Download The Living World

The Living World
Fourth Edition
GEORGE B. JOHNSON
32
Populations and
Communities
PowerPoint® Lectures prepared by Johnny El-Rady
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
32.1 Population Growth
A population is a group of individuals of the same
species living together
Critical properties of a population include
Population size
The number of individuals in a population
Population density
Population size per unit area
Population dispersion
Scatter of individuals within a population’s range
Population growth
How populations grow and the factors affecting growth
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
The Exponential Growth Model
Assumes a population is growing without limits at its
maximal rate
Rate is symbolized r and called the biotic potential
Change
over time
Intrinsic rate
of increase
Growth rate = dN/dt = riN
No. of individuals
in a population
The actual rate of population increase is
Birthrate
Deathrate
Net immigration
r = (b – d) + (i – e)
Net emigration
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Carrying Capacity
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
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
The Logistics Growth Model
As a population approaches its carrying capacity, the
growth rate slows because of limiting resources
The logistic growth equation accounts for this
dN/dt = rN ( K – N
K
Growth rate begins
to slow as N
approaches K
It reaches 0
when N = K
)
Fig. 32.2
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
The Logistics Growth Model
A graphical plot of N versus t (time) gives an
S-shaped sigmoid growth curve
History of a fur
seal population
on St. Paul
Island, Alaska
Fig. 32.3
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
32.2 The Influence of
Population 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
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
32.2 The Influence of
Population Density
Density-dependent effects
Effects that are dependent on population size and
act to regulate growth
Reproductive
success decreases
as population size
increases
These effects
have an
increasing
effect as
population
size increases
Song
sparrow
Fig. 32.4
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
32.2 The Influence of
Population Density
Maximizing population productivity
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
Fig. 32.5
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Most natural
populations
exhibit a
combination
of the r/k
adaptations
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
32.4 Population Demography
Greek demos, “people”
Demography is the statistical study of populations
Greek graphos, “measurement”
It helps predict how population sizes will change
in the future
Growth rate sensitive to
Age structure
Sex ratio
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Age structure
Cohort = 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 ratio
The proportion of males and females in a population
The number of births is usually directly related to the
number of females
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Survivorship curves
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
Type I
Mortality rises in
postreproductive years
Type II
Mortality constant
throughout life
Type III
Mortality low after
establishment
Fig. 32.7
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Competitive Exclusion
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
Fig. 32.10
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
However, P. caudatum
declined to extinction when
grown with P. aurelia
The two shared the
same realized niche and
the latter was better!
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!
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
P. caudatum and P. bursaria
were able to coexist
The two have different
realized niches and thus
avoid competition
Fig. 32.10
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
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Resource Partitioning
Persistent competition
is rare in natural
communities
Either one species drives
the other to extinction
Or natural selection
reduces the competition
between them
Five species of warblers
subdivided a niche to
avoid direct competition
with one another
Fig. 32.11
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Resource Partitioning
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
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Resource Partitioning
Feeds on both
resources
Feeds on
plankton
Feeds on
larger prey
Fig. 32.12 Character displacement in stickleback fish
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Mutualism
Symbiotic relationship in which both species benefit
Ants and Aphids
Aphids provide the
ants with food in the
form of continuously
excreted “honeydew”
Fig. 32.14
Ants transport the aphids and protect them from
predators
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Mutualism
Symbiotic relationship in which both species benefit
Beltian body
Ants and Acacias
Acacias provide the ants
with food in the form of
Beltian bodies
Ants provide the acacias
with organic nutrients
and protect it from
herbivores and shading
from other plants
Fig. 32.15
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Parasitism
Symbiotic relationship that is a form of predation
The predator (parasite) is much smaller than the prey
The prey does not necessarily die
External parasites
Ectoparasites feed on the
exterior surface of an
organism
Parasitoids are insects
that lay eggs on living
hosts
Wasps
Fig. 32.16a
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Dodder is a
chlorophyll-less
parasitic plant
Internal parasites
Endoparasites live within
the bodies of vertebrates
and invertebrates
Marked by much more
extreme specialization
than external parasites
Brood parasitism
Birds lay their eggs in the
nests of other species
Brood parasites reduce
the reproductive
success of the foster
parent hosts
Sarcocystis
Fig. 32.16
Meadow pipit
Foster
parent
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Brood
parasite
Cuckoo
Commensalism
Symbiotic relationship that benefits one species and
neither harms nor benefits the other
Clownfishes and Sea
anemones
Clownfishes gain
protection by
remaining among the
anemone’s tentacles
They also glean
scraps from the
anemone’s food
Fig. 32.17
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Cattle egrets and
African cape buffalo
Egrets eat insects off
of the buffalo
Fig. 32.18
Note:
No clear distinction between commensalism and mutualism
Difficult to determine if second partner benefits at all
Indeed, the relationship maybe even parasitic
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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!
Fig. 32.20
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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 plants
Willow and birch
twigs
2. Predators
Canada lynx (Lynx
Fig. 32.21a
canadensis)
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
32.8 Predator-Prey Interactions
Fig. 32.21b
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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)
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
The Evolutionary Response of Herbivores
Mustard oils protected plants from herbivores at first
At some point, however, certain insects evolved
the ability to break down mustard oil
These insects were
able to use a new
resource without
competing with other
herbivores for it
Cabbage butterfly
caterpillars
Adult
Green caterpillar
Fig. 32.23
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Animal Defenses
Some animals receive an added benefit from eating
plants rich in secondary chemical compounds
Caterpillars of monarch butterflies concentrate and store
these compounds
They then pass
them to the adult
and even to eggs
of next generation
Birds that eat the
butterflies
regurgitate them
Blue jay
Fig. 32.24
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
I’m not eating
this again!
Defensive coloration
Cryptic coloration
Color that blends
with surrounding
Aposematic coloration
Showy color advertising
poisonous nature
Inchworm caterpillar
Camouflage!
Warning!
Fig. 32.25
Chemical defenses
Dendrobatid frog
Stings – Bees and wasps
Toxic alkaloids – Dendrobatid frogs
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Fig. 32.26
32.10 Mimicry
Many non-poisonous species have evolved to
resemble poisonous ones with aposematic coloration
Two types of mimicry have been identified
Batesian mimicry
After Henry Bates, a 19th century British naturalist
Müllerian mimicry
After Fritz Müller, a 19th century German biologist
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Batesian Mimicry
A harmless unprotected species (mimic) resembles a
poisonous model that exhibits aposematic coloration
Monarch butterfly
Fig. 32.27
If the mimics
are relatively
scarce, they
will be
avoided by
predators
Viceroy butterfly
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Müllerian Mimicry
Two or more unrelated but protected (toxic) species
come to resemble one another
Yellow jacket
Thus a group
defense is
achieved
Masarid wasp
Fig. 32.28
Sand wasp
Anthidiine bee
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Self Mimicry
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
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Why Succession Happens
Three dynamic critical concepts
1. Tolerance
First to come are weedy r-selected species that are
tolerant of the harsh abiotic conditions
2. Facilitation
Habitat changes are introduced that favor other, less
weedy species
3. Inhibition
Habitat changes may inhibit the growth of the
species that caused them
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Why Succession Happens
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
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
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
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Ecosystem Size
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
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Latitude
The number of
species in the tropics
is far more than that
in the arctic region
Two principal reasons
1. Length of
growing season
2. Climatic stability
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Fig. 32.32
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 size
Larger islands have more species than
smaller ones
Distance from mainland
Distant islands have less species than those
near the mainland
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
Fig. 32.33 The equilibrium model of island biogeography
Equilibrium
Shifting
equilibrium
Small distant
islands have fewer
bird species
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display