updates

1
updates

• No make-up session tomorrow
• Quiz on Wednesday
• Make-up session Thursday

2
Chapter 4 Variations in the Physical Environment

• Robert E. Ricklefs
• The Economy of Nature, Fifth Edition

3
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.

4
Background, Contd

• The physical environment varies widely over the
  earths 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

5
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

6
Focus on Climate - Temporal Variation

• Climate has predictable and unpredictable
  components of temporal variation
• predictable
• seasonal variation
• diurnal variation
• unpredictable
• large-scale events (El Niño, cyclonic storms)
• small-scale events (variable weather patterns)

7
Earth as a Solar-powered Machine

• Earths 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
  precipitation
• general patterns of circulation of winds and
  oceans

8
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

9
Temporal Variation in Climate with Latitude

• Temporal patterns are predictable (diurnal,
  lunar, and seasonal cycles).
• Earths 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 earths surface
• general increase in seasonal variation from
  equator poleward, especially in N hemisphere

10
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)

11
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.

12
Surface Winds

• Surface flow of air in Hadley cells is deflected
  by earths 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

13
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).

14
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 moisture
• Coastal areas have less variable maritime
  climates than continental interiors.

15
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.

16
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.

17
Seasonal Variation in Climate

• Seasonal progression of suns 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

18
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

19
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)

20
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

21
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 water
• thermocline - zone of rapid temperature change
• hypolimnion - cool, denser bottom water (may
  become oxygen-depleted)
• fall water cooling at surface sinks, destroying
  stratification, once again permitting wind-driven
  fall overturn

22
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)

23
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

24
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 ecosystem
• dramatic 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).

25
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

26
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

27
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.

28
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

29
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 earths
  surface
• soil contains unaltered and modified minerals,
  organic matter, air, water, living organisms

30
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)

31
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 rock
• Rate of development varies
• in arid regions, soils may be shallow
• in humid tropics, soils may develop to 100 m

32
Weathering

• Weathering physical and chemical alteration of
  rock or other parent material near earths
  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

33
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

34
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.

35
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

36
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

37
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.

38
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.

39
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.

40
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.