Biological Communities: The Biome Concept

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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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Figure 5.1
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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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Figure 5.2
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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.

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Biomes - Terrestrial Examples

• 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
• for 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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Figure 5.3
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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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Figure 5.4
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Variations in topography and soils influence
plant distributions.

• Topography in mountains creates a wide range of
  moisture conditions
• each species exhibits a local and distinctive
  optimum in northern California
• 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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Figure 5.5
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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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Figure 5.6
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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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Table 5.1
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Figure 5.7
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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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Figure 5.8
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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 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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Figure 5.9
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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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Figure 5.10
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Figure 5.11a
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Figure 5.11b
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Figure 5.11c
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Figure 5.12a
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Figure 5.12b
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Figure 5.12c
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Figure 5.13
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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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Figure 5.14a
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Figure 5.14b
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Figure 5.14c
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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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Figure 5.15a
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Figure 5.15b
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Figure 5.15c
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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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Figure 5.16a
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Figure 5.16b
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Figure 5.16c
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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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Figure 5.17a
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Figure 5.17b
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Figure 5.17c
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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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Figure 5.18a
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Figure 5.18b
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Figure 5.18c
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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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Figure 5.19a
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Figure 5.19b
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Figure 5.19c
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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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Figure 5.20a
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Figure 5.20b
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Figure 5.20c
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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
  lianas and epiphytes.

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Figure 5.21a
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Figure 5.21b
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Figure 5.21c
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Tropical Seasonal Forest/Savanna Biomes 1

• Found where climate is seasonally dry, but has
  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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Figure 5.22a
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Figure 5.22b
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Figure 5.22c
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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)
• runs (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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Figure 5.23
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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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Figure 5.24
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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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Figure 5.25
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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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Figure 5.26
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Figure 5.27
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Figure 5.28
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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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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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Figure 5.29