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, 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

5. Focus on Climate - Spatial Variation • Climate has predictable and unpredictable components of spatialvariation: • 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 temporalvariation: • 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 • 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 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). • 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

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 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

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 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

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 earth’s 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 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

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.