side of mountain.

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Life Zones in Southwestern Mountains

Nineteenth-century naturalist, C.H. Merriam, recognized life zones, prominent in the American Southwest: in Lower Sonoran Zone, subtropical plants

and animals (hummingbirds, ring-tailed cats, etc.) make their only Temperate Zone appearances

In Alpine Zone, 2,600 m higher, landscape resembles tundra of northern Canada, 2,000 km to the north

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Climate and Soil

Climate exerts indirect effect on distributions of plants and animals through its influence on development of soils.

What are soils? chemically and biologically altered materials

overlying unaltered parent materials at earth’s surface

soil contains unaltered and modified minerals, organic matter, air, water, living organisms

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Soil Characteristics

Soils are the product of climate, parent material, vegetation and other organisms, local topography, and time.

Soils often have distinct layers or horizons: O (dead organic matter) A1 (humus rich) and A2 (zone of leaching) B (low organic matter, deposition of clays) C (weakly altered material resembling parent

material)

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Soils exist in a dynamic state.

Soils change through time: water leaches materials vegetation adds organic material other materials enter through precipitation,

dust, and from underlying rockRate of development varies:

in arid regions, soils may be shallow in humid tropics, soils may develop to 100

m

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Weathering

Weathering = physical and chemical alteration of rock or other parent material near earth’s surface.

Various processes characterize weathering: freeze/thaw cycles break rock and expose it to

chemical action water dissolves readily soluble materials other processes lead to synthesis of new

minerals, such as clays

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Synthesis of Clay Minerals

Common minerals, such as feldspar and mica, can be chemically altered to form clay minerals: these minerals are K, Mg, Fe aluminosilicates H+ ions displace K and Mg Fe, Al, and Si form new insoluble clay minerals clay minerals are important to water-holding

and cation-exchange properties of soils

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H+ ions are essential for clay synthesis.

What is the source of this acidity? rainwater is naturally acidic; carbonic acid is

formed when CO2 dissolved in rainwater; results in natural pH of about 5.

additional acidity is produced by oxidation of biological materials, producing CO2 and more carbonic acid.

acidity formed by oxidation of biological materials is more significant in the tropics.

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Podzolization

Podzolization occurs when clay particles break down in the A horizon and their soluble ions are transported downward.

This process is most likely to occur in cold regions where needle-leaved trees predominate: organic acids percolate through soil under

humid climate regime, leaving leached A2, with deposition in B horizon below

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Laterization

In warm, wet conditions of tropics and subtropics, soils weather to great depths: clay particles break down silica is leached from soil residue is rich in oxides of iron and

aluminum

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Consequences of Laterization

Lateritic soils are usually not acidic are infertile; they contain little clay or humus

to hold cations, which are easily leached are deeply weathered, so minerals released

from weathering of parent material are not accessible to plants

Rich soils do develop in tropics, in mountainous areas and on volcanic deposits.

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Soils -- Bottom Line

Soil formation emphasizes the role of the physical environment, particularly climate, geology, and landforms, in creating the tremendous variety of environments for life that exist at the surface of the earth and in its waters.

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Summary 1

Global environmental patterns are the result of differential input of solar irradiation in different regions and redistribution of heat energy by winds and ocean currents.

Seasonality in terrestrial environments is caused by the latitudinal movement of the solar equator. Seasonal changes in energy budgets profoundly affect temperate lakes.

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Summary 2

Irregular and unpredictable variations in climate, such as severe El Niño-Southern Oscillation events, may disrupt biological communities on a global scale.

Topography and geology superimpose local environmental variation on more general climatic patterns.

Soil properties contribute to local variation in terrestrial environments.

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