Conditions, resources and the world’s communities

Chapter 4

introduction

• We’ve discussed how individual organisms are affected by conditions and resources

• How does the interplay of conditions and resources influences whole communities [remember: community = assemblage of species that occur together.]

• Answer: it depends– Depends on the scale at which we choose to study

communities

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Large-scale patterns in conditions and resources

• Large-scale climatic patterns• Geography of life: mainly a consequence of

the planet’s movement through space• Tilt of the Earth: causes solar radiation to

strike at different intensities• Equator: tilted toward the sun therefore

receive more direct sunlight and are warmer than other

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Warm air – cold air

• warm air holds more moisture than cold air – thus water-holding capacity greater

• Solar radiation: draws water from vegetation by evaporation

• Warm and wet air –> much of the water condenses and falls back as rain

• Rotation of Earth causes air masses from the tropics to curve to the north and south.

• Thus…

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ITCZ

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Ocean currents

• Another powerful influences on climatic patterns

• Southern waters circulate counter clockwise. Carry cold Antarctic waters up along western coasts of continents. Distribute warmer waters from the tropics along the eastern coasts

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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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• 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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Mountain ranges

• Topography of the land: additional consequences, more so on the intermediate scale, for the pattern of terrestrial climates

• A winds meet mountain ranges…

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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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• 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 re-warms, resulting in warm and dry air at base of lee side of mountain.

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• These variety of influences produces a mosaic of dry, wet, cool and warm climates over the surface of the globe

• Within the patches of this mosaic distinctive terrestrial associations of vegetation and animals have formed

• Biomes characteristic types of vegetation

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Small-scale patterns in conditions and resources

• Biomes are not homogenous within their hypothetical boundaries

• Every biome has gradients of physicochemical conditions related to local topography and geology

• Local variations in topography can > broad climatic patterns– Eg: temperature and altitude

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Small-scale patterns in conditions and resources

• So local topography… plus local geology and soil• Rocks differ in their mineral composition giving rise

to a variety of types of soil• Limestone rocks and chalk from marine deposits of

calcium carbonate; basis for neutral or slightly alkaline calcareous soils; flora love calcium (like Lebanon)

• Plants normally found on more acid soils are unsuccessful on calcareous soils and strict calcium lovers suffer on acidic soils where they are intolerant of aluminum ions released at low pH

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Small-scale patterns in conditions and resources

• So local geology influences soil; soil influences vegetation

• And: organic matter component of soil also influences vegetation

• Organic matter accumulates at different rates in different soils

• Eg: acidic rocks + low temperatures and/or waterlogged soil v. low rates of decomposition formation of peat bogs

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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 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

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patchiness

• A patch in a community = an area in which a single variable distinguishes it from its surroundings– Eg: a fallen tree

• Think of patches as the scale at which particular organisms experience the environment around them

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Aquatic environments• Differences not as visible as in terrestrial biomes• Exceptions that are as distinctive– Ocean’s edge Tropical mangrove– Coral reefs Temperate kelp forests

• Why? Because of their close relationship with major terrestrial climates

• Large open oceans: continuum across the globe– High thermal capacity of water makes oceans slow to heat

and slow to cool– Thus: temperature of water at one point on the globe is a

better reflection of where the water has come from (along ocean currents) than of the local climate

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Aquatic environments

• Large lakes – can be distinguished and classified according to their physical conditions

• Are they stratified? [stratification = distinct layers of water at particular temperatures]

• Seasonal patterns of stratification: norm in temperate regions

• Permanent stratification: equatorial regions• Polar circles: permanent ice cover; no mixing

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• 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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– 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

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Temporal patterns in conditions and resources

• Composition of communities: changes over time, from hours to millennia, as conditions and resources change

• Remember: changes in climate during Pleistocene ice ages bear much of the responsibility for present patterns of distribution of plants and animals

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• Intermediate temporal scales• - successional sequence that occurs on cooled

volcanic lava takes several centuries to turn its course– Documented by comparing the plants living on

lava flows from eruptions that occurred at different times

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Terrestrial biomes

• Character (plant and animal life) of natural communities is determined by climate, topography, and soil (or parallel influences in aquatic environments).

• Because of convergence, similar dominant plant forms occur under similar conditions. Biomes are categories that group communities by dominant plant forms.

• Biomes In North America:– tundra, boreal forest, temperate seasonal forest, temperate rain

forest, shrubland, grassland, and subtropical desert• Biomes In Mexico and Central America:

– tropical rain forest, tropical deciduous forest, and tropical savanna

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• Geographic distributions of biomes correspond closely to major climate zones.

• Not all biome classifications are the same:– some recognize finer or coarser detail– various biomes intergrade continuously and recognizing

boundaries is difficult• Matching of biomes and environment occurs

because no single type of plant can endure the entire range of conditions on earth.

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• A widely adopted climatic classification is that of Heinrich Walter:– Walter’s scheme is based on the annual course of

temperature and precipitation:• focuses on conditions of moisture and temperature

stress that determine plant form• recognizes 9 zones, from Equatorial (Tropical rain

forest) to Polar (Tundra)

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• Difference: focus on vegetation structure as definition of biomes

• Whittaker related major biomes to annual temperature and precipitation.

• The biomes fall in a triangular area with corners representing following conditions:– warm-moist– warm-dry– cool-dry

• Whittaker’s scheme is similar in many respects to Walter’s:– Whittaker starts with vegetation and relates climate

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• Equatorial and tropical climate zones (mean temperatures between 20oC and 30oC)– precipitation ranges from 0 to 400+ cm/yr

• Temperate climate zones (mean temperatures between 5oC and 20oC)– precipitation ranges from 0 to 300+ cm/yr

• Boreal and polar climate zones (mean temperatures less than 5oC)– precipitation typically below 200 cm/yr

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• Climate is continually warm and moist:– precipitation is in excess of 200 cm/yr, biseasonal, but never

less than 10 cm in any month• Occupies three important regions, in South/Central

America, West Africa, Indo-Malayan region.• These are exceedingly diverse forests, dominated by

evergreen or seasonally deciduous broad-leaved trees, featuring diverse growth forms including climbing lianas (woody vines) and epiphytes (plants that grow on the branches of other plants).

• Most productive of the Earth’s biomes; photosynthetic productivity > 1000 g of carbon fixed per square meter per year. Why?

• Most animal and plant species are active throughout the year; plants may flower and ripen fruit in sequence

• Dramatically high species richness is the norm04/10/23 39

• climate is seasonally dry, but sufficient moisture to support forest:– progressively drier tropical habitats support dry

forests, thorn scrub, and true deserts

• Occur worldwide within the tropics, but typically beyond 10oN or S of the equator.

• Tropical seasonal forests have a preponderance of deciduous species.

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• Savannas are grasslands with scattered trees.• These are typical of large areas of semiarid

tropics, especially at high elevations in East Africa.

• Rainfall is strongly seasonal: plant growth limited for part of the year by drought; seasonal glut of food– 90-150 cm/yr but driest 3-4 months receive less

than 5 cm each– fire and grazing play important roles in

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• Found in continental climate zones:– summers are hot and wet; winters are cold– growing season is 120-300 days– fires are a dominant influence

• Natural vegetation over large areas in every continent

• Extensive grasslands develop, called prairies in North America, steppes in central Asia.

• Vegetation is dominated by grasses and forbs:– fire is frequent and most species have underground fire-

resistant stems

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• Grasslands grade into deserts in arid continental climates:– winters are cold and summers hot– precipitation is 25-50 cm/yr– fires are infrequent because of low fuel accumulation– grazing can exert strong pressure on vegetation

• Grasslands are widespread in the western US, from Great Basin southward.

• Vegetation is dominated by shrubs, such as sagebrush, or small trees, such as piñon pine and juniper.

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Grasslands

• Many of these grassland have been cultivated and replaced by arable annual ‘grasslands’ of wheat, oats, barley, rye, and corn

• Together with rice in the tropics -> provide the staple food of human populations worldwide

• Vast increase of human pop has depended on the domestication of grasses for human food or feed for domestic animals

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Desert

• Hot deserts, in their most extreme, too dry to bear any vegetation; as bare as the cold deserts of Antarctica

• Vegetation falls into 2 sharply contrasted patterns of behavior:– Opportunistic lifestyle – Long-lived with sluggish physiological processes (cacti)

Animal diversity: low; nomadic bird species

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• These are highly variable systems found under extreme aridity:– develop at 20o-30o north and south latitude– rainfall is sparse (less than 25 mm)– creosote bush is common in subtropical American

deserts, with associated cacti, shrubs, and small trees:• subtropical deserts typically have summer rainfall, with

high species diversity, prominent annual flora

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• These zones have average temperatures below 5oC.

• Boreal forest (taiga) develops between temperatures of 5oC and -5oC.

• Tundra develops at temperatures below -5oC.

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• Climate is extremely cold, with temperatures as low as -60oC in winter:– average annual temperature is below 5oC,

precipitation 40-100 cm/yr– growing season is 50-100 days

• Boreal forest is centered on a broad belt at 50-60oN latitude across North America and Eurasia.

• Also called taiga, vegetation of low diversity dominated by evergreen needle-leaved trees, typically spruce and fir.

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• Exceedingly cold climate, with brief, but active, growing season in summer:– soils are permanently frozen, thaw to depth of 0.5-1 m during brief

summer growing season– precipitation is less than 60 cm/yr, but soils may be saturated because

of impeded drainage

• Found at high latitudes, north of boreal forest belt (but superficially similar systems occur in alpine zones).

• Tundra is a treeless expanse of dwarf, prostrate woody shrubs.

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• Temperate zone is characterized by temperatures between 5o-20oC at low elevations, with frost throughout the zone:– found between 30oN and 45oN in North America

and between 40oN and 60oN in Europe– biomes differentiated by:

• total amounts and seasonality of precipitation• length of frost-free season or growing season

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• Develops under moderate climates with winter freezing:– growing season is 130-180 days– precipitation exceeds evapotranspiration

• Found principally in eastern North America, Europe, and eastern Asia.

• Vegetation is dominated by deciduous trees with understory of small trees and shrubs, often abundant herbs.

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• Warmer and drier parts of the temperate seasonal forest biome are dominated by needle-leaved trees, typically pines:– found principally in North America along the Atlantic and

Gulf coasts and at higher elevations in the western states– needle-leaved forests typically develop under conditions

of drought and nutrient stress– fires may be frequent and species can resist fire damage

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• Develops primarily in warm temperate climates:– mild winters, heavy winter rains, summer fogs common

• Found principally in the northwestern US, adjacent British Columbia, southern Chile, New Zealand, Tasmania.

• Vegetation is dominated by tall evergreen trees, such as Douglas fir and coastal redwood:– extensive during Mesozoic era– not as diverse as its tropical counterparts

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• Develops in Mediterranean-type climate (cool, wet winter, warm dry summer):– fires are frequent and most plants have adaptations to fire

(resistant seeds or root crowns)• Typically found at 30-40o latitude, west coasts,

common in southern Europe, southern California, central Chile, Cape region of South Africa.

• Vegetation is dominated by sclerophyllous evergreen shrubs.

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Future distribution of biomes

• Predicted changes in global climate over the next few decades can be expected to result in dramatic changes to the distribution of biomes over the face of the Earth

• Exact nature of changes is uncertain

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Distribution of major biomes under current

climate

Potential distribution of major biomes from climate

change – reduction in

area of northern biomes of

tundra and taiga

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Anthropogenic biomes

“Anthropogenic biomes describe globally-significant ecological patterns within the terrestrial biosphere caused by sustained direct human interaction with ecosystems, including agriculture, urbanization, forestry and other land uses. Conventional biomes, such as tropical rainforests or grasslands, are based on global vegetation patterns related to climate. Now that humans have fundamentally altered global patterns of ecosystem form, process, and biodiversity, anthropogenic biomes provide a contemporary view of the terrestrial biosphere in its human-altered form. Anthropogenic biomes may also be termed “anthromes” to distinguish them from conventional biome systems, or “human biomes” (a simpler but less precise term).” -Erle Ellis and Navin Ramankutty

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