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CHAPTER 28: DYNAMICS OF ECOSYSTEMS
C HAPTER S YNOPSIS
photosynthesizers, primary consumers, secondary
consumers, decomposers, and detrivores. Primary
productivity is the total amount of organic matter
produced from the sun’s energy within a given
system. Some energy is lost in the metabolic
activities of the initial photosynthesizers,
resulting in the net primary productivity value.
The weight of all of the organisms living in an
ecosystem is the biomass of that ecosystem. This
figure increases as a result of net productivity.
Many ecosystems have both high productivity
and high biomass. Others have high
productivity but low biomass. Tropical forests
and wetlands have the highest biomass ratios,
while deserts have the lowest.
Ecosystems are the most complex level of
organization because they include living and
nonliving factors. The earth is a closed system
with respect to chemicals and nutrients, but an
open system with regard to energy from the sun.
All substances in organisms cycle through the
ecosystem. Many of these substances have
atmospheric reservoirs while others are
contained within rocks. Rarely is the bulk of any
substance contained within bodies of organisms.
Among the more important biogeochemical cycles
are those that involve water, carbon, nitrogen,
and phosphorus.
Life depends on water, 98% of which is found in
oceans, in surface fresh water, and as ground
water. Life also depends on carbon, most of which
is contained in the carbon dioxide of the
atmosphere. Various photosynthesizers fix
carbon into organic substances which are then
passed to various heterotrophs. Carbon is
returned to the atmosphere via respiration and
when an organism dies and decomposes. Until
recently, the processes of photosynthesis and
respiration/decomposition have roughly
balanced each other. The burning of fossil fuels
and the destruction of vast areas of
photosynthesizing plants may be tipping that
scale in the direction of increased atmospheric
carbon dioxide. Nitrogen gas comprises 78% of
the atmosphere. It is only available to organisms
by the action of various nitrogen-fixing bacteria.
Leguminous plants harbor such bacteria, which
are readily visible in their root nodules. They fix
sufficient nitrogen for their own growth and
release the excess into the soil, fertilizing it
without the addition of costly chemicals. The
phosphorus cycle is typical of many of the other
mineral-oriented biogeochemical cycles. Much
phosphorus is taken up by marine organisms
which are then eaten by ocean birds. Guano is
among the richest fertilizers available.
Energy is passed through an ecosystem in a series
of small steps comprising a food chain or a food
web. In theory, higher productivity should
support longer food chains, but in reality little
energy remains after only four steps. The
relationships of these trophic levels are
diagrammatically represented by biomass and
energy pyramids. Occasionally a biomass
pyramid may be inverted, but the energy
pyramid cannot be inverted. Trophic levels
dramatically affect each other through a
phenomenon called a trophic cascade. Lower
trophic levels can influence higher levels via
bottom-up effects. In addition lower and higher
trophic levels can cancel each other out or
reinforce each other, producing extremely
complicated food web interactions.
Biodiversity, species richness, and species
diversity promote community stability. Factors
that promote species richness include ecosystem
productivity, spatial heterogeneity, and climate.
The tropics exhibit a greater number of species
than any other biome. This may be due to the
long evolutionary age of the tropics combined
with high productivity, predictability of
climate, intense predation, and spatial
heterogeneity. A predictable species-area
relationship has been observed as one of the most
predictable patterns in ecology. The equilibrium
model of island biogeography proposes that, in
time, the number of species extinctions and
colonizations balance each other out and the
number of species remains constant.
Only a small percentage of the sun’s energy is
captured by photosynthesizers; even less of this
energy is passed on to animals. In terms of energy
utilization, it is better to be a vegetarian or
herbivore than a carnivore. Each successive level
of consumer is a trophic level and includes
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DYNAMICS OF ECOSYSTEMS
291
C HAPTER O BJECTIVES
➤ Understand the nature of an ecosystem.
➤ Discuss the five primary biogeochemical
cycles, including the reservoirs and unique
organisms upon which each cycle depends.
➤ Discuss the effects of deforestation on the
water cycle, flood control, and overall
fertility.
➤ Understand the relationship of one trophic
level with another in terms of energy and
biomass.
➤ Differentiate between gross and net
productivity and explain how each is related
to biomass.
➤ Identify the components of a general food
chain.
➤ Explain what an ecological pyramid is and
what different shapes indicate.
➤ Understand how trophic cascades and
bottom-up effects influence populations at
other trophic levels and how the two effects
interact.
➤ Compare species richness and species
diversity and the factors that promote each.
➤ Understand the importance of the
equilibrium model in island biogeography.
➤ Compare the overall productivity of each of
the major ecosystems to one another.
KEY TERMS
ammonification
autotroph
biogeochemical cycle
biomass
bottom-up effects
carbon cycle
consumer
decomposer
denitrification
detrivore
food chain
food web
gross primary productivity
heterotroph
net primary productivity (NPP)
nitrogen cycle
phosphorus cycle
primary consumer
primary producer
primary production
primary productivity
secondary consumer
secondary productivity
species-area relationship
species diversity cline
species turnover
superphosphate
trophic cascade
trophic level
C HAPTER O UTLINE
28.0
Introduction
I.
Ecosystems Are the Most Complex Level of Biological Organization
A. Transfer of Energy Is Regulated, Nutrients Are Cycled
1. Earth is a closed system with respect to nutrients and chemicals
2. System is open with respect to energy
B. All Organisms Depend on Other Organisms to Recycle Basic Components
fig 28.1
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CHAPTER 28
28.1
Chemicals cycle within ecosystems
I.
B IOGEOCHEMICAL CYCLES
A. All Substances in Organisms Cycle Through Ecosystems
1. Cycle via biogeochemical cycles
2. The bulk of these are not contained within the bodies of organisms
a. Contained within the atmosphere: Carbon, nitrogen, and oxygen
b. Contained within rocks: Phosphorus, potassium, and other minerals
B. Substances Are Incorporated from Nonliving Sources into Organisms
1. Minerals enter organisms through drinking water
2. Returned to nonliving world through decomposition
II. THE W ATER CYCLE
A. The Most Familiar Biogeochemical Cycle
1. All life depends directly on the presence of water
2. Water is source of hydrogens used in generating ATP
B. The Path of Free Water
1. Energy from sun powers the evaporation of water into atmosphere
a. Most falls back into the oceans or subsurface bodies of water
b. Most of water over land evaporates from plants through transpiration
2. 98% of earth’s water is free, only 2% is fixed
C. The Importance of Water to Organisms
1. All organisms require water to live
a. Plants obtain water from the earth
b. Animals drink water or obtain it by eating plants
2. Free water determines nature and abundance of life in any particular place
D. Groundwater
1. Water occurs as surface and ground water
a. Aquifers are permeable saturated layers of rock, sand, and gravel
b. Ground water is an important reservoir of water
c. Water table: Upper, unconfined portion of ground water
2. Ground water flows more slowly than surface water
a. Rate of use is increasing enormously
b. Many aquifers are threatened with depletion
3. Pollution in groundwater is a serious problem
a. Estimated that 2% of the U.S. groundwater is polluted
b. Includes pesticides, herbicides, fertilizers, chemical waste pits
4. Removing pollutants from aquifers virtually impossible
a. Large volume of water with slow turnover rate
b. Generally inaccessible
E. Breaking the Water Cycle
1. Cycling in forest ecosystems is mediated by plants
a. 90% of moisture taken up by plants, transpires back into air
b. Vegetation is primary source of rainfall
2. Removal of forests affects water cycle
a. Moisture not returned to air
b. Drains away from region
*fig 28.2
DYNAMICS OF ECOSYSTEMS
3.
4.
5.
von Humbolt study in Columbia in 1700s
a. Stripping trees from rainforest prevents water return to atmosphere
b. Area became semiarid desert
Transformation occurring today in the name of “development”
a. Much of Madagascar was tropical forest, now semiarid desert
b. No practical way to reforest land
Once broken, hard to reestablish water cycle
III. THE CARBON CYCLE
293
fig 28.3
*fig 28.4
A. Based on Atmospheric Carbon Dioxide
1. Carbon dioxide comprises 0.03% of atmosphere, 700 billion metric tons
2. Synthesis of organic compounds fixes 10% of atmospheric carbon dioxide yearly
a. Accomplished by various photosynthesizers
b. All heterotrophic nonphotosynthesizers depend on their activity
3. Carbon dioxide released into atmosphere when organisms decompose
B. Most Carbon-Containing Compounds Ultimately Break Down
1. Some carbon compounds are accumulated
a. Cellulose is more resistant to breakdown
b. May eventually be incorporated into fossil fuels or minerals
2. One trillion metric tons of CO2 are dissolved in the ocean
a. Fossil fuels contain 5 trillion metric tons
b. 600 billion to 1 trillion metric tons contained within organisms
3. Processes of respiration and photosynthesis are roughly balanced
a. Carbon dioxide increasing as a result of burning fossil fuels
b. May be altering global climates
IV. THE NITROGEN CYCLE
*fig 28.5
A. Organisms Depend on Nitrogen
1. Nitrogen gas constitutes 78% of the atmosphere
2. Only bacteria can fix nitrogen into forms used by other organisms
3. Nitrogen cycles between organisms and reservoirs
B. Nitrogen Fixation
1. Few organisms convert atmospheric nitrogen into biologically useful forms
a. All are nitrogen-fixing bacteria
b. Triple bond linking nitrogen atoms makes the gas very stable
c. Process is enzyme catalyzed and utilizes ATP
d. N 2 + 3H2 → 2NH 3
2. Some nitrogen-fixing bacteria are free-living
3. Some form symbiotic relationships with roots of legume plants
a. Fix enough nitrogen to be of significance
b. Large reservoir of ammonia and nitrates now exists
4. Bacteria and fungi rapidly decompose nitrogen-containing compounds
a. Use products to synthesize own proteins, release excess as ammonium (NH4 +)
b. Process called ammonification
5. Fixed nitrogen is lost to the atmosphere by denitrification
a. Accomplished by certain anaerobic bacteria
b. Nitrate converted to N2 gas and N2 O (nitrous oxide), both return to air
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V. THE PHOSPHORUS CYCLE
*fig 28.6
A. Exemplifies Other Mineral Cycles
1. Most biogeochemical cycle reservoirs in minerals, not atmosphere
2. Phosphorus plays critical role in plant nutrition
3. Phosphates exist in the soil in only small amounts
a. Are relatively insoluble and contained in only certain kinds of rocks
b. Weather out of rocks, transported to oceans
c. Brought up by natural uplift of land masses or by marine animals
d. Form rich natural deposits of guano from sea birds that eat marine animals
4. Examples of phosphorous fertilizers
a. Guano deposits
b. Crushed phosphate-rich rocks
c. Seas are only inexhaustible source, value of seabed mining
B. Value of Phosphorous Fertilizers
1. Millions of tons of phosphates added to farm land each year
a. Calcium dihydrogen phosphate [Ca(H2 PO4 ) 2 ]is called superphosphate
b. Made by treating calcium phosphate with sulfuric acid
2. Not leading to proportional increase in crops
VI. B IOGEOCHEMICAL CYCLES ILLUSTRATED: R ECYCLING IN A FORESTED ECOSYSTEM
A. Studies of Hubbard Brook Experimental Forest
1. Reveals details about overall recycling patterns in an ecosystem
a. Studied since 1962
b. Provides much data about nutrient cycling in forest ecosystems
c. Basis of development of experimental methodology
2. Description of region and methodology
a. Central stream of large temperate deciduous forest watershed
b. Measure water and nutrient flow made through concrete weirs on six streams
c. Determined minerals present in the streams
3. Conclusions: Undisturbed forests efficiently retained nutrients
a. Amount gained in snow and rain equaled amount lost in runoff
b. Quantities low compared to nutrients within system
c. Small net loss of calcium
d. Small net gain of nitrogen and potassium
4. Instructive with regard to loss of rainforest area to crops
B. Experiment to Determine Effect of Loss of Trees
1. Cut down of trees and shrubs in one of six watersheds
2. Prevented regrowth by use of herbicides
3. Deleterious effects
a. Amount of water runoff increased by 40%, not taken up by plants
b. Amount of nutrients lost was greatly increased
c. Ten times greater loss of calcium
d. Phosphorous not in run-off water, locked up in soil
e. Nitrogen lost at rate of 120 kilograms per hectare per year
f. Nitrate level in water increased to level unfit for drinking
g. Generated massive blooms of cyanobacteria and algae
4. Conclusion: Fertility lost, danger of flooding increased
*fig 28.7
DYNAMICS OF ECOSYSTEMS
28.2
295
Ecosystems are structured by who eats who
I.
TROPHIC LEVELS
A. Ecosystems Contain Autotrophs and Heterotrophs
1. Autotrophs capture light energy and manufacture own food
2. Heterotrophs obtain organic molecules synthesized by autotrophs
3. Autotrophs are primary producers, heterotrophs are consumers
B. Cycling of Metabolic Energy
1. Energy captured is slowly released through metabolic processes
a. Green plants convert 1% of the sun’s energy
b. Less energy converted by the animals that eat plants
2. Levels of consumers
a. Primary consumers: Herbivores, feed on green plants
b. Secondary consumers: Carnivores and parasites feed on herbivores
c. Decomposers: Break down matter accumulated in bodies of organisms
d. Detrivores: Live on dead organisms and cast-off parts of organisms
3. Such trophic levels exist in all complicated ecosystems
a. Organisms from each level compose food chain
b. Relationships are more accurately branching food webs
4. Some energy ingested is lost at each successive trophic level
a. Much goes to heat production, some lost for digestion and work
b. Less than 40% goes toward growth and reproduction
28.3
*fig 28.8
*fig 28.9
Energy flows through ecosystems
I.
PRIMARY PRODUCTIVITY
A. Productivity Terms Defined
1. Primary production
a. Also called primary productivity
b. Amount of organic matter produced from solar energy per area per time
2. Gross primary productivity (GPP)
a. Total amount of organic matter produced
b. Includes what is used by photosynthetic organisms for respiration
3. Net primary productivity (NPP)
a. Amount of organic matter produced in a community at a given time
b. Available to be used by heterotrophs
c. NPP = GPP - amount of energy expended in photosynthesizers metabolisms
4. Biomass
a. Net weight of all organisms living in the ecosystem
b. Increases as a result of net productivity
B. Productive Biological Communities
1. Tropical forests and wetlands have high net primary productivity
2. Comparative net primary productivity: Biomass ratios
a. Tropical forest and wetlands = 1500 to 3000 grams/year
b. Temperate forest = 1200 to 1300 grams/year
c. Savanna = 900 grams/year
d. Desert = 90 grams/year
*tbl 28.1
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C. Secondary Productivity
1. Rate of production by heterotrophs
a. Cannot manufacture own biomolecules
b. Obtain by eating plants or other heterotrophs
2. About one magnitude less than primary productivity in same system
3. Loss of energy from plants to herbivores
a. Much of biomass not consumed, supports decomposers
b. Some energy not assimilated, passed out as feces to decomposers
c. Some energy assimilated is lost in heat
fig 28.10
II. THE ENERGY IN FOOD CHAINS
A. Consist of Only a Few Steps
1. Much energy lost at each step
2. Little remains after four successive steps
B. Community Energy Budgets
1. Cole’s experimental studies of freshwater ecosystem in upstate New York
2. For each 1000 calories of energy fixed by photosynthesizers
a. 150 calories transferred to small heterotrophs
b. 30 calories of that transferred to smelt
c. 6 calories of that transferred to trout (or humans)
d. 1.2 calories from trout transferred to humans
3. More efficient to eat organisms at lower level in food chain
fig 28.11
*fig 28.12
C. Factors Limiting Community Productivity
1. Higher productivity supports longer food chains in theory
2. Amount of sunlight is ultimate limit of a community’s productivity
a. Primary productivity of deciduous forests increases as growing season increases
b. NPP higher in warm climates than cold ones
1) Longer growing season
2) More nitrogen available via activity of nitrogen fixing bacteria
III. ECOLOGICAL PYRAMIDS
A. Less Energy Available at Successive Levels
1. Comparison of values at different levels
a. Plant fixes about 1% of the energy received by its leaves
b. Heterotrophs process 10% of the energy available from their food sources
c. More individuals at lower levels of food chain than higher levels
d. Successive levels have lower biomass
2. Diagrammatic representations of such relationships form pyramids
*fig 28.13
B. Inverted Pyramids
1. Some aquatic ecosystems have inverted biomass pyramids
2. Planktonic ecosystems dominated by small organisms floating in water
a. Turnover of lowest level, photosynthetic phytoplankton, very rapid
b. Cannot develop large population size
c. Phytoplankton reproduce rapidly
d. Community supports larger population of heterotrophs
*fig 28.13b
DYNAMICS OF ECOSYSTEMS
297
C. Top Carnivores
1. Loss of energy at each level limits number of top carnivores
a. One-thousandth of energy from photosynthesis passes to third stage
b. No predators for high-level organisms, not enough biomass to support it
2. Top-level predators tend to be large animals
3. Small residual biomass concentrated in small number of individuals
IV. I NTERACTIONS AMONG DIFFERENT TROPHIC LEVELS
A. Species at Different Trophic Levels Interact Due to Food Webs
1. Predators affect prey and plants eaten by their prey
2. Increase in primary productivity provides food for herbivores and carnivores
B. Trophic Cascades
1. Predators keep herbivore populations in check
2. Effect of one trophic level flowing to others is called trophic cascade
3. Existence confirmed by experimental studies — New Zealand
*fig 28.14
a. Stream sections isolated with mesh, prevent fish from entering
b. Large brown trout added to some sections and not others (controls)
c. Number of invertebrates in trout areas one-half of that in control areas
d. Algae biomass 5 times greater in trout sections than in control areas
4. Predict carnivore-eating-carnivore trophic level would cause cascade effects
a. Top predators keep lower-level predators in check
b. Leads to profusion of herbivores, decrease in vegetation
c. Experimental support via study in California
*fig 28.15
C. Human Effects on Trophic Cascades
1. Humans remove top predators, test trophic cascade hypothesis
2. Leopold’s observations in Sand Country Almanac about loss of wolves
a. Loss of wolves increase number of deer
b. Increase in deer reduced amount of vegetation
3. Barro Colorado Island formed by construction of Panama Canal
a. Absence of large predators increases numbers of small predators
b. Ground-nesting birds have greatly decreased in number
4. Example of woodlots in U.S.
a. Elimination of large predators
b. Abundance of raccoons, opossums and foxes; decline in ground-nesting birds
D. Bottom-Up Effects
1. Factors acting at bottom of food webs also effect higher trophic levels
*fig 28.16
a. Ecosystem productivity low, herbivore populations too small to support predators
1) Productivity increases devoured by herbivores, their populations increase
2) Herbivore populations now large enough to support predators
b. Predators, not herbivores, increased with further increase in productivity
1) Further increase allows development of higher-level predators
2) Herbivore populations can increase with their predators kept in check
2. Experimental evidence via Eel River study in California
*fig 28.17
a. Constructed enclosures to exclude large fish
b. Roofed over to control level of light in enclosure
1) Primary productivity greatest in unshaded enclosures
2) Increase in productivity led to more vegetation and more predators
3) As predicted, number of herbivores did not increase
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E. Relative Importance of Trophic Cascades and Bottom-Up Effects
1. Top-down effects may not propagate to lower levels
a. Strongly competing herbivores, one preyed upon more greatly
b. Increased predation will only decrease population of vulnerable species
c. No change in other herbivore or vegetation
2. Productivity increases may not move upward to all levels
a. Prey populations increase too quickly for predator control
b. Productivity increases do not move upward
3. Trophic cascades and bottom-up effects may reinforce each other
a. Lakes with opposite levels of minnows and zooplankton
b. Added nutrients to both lakes
c. In minnow lake with few zooplankton
1) Algae productivity did not move up food chain
2) Formed large algae mats
d. In large-fish lake with few minnows and much zooplankton
1) Increased productivity moved up food chain
2) Amount of algae controlled
4. Nature is generally more complicated than a simple food chain
a. Forms complicated, interacting food webs
*fig 28.9
b. Scientists working to apply theories to more complicated patterns
28.4
Biodiversity promotes community stability
I.
EFFECTS OF SPECIES RICHNESS
A. Some Communities Are More Stable than Others
1. Biologically diverse ecosystems are more stable
a. More constant in composition
b. More able to resist disturbance
2. Cedar Creek Natural History Area studies
a. 207 small rectangular plots studied for 11 years
fig 28.18a
b. Counted number of species and measured plant biomass per plot
c. Plots with more species showed less year-to-year biomass variation *fig 28.18b
d. Plots with more species less disturbed by 2 years of drought
e. More diverse communities better able to resist invasion by new species
B. Species Richness Affects Other Ecosystem Processes
1. 147 plots with experimentally varied number of species
2. Plot growth and nitrogen uptake monitored
a. Greater nitrogen uptake if more species
b. Total amount of biomass produced also higher
3. Increased diversity leads to greater productivity
C. Artificial Ecosystem Studies Produce Similar Results
1. Ecosystems constructed in environmentally-controlled growth chambers
a. Variety of species added with differing diversity
b. Amount of biomass produced reflected species richness
c. Amount of carbon dioxide produced paralleled species richness
*fig 28.19
DYNAMICS OF ECOSYSTEMS
299
II. CAUSES OF SPECIES RICHNESS
A. Understanding Ecosystem Stability Helps Avoid Permanent Changes
1. Increase ability to return to normal after disturbance
2. Diverse systems are more stable
a. Support more complex web interactions
b. Alternative niche more likely to exist
B. Factors Promoting Species Richness
1. Difficult to sort out contribution of all factors
a. Variables include ecosystem productivity, spatial heterogeneity, climate
b. Additional factors: evolutionary age, degree of disturbance
2. Ecosystem productivity
a. Species richness correlation with productivity is not linear
*fig 28.20
b. Communities with intermediate levels have most species
1) Productivity levels may be linked to predators
2) Fewer predators with low productivity
3) Superior competitors eliminate most species
4) Many predators with high productivity; predation-resistant species survive
5) Predators act as keystone species at intermediate levels
3. Spatial heterogeneity
a. When heterogenous, contain more kinds of variations
b. Accommodate more species by having more microhabitats
c. Species richness of animals reflects species richness of plants
d. Plants provide spatial heterogeneity to the animals
e. Example: Lizard/plant species in southwest
*fig 28.21
4. Climate
a. Role more difficult to assess
b. Might expect more species to exist where there is seasonal variation
c. Stable environments support specialized species
d. Western mammal species increase as temperature range decreases
*fig 28.22
III. B IOGEOGRAPHIC PATTERNS OF SPECIES DIVERSITY
A. Species Diversity Cline
1. Increase in species richness from arctic to tropics
2. Gradient in number of species correlated with latitude
*fig 28.23
B. Why Are There More Species in the Tropics?
1. Evolutionary age
a. Tropics have existed over long, uninterrupted periods
b. Temperate regions have experienced glaciation
c. Greater age of tropics allows for evolution of more complex population interactions
d. Fosters greater variety of plants and animals
e. Long-term stability of tropics now disputed, greatly exaggerated
2. Higher productivity
a. Productivity due to greater solar radiation
b. Increases overall photosynthesis of plants
c. Field studies indicate greatest species richness at intermediate productivity
d. Filtering of light in tropical forest may lead to variety of light environments
3. Predictability
a. Climates are stable and predictable
b. Unchanging environment may encourage specialization, reduce competition
c. Produces larger number of specialized species in tropics
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4.
5.
Predation
a. More intense in tropics
b. Could reduce importance of competition
c. Permits niche overlap, promotes species richness
Spatial heterogeneity
a. Promotes species richness
b. Provides great number of microhabitats, fosters larger number of species
IV. I SLAND B IOGEOGRAPHY
A. Larger Islands Contain More Species than Smaller Islands
1. Examined by MacArthur and Wilson
2. Species-area relationship effects species extinction and colonization
B. The Equilibrium Model
1. Species constantly dispersed to islands
a. Islands accumulate more and more species
b. Other species lost to extinction
2. When number of species filled to capacity, no new ones established without extinction
a. Assumes characteristic equilibrium number
*fig 28.24a
b. Identity of species may change
3. Island species richness is a dynamic equilibrium between colonization and extinction
a. Dependent on island size and distance from mainland
b. Small islands have higher rate of extinction
c. Less colonization in islands farther from mainland
d. Small islands far from mainland have fewest species
*fig 28.24b
e. Large islands closer to mainland have more species
4. Example: Asian Pacific bird species comparison
*fig 28.24c
a. Positive correlation of species richness with island size
b. Negative correlation of species richness and distance from mainland
C. Testing the Equilibrium Model
1. Field studies of small islands support equilibrium model
2. Long-term studies suggest more complicated situation
a. Theory supports high level of species turnover
b. Studies on birds and spiders show little yearly turnover
c. Species that change are subset of species with generally low populations
3. Over periods of centuries the equilibrium theory may gain strength
INSTRUCTIONAL S TRATEGY
PRESENTATION A SSISTANCE :
It is most valuable to present the cyclic nature of
biogeochemical cycles, their reservoirs, and how
they are affected by organisms.
This is another appropriate location to discuss
the greenhouse effect as it relates to the carbon
V ISUAL R ESOURCES:
Bring in photos of organisms that exemplify
various kinds of trophic levels and/or food webs.
cycle. Photosynthesis and respiration should be
in balance. When forests are destroyed,
photosynthesis decreases. Carbon dioxide
increases if they are burned or if they decompose
naturally. The ultimate result is increased
atmospheric carbon dioxide.