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,

EuphorbiaceaeThis situation illustrates convergence of

species descended from different ancestors.

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ConvergenceConvergence 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

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 difficultMatching 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 8

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 calciumsilver: moist, well-drained soilsred: 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

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

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: Walter’s scheme is based on the annual

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

temperature stress that determine plant formrecognizes 9 zones, from Equatorial (Tropical

rain forest) to Polar (Tundra)

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Walter’s climate zone classification

Whittaker’s 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

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

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Whittaker’s Scheme 2Equatorial 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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Whittaker’s biomes

Whittaker’s 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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Walter’s Climate Diagrams

Walter’s 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

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 precipitationlength 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 desertsOccur 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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04/20/23 39

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

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

Aquatic Ecosystems - LakesLakes 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

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 …

Aquatic Ecosystems - OceansOceans 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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Ocean’s ecological zones

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 sidenutrients forced up as deeper-water currents

encounter the reef

are typically highly diverse

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Coral reefs in the Red Sea

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.

Walter’s approach classifies regions on the basis of climates having characteristic vegetation.

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Summary 2

Whittaker’s 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 MondayReminder: quiz – chapters 1 to 4,

March 17 (for BIOL 207) and March 18 (for PDHP 212)

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