the living world fourth edition george b. johnson copyright ©the mcgraw-hill companies, inc....
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The Living WorldFourth Edition
GEORGE B. JOHNSON
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PowerPoint® Lectures prepared by Johnny El-Rady
32 Populations andCommunities
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32.1 Population Growth
A population is a group of individuals of the same species living together
Critical properties of a population include
Population sizeThe number of individuals in a population
Population densityPopulation size per unit area
Population dispersionScatter of individuals within a population’s range
Population growthHow populations grow and the factors affecting growth
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Assumes a population is growing without limits at its maximal rate
Rate is symbolized r and called the biotic potential
The Exponential Growth Model
Growth rate = dN/dt = riN No. of individuals in a population
Intrinsic rate of increase
Change over time
The actual rate of population increase is
r = (b – d) + (i – e)
Birthrate Deathrate Net immigration
Net emigration
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No matter how fast populations grow, they eventually reach a limit
This is imposed by shortages of important environmental factors
Nutrients, water, space, light
The carrying capacity is the maximum number of individuals that an area can support
It is symbolized by k
Carrying Capacity
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As a population approaches its carrying capacity, the growth rate slows because of limiting resources
The Logistics Growth Model
Growth rate begins to slow as N approaches K
It reaches 0 when N = K
Fig. 32.2
dN/dt = rN K – NK
( )
The logistic growth equation accounts for this
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A graphical plot of N versus t (time) gives an S-shaped sigmoid growth curve
The Logistics Growth Model
History of a fur seal population on St. Paul Island, Alaska
Fig. 32.3
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32.2 The Influence ofPopulation Density
Density-independent effects
Effects that are independent of population size but still regulate growth
Most are aspects of the external environment
Weather
Droughts, storms, floods
Physical disruptions
Fire, road construction
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Effects that are dependent on population size and act to regulate growth
32.2 The Influence ofPopulation Density
Density-dependent effects
These effects have an increasing effect as population size increases Song
sparrow
Fig. 32.4
Reproductive success decreases as population size
increases
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The goal of harvesting organisms for commercial purposes is to maximize net productivity
The point of maximal sustainable yield lies partly up the sigmoid curve
32.2 The Influence ofPopulation Density
Maximizing population productivity
Fig. 32.5
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32.3 Life History Adaptations
Life history = The complete life cycle of an animal
Life histories are diverse, with different organisms having different adaptations to their environments
r-selected adaptations
Populations favor the exponential growth model
Have a high rate of increase
K-selected adaptations
Populations experience competitive logistic growth
Favor reproduction near carrying capacity
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Most natural populations exhibit a combination of the r/k adaptations
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32.4 Population Demography
Demography is the statistical study of populations
Greek demos, “people”
Greek graphos, “measurement”
It helps predict how population sizes will change in the future
Growth rate sensitive to
Age structure
Sex ratio
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Age structureCohort = A group of individuals of the same age
Has a characteristic
Birth rate or fecundity
Number of offspring born in a standard time
Death rate or mortality
Number of individuals that die in that period
The relative number of individuals in each cohort defines a population’s age structure
Sex ratioThe proportion of males and females in a population
The number of births is usually directly related to the number of females
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Survivorship curves
Type I
Mortality rises in postreproductive years
Type II
Mortality constant throughout life
Type III
Mortality low after establishment Fig. 32.7
Provide a way to express the age distribution characteristics of populations
Survivorship is the percentage of an original population that survives to a given age
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32.5 Communities
All organisms that live together in an area are called a community
The different species compete and cooperate with each other to make the community stable
A community is often identified by the presence of its dominant species
The distribution of the other organisms may differ a good deal
However, the ranges of all organisms overlap
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32.6 The Niche and Competition
A niche is the particular biological role of an organism in a community
It is a pattern of living
Competition is the struggle of two organisms to use the same resource
Interspecific competition occurs between individuals of different species
Intraspecific competition occurs between individuals of a single species
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Because of competition, organisms may not be able to occupy their fundamental (theoretical) niche
Instead, they occupy their realized (actual) niche
Fig. 32.9 Competition among two species of barnacles limits niche use
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In the 1930s, G.F. Gause studied interspecific competition among three species of Paramecium
P. aurelia; P. caudatum; P. bursaria
All three grew well alone in culture tubes
Competitive Exclusion
Fig. 32.10
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However, P. caudatum declined to extinction when grown with P. aurelia
Fig. 32.10
Gause formulated the principle of competitive exclusion
No two species with the same niche can coexist
But is one competitor always eliminated?
No, as we shall soon see!
The two shared the same realized niche and the latter was better!
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P. caudatum and P. bursaria were able to coexist
Gause’s principle of competitive exclusion can be restated
No two species can occupy the same niche indefinitely
When niches overlap, two outcomes are possible
Competitive exclusion or resource partitioning
Fig. 32.10
The two have different realized niches and thus avoid competition
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Persistent competition is rare in natural communities
Either one species drives the other to extinction
Or natural selection reduces the competition between them
Resource Partitioning
Fig. 32.11
Five species of warblers subdivided a niche to avoid direct competition with one another
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Sympatric species occupy same geographical area
Avoid competition by partitioning resources
Allopatric species do not live in the same geographical area and thus are not in competition
Sympatric species tend to exhibit greater differences than allopatric species do
Character displacement facilitates habitat partitioning and thus reduces competition
Resource Partitioning
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Fig. 32.12 Character displacement in stickleback fish
Resource Partitioning
Feeds on plankton
Feeds on both resources
Feeds on larger prey
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32.7 Coevolution and Symbiosis
Coevolution is a term that describes the long-term evolutionary adjustments of species to one another
Symbiosis is the condition in which two (or more) kinds of organisms live together in close associations
Major kinds include
Mutualism – Both participating species benefit
Parasitism – One species benefits while the other is harmed
Commensalism – One species benefits and the other neither benefits nor is harmed
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Symbiotic relationship in which both species benefit
Mutualism
Fig. 32.14
Ants and Aphids
Ants transport the aphids and protect them from predators
Aphids provide the ants with food in the form of continuously excreted “honeydew”
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Fig. 32.15
Ants and Acacias
Symbiotic relationship in which both species benefit
Mutualism
Acacias provide the ants with food in the form of Beltian bodies
Beltian body
Ants provide the acacias with organic nutrients and protect it from herbivores and shading from other plants
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Fig. 32.16a
Symbiotic relationship that is a form of predationThe predator (parasite) is much smaller than the prey
The prey does not necessarily die
Parasitism
External parasitesEctoparasites feed on the exterior surface of an organism
Dodder is a chlorophyll-less parasitic plant
Parasitoids are insects that lay eggs on living hosts
Wasps
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Cuckoo
Meadow pipit
Internal parasites
Brood parasitismBirds lay their eggs in the nests of other species
Brood parasite
Foster parent
Fig. 32.16
Sarcocystis
Endoparasites live within the bodies of vertebrates and invertebrates
Marked by much more extreme specialization than external parasites
Brood parasites reduce the reproductive success of the foster parent hosts
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Symbiotic relationship that benefits one species and neither harms nor benefits the other
Commensalism
Clownfishes and Sea anemonesClownfishes gain protection by remaining among the anemone’s tentacles
They also glean scraps from the anemone’s food
Fig. 32.17
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Fig. 32.18
Cattle egrets and African cape buffalo
Note:
No clear distinction between commensalism and mutualism
Difficult to determine if second partner benefits at all
Indeed, the relationship maybe even parasitic
Egrets eat insects off of the buffalo
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Fig. 32.20
32.8 Predator-Prey Interactions
Predation is the consuming of one organism by another, usually of a similar or larger size
Under simple laboratory conditions, the predator often exterminates its prey
It then becomes extinct itself having run out of food!
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32.8 Predator-Prey Interactions
In nature, predator and prey populations often exhibit cyclic oscillations
The North American snowshoe hare (Lepus americanus) follows a “10-year cycle”
Two factors involved
1. Food plantsWillow and birch twigs
2. PredatorsCanada lynx (Lynx canadensis)
Fig. 32.21a
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32.8 Predator-Prey Interactions
Fig. 32.21b
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32.8 Predator-Prey Interactions
Predator-prey interactions are essential in the maintenance of species-diverse communities
Predators greatly reduce competitive exclusion by reducing the individuals of competing species
For example, sea stars prevent bivalves from dominating intertidal habitats
Other organisms can share their habitat
Keystone species are species that play key roles in their communities
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32.9 Plant and Animal Defenses
Plants have evolved many mechanisms to defend themselves from herbivores
Morphological (structural) defenses
Thorns, spines and prickles
Chemical defenses
Secondary chemical compounds
Found in most algae as well
Mustard oils
Found in the mustard family (Brassicaceae)
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Mustard oils protected plants from herbivores at first
At some point, however, certain insects evolved the ability to break down mustard oil
The Evolutionary Response of Herbivores
These insects were able to use a new resource without competing with other herbivores for it
Cabbage butterfly caterpillars
Fig. 32.23
Adult
Green caterpillar
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Some animals receive an added benefit from eating plants rich in secondary chemical compounds
Caterpillars of monarch butterflies concentrate and store these compounds
Animal Defenses
They then pass them to the adult and even to eggs of next generation
Birds that eat the butterflies regurgitate them
Fig. 32.24
Blue jay
I’m not eating this again!
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Fig. 32.26
Dendrobatid frog
Defensive coloration
Cryptic colorationColor that blends with surrounding
Aposematic colorationShowy color advertising poisonous nature
Fig. 32.25
Inchworm caterpillar
Camouflage!
Warning!
Chemical defensesStings – Bees and wasps
Toxic alkaloids – Dendrobatid frogs
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32.10 Mimicry
Many non-poisonous species have evolved to resemble poisonous ones with aposematic coloration
Two types of mimicry have been identified
Batesian mimicryAfter Henry Bates, a 19th century British naturalist
Müllerian mimicryAfter Fritz Müller, a 19th century German biologist
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A harmless unprotected species (mimic) resembles a poisonous model that exhibits aposematic coloration
Batesian Mimicry
If the mimics are relatively scarce, they will be avoided by predators
Monarch butterfly
Fig. 32.27
Viceroy butterfly
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Two or more unrelated but protected (toxic) species come to resemble one another
Müllerian Mimicry
Thus a group defense is achieved
Yellow jacket
Fig. 32.28
Masarid wasp
Sand wasp Anthidiine bee
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Involves adaptations where one animal body part comes to resemble another
This type of mimicry is used by both predator and prey
Example
“Eye-spots” found in many butterflies, moths and fish
Self Mimicry
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32.11 Ecological Succession
Succession is the orderly progression of changes in community composition that occur over time
Secondary succession
Occurs in areas where an existing community has been disturbed
Primary succession
Occurs on bare lifeless substrates, like rocks
The first plants to appear from a pioneering community
The climax community comes at the end
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Three dynamic critical concepts
1. ToleranceFirst to come are weedy r-selected species that are tolerant of the harsh abiotic conditions
2. FacilitationHabitat changes are introduced that favor other, less weedy species
3. InhibitionHabitat changes may inhibit the growth of the species that caused them
Why Succession Happens
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As ecosystems mature, more K-selected species replace r-selected ones
Species richness and total biomass increase
However, net productivity decreases
Thus, agricultural systems are maintained in early successional stages to keep net productivity high
Why Succession Happens
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32.12 Biodiversity
Biologically diverse ecosystems are in general more stable than simple ones
Species richness refers to the number of species in an ecosystem
It is the quantity usually measured by biologists to characterize an ecosystem’s biodiversity
Two factors are important in promoting biodiversity
Ecosystem size
Latitude
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Larger ecosystems contain more diverse habitats and therefore have greater number of species
A reduction in an ecosystem size, will reduce the number of species it can support
Faunal collapse (extinction) may occur in extreme cases
Ecosystem Size
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The number of species in the tropics is far more than that in the arctic region
Latitude
Fig. 32.32
Two principal reasons
1. Length of growing season
2. Climatic stability
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Island Biodiversity
In 1967, Robert MacArthur and Edward O. Wilson proposed the equilibrium model
The species richness on islands is a dynamic equilibrium between colonization and extinction
Two important factors
Island sizeLarger islands have more species than smaller ones
Distance from mainlandDistant islands have less species than those near the mainland
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Fig. 32.33 The equilibrium model of island biogeography
Equilibrium
Shifting equilibrium
Small distant islands have fewer
bird species
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