Chapter 5: Biological Communities: The Biome Concept

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Chapter 5 Biological Communities The Biome
Concept
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The Case of the Cactus Look-Alike

• Cactus-like plants are common in Africa.
• These plants do not belong to the cactus family,
  Cactaceae
• Cactaceae are native only to New World
• look-alikes may be in spurge family,
  Euphorbiaceae
• This situation illustrates convergence of species
  descended from different ancestors.

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Convergence

• Convergence is the process by which unrelated
  organisms evolve a resemblance to each other in
  response to common environmental conditions
• similar adaptive responses emerge in response to
  particular selective conditions
• an example mangroves worldwide typically have
  thick, leathery leaves, root projections, and
  viviparity

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(a) A tree-forming cactus in Mexico (b) an East
African euphorb tree. Both converged in response
to dry climate
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The Biome Concept

• 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.
• In North America
• tundra, boreal forest, temperate seasonal forest,
  temperate rain forest, shrubland, grassland, and
  subtropical desert
• In Mexico and Central America
• tropical rain forest, tropical deciduous forest,
  and tropical savanna

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Biomes - Key Points

• 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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Adaptations and Environment -- Not the Whole Story

• Distributions of species are not solely a
  function of relationships to physical
  environment
• biotic interactions shape these distributions
• chance and history play important roles

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Climate is the major determinant of plant
distribution.

• Climatic factors typically establish limits of
  plant distributions
• the sugar maple, Acer saccharum, in eastern North
  America, is limited by
• cold winter temperatures to the north
• hot summer temperatures to the south
• summer drought to the west

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Ecological Tolerances

• Several tree-sized maples in eastern North
  America have distributions that broadly overlap
  that of sugar maple
• because of different ecological tolerances, these
  other species exhibit distinctive environmental
  preferences, even when their ranges overlap
• black drier, better-drained soils high in
  calcium
• silver moist, well-drained soils
• red wet and swampy or dry, poorly-developed soils

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Related species may differ in their ecological
tolerances
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Topography in mountains creates a wide range of
moisture conditions

• each species exhibits a local and distinctive
  optimum the type of site in which it does best
• coast redwood dominates center of moisture
  gradient
• cedar, Douglas fir, madrone occur at drier end of
  the moisture gradient
• big-leaf maple, California bay tree occur at
  wetter end of moisture gradient

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Form and function match the environment.

• Adaptations match each species to the environment
  where it lives
• all species are to some extent specialized
• insect larvae from ditches and sloughs survive
  without oxygen longer than related species from
  well-aerated streams
• marine snails from the upper intertidal tolerate
  desiccation better than their relatives from
  lower levels
• we recognize both specialists and generalists

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(a) Mesquite leaves are subdivided into leaflets
that facilitate dissipation of heat
(b) Paloverde leaflets are tiny thick stems are
responsible for photosynthesis
(c) Limberbrush has broad, succulent leaves
produced for only a few weeks
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Other Considerations

• Certain species make their environments more
  favorable for themselves
• decaying foliage of evergreen species of poor
  soils produces organic acids, leaching minerals
  from soil
• Availability of moisture is the single most
  important climatic factor defining biomes
• because heat influences moisture stress,
  temperature and precipitation together are the
  determinants of boundaries of major biomes

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Climate defines the boundaries of terrestrial
biomes.

• A widely adopted climatic classification is that
  of Heinrich Walter
• Walters 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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Walters climate zone classification
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Whittakers Scheme 1

• 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
• Whittakers scheme is similar in many respects to
  Walters
• Whittaker starts with vegetation and relates
  climate

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Whittakers Scheme 2

• 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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Whittakers biomes
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Whittakers Scheme - Other Considerations

• Fire shapes vegetation toward drier end of
  spectrum within each temperature range
• typically in grassland and shrub biomes where
• moisture is intermediate (sufficient productivity
  for fuels to accumulate)
• seasonal droughts occur (fuels dry out
  sufficiently to burn)
• fire favors grasses and forbs over woody plants
• species of these systems are adapted to or are
  specialized for frequent fires

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Walters Climate Diagrams

• Walters climate diagrams relate monthly
  temperature and precipitation through the year
• 20 mm of monthly precipitation is equated with
  10oC in temperature
• vertical scales permit ready identification of
  periods of water deficit and water abundance
• Localities within the same climate zone have
  similar climates worldwide.

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Global distribution of major biomes
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Temperate Climate Zones

• 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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Temperate Seasonal Forest Biome 1

• 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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Temperate Seasonal Forest Biome 2

• 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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Temperate Rain Forest Biome

• 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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Temperate Grassland/Desert Biome 1

• Found in continental climate zones
• summers are hot and wet winters are cold
• growing season is 120-300 days
• fires are a dominant influence
• 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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Temperate Grassland/Desert Biome 2

• 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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Woodland/Shrubland Biome

• 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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Subtropical Desert Biome

• 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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Boreal and Polar Climate Zones

• 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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Boreal Forest Biome

• 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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Tundra Biome

• 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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Equatorial and Tropical Climate Zones

• Located within 20o of the equator.
• Daily temperature variation exceeds monthly
  variation through the year.
• Environments are largely distinguished by
  differences in the seasonal pattern of rainfall.
• Frost is not a factor plants and animals cannot
  tolerate freezing.

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Tropical Rain Forest Biome

• 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).

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Tropical Seasonal Forest/Savanna Biomes 1

• 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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Tropical Seasonal Forest/Savanna Biomes 2

• 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
• 90-150 cm/yr but driest 3-4 months receive less
  than 5 cm each
• fire and grazing play important roles in
  maintenance of these system

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A pause

• Rainforests today

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Rain forests today

• More than one half of tropical forests have
  already been destroyed
• Rainforests once covered 14 of the earth's land
  surface now they cover a mere 6 and experts
  estimate that the last remaining rainforests
  could be consumed in less than 40 years.
• each day at least 80,000 acres (32,300 ha) of
  forest disappear from Earth. At least another
  80,000 acres (32,300 ha) of forest are degraded.
  Hundreds of species driven to extinction
• FAO estimates that 10.4 million hectares of
  tropical forest were permanently destroyed each
  year in the period from 2000 to 2005, an increase
  since the 1990-2000 period, when around 10.16
  million hectares of forest were lost
• Experts estimates that we are losing 137 plant,
  animal and insect species every single day due to
  rainforest deforestation. That equates to 50,000
  species a year. As the rainforest species
  disappear, so do many possible cures for
  life-threatening diseases. Currently, 121
  prescription drugs sold worldwide come from
  plant-derived sources. While 25 of Western
  pharmaceuticals are derived from rainforest
  ingredients, less that 1 of these tropical trees
  and plants have been tested by scientists.
• There were an estimated ten million Indians
  living in the Amazonian Rainforest five centuries
  ago. Today there are less than 200,000.

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Deforestation for cattle grazing
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Modification of the Biome Concept for Aquatic
Ecosystems

• The biome concept in its strict sense does not
  exist for aquatic ecosystems
• biomes were developed for terrestrial ecosystems,
  where growth form of dominant vegetation is
  distinguishing factor
• aquatic ecologists have tended to develop
  independent classifications for aquatic systems,
  focused predominantly on physical factors

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Aquatic Ecosystems - Streams

• Streams form wherever precipitation exceeds
  evaporation, draining excess water.
• Streams may be divided into principal habitats
• riffles (where water runs over rocky substrate)
• pools (deeper stretches of slow-moving water)
• Streams exhibit continuous change in conditions
  from headwaters downstream, captured in river
  continuum concept.
• Streams exhibit downstream drift of
  organisms/material.

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Conditions of a stream differ in pools and riffles
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Aquatic Ecosystems - Lakes

• Lakes form in any kind of depression (typically
  effects of glaciation or geological activity).
• May be divided into principal habitats
• littoral zone (shallow zone with rooted
  vegetation)
• limnetic zone (open water beyond littoral zone)
• benthic zone (bottom sediments, habitat for
  burrowing animals and microorganisms)

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Ecological zones in a lake
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Aquatic Ecosystems - Estuaries

• Are special environments at the mouths of rivers,
  especially where outflow is partially enclosed
  (such as barrier islands).
• Unique because they are the interface between
  fresh and salt water habitats
• typically highly productive because of influx of
  nutrients and their rapid exchange between
  sediments and surface waters
• often edged by extensive tidal marshes with
  emergent vegetation

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Estuaries
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Aquatic Ecosystems - Oceans

• Oceans are enormously complex systems, with
  conditions varying with temperature, depth,
  current, substrate, tides.
• Oceans are often classified according to depth
• littoral zone (between high and low tides,
  exhibits dramatic zonation)
• neritic zone (beyond low tide to edge of
  continental shelf, often subdivided into photic
  and aphotic zones, typically productive)
• oceanic zone (deeper waters, also divided into
  photic and aphotic zones, typically unproductive)

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Oceans ecological zones
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Aquatic Ecosystems - Coral Reefs

• Coral reefs are special oceanic systems
• develop in shallow waters of warm oceans
• may develop around volcanoes, which may subside
  or erode, leaving a ring-like atoll
• are typically highly productive
• nutrients released from erosion on landward side
• nutrients forced up as deeper-water currents
  encounter the reef
• are typically highly diverse

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Coral reefs in the Red Sea
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Summary 1

• Climate has profound effects on dominant plant
  forms. Each region has characteristic vegetation
  differing in growth form, leaf morphology, and
  seasonality of foliage.
• Major vegetation types can be used to classify
  ecosystems into biomes associated with major
  climatic classes.
• Walters approach classifies regions on the basis
  of climates having characteristic vegetation.

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Summary 2

• Whittakers approach classifies regions on the
  basis of vegetation having characteristic
  climates.
• Principal climatic zones (tropical, temperate,
  boreal, polar) and their biomes are distinguished
  first on the basis of temperature, then
  precipitation, and its seasonality.
• Aquatic systems are not classified into biomes,
  but we distinguish streams, lakes, estuaries,
  oceans, and coral reefs.

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As usual

• Do quiz for chapter 5 due Monday
• Reminder quiz chapters 1 to 4, March 17 (for
  BIOL 207) and March 18 (for PDHP 212)