Download Ch. 38

The Living World
Fifth Edition
George B. Johnson
Jonathan B. Losos
Chapter 38
Populations and Communities
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
38.1 Population Growth
• A population is a group of individuals of a species that
live together and influence each other’s survival
• Populations have several properties that can describe
them
 population size is the number of individuals in the population
 population density is the population size that occurs in a given
area
 population dispersion is the scatter of individuals within the
population’s range
Figure 38.1 Population dispersion
38.1 Population Growth
• Another characteristic about any
population is its capacity to grow
• Population growth can be modeled in
different ways that identify what factors in
nature limit growth
38.1 Population Growth
• Biotic potential, symbolized by r, is the
rate at which a population of a given
species will increase when no limits are
placed on its rate of growth
• The simplest model of population growth
assumes a population growing without
limits at its maximal rate
38.1 Population Growth
• The exponential growth model is defined by the
following formula
growth rate = dN/dT = riN
N is the population size
dN/dT is the rate of change over time
ri is the intrinsic rate of natural increase
38.1 Population Growth
• The actual rate of population increase, r, is
defined as
r = (b – d) + (i – e)
• b is the birthrate, d is the death rate
• e is the amount of emigration out of the
area and i is the amount of immigration
into the area
38.1 Population Growth
• The innate capacity for growth of any
population is exponential
 even when the rate of increase remains
constant, the actual increase in the number of
individuals accelerates rapidly as the
population grows
 in practice, such patterns prevail for only short
periods, usually when an organism reaches a
new habitat with abundant resources
38.1 Population Growth
• No matter how rapidly populations grow, they
eventually reach a limit imposed by shortages of
important environmental factors
• A population ultimately stabilizes at a certain
size, called the carrying capacity
 the carrying capacity is symbolized by K and is
defined as the maximum number of individuals that
an area can support
38.1 Population Growth
• The growth curve of a population that is
approaching its carrying capacity can be
approximated by the logistic growth equation
dN/dt = rN (1 – N/K)
• As N approaches K, the rate of population
growth (dN/dt) begins to slow, until it reaches
zero at N = K
Figure 38.2 Two models of
population growth
38.1 Population Growth
• The sigmoid growth
curve is characteristic of
most biological
populations
• The processes of
competition and
emigration tend to
increase as a population
approaches its carrying
capacity
Figure 38.3 Most natural
populations exhibit logistic growth
38.2 The Influence of Population
Density
• Many factors act to regulate the growth of
populations in nature
 density-independent effects
• these effects regulate population growth regardless of
population size
• for example, weather effects or geological events (i.e.,
volcanoes)
 density-dependent effects
• the effect that these factors have on population growth
depends on population size
• these effects grow stronger as the population size increases
Figure 38.4 Density-dependent
effects
38.2 The Influence of Population
Density
• In natural systems that are exploited by
humans, the aim is to maximize
productivity by exploiting the population
early in the rising portion of its sigmoid
growth curve
 commercial fisheries, for example, attempt to
operate so that they harvest a population near
its point of maximal sustainable yield
Figure 38.5 Maximal sustainable
yield
38.3 Life History Adaptations
• Life history describes the complete life cycle of
an organism
 r-selected adaptations
• these favor rapid growth in a habitat with unlimited resources
or in unpredictable environments
• or, in unpredictable environments, organisms have to take
advantage of resources when they are available
 K-selected adaptations
• favor reproduction near the carrying capacity of the
environment
• help survival in an environment in which individuals are
competing for limited resources
Table 38.1 r-selected and K-selected
life history adaptations
38.3 Life History Adaptations
• The r/K concept of life histories provides a
way to examine more closely related
organisms living in different types of
habitats
 populations living in rapidly changing
environments tend to exhibit r-selected
adaptations
 populations living in more stable and
competitive habitats tend to exhibit more Kselected adaptations
Figure 38.6 The consequences of
exponential growth
38.4 Population Demography
• demography is the statistical study of
populations
 measures characteristics of populations and
helps predict how population sizes will change
in the future
• populations grow if births outnumber deaths and
shrink if deaths outnumber births
• birth and death rates are dependent on age and
sex
38.4 Population Demography
• A cohort is a group of individuals of the
same age
 within a population, every cohort has the
following characteristics
• fecundity, or birthrate, which is defined as the
number of offspring produced in a standard time
• mortality, or deathrate, which is the number of
individuals that die in that period
 the relative number of individual’s in each
cohort defines a populations age structure
38.4 Population Demography
• Sex ratio is the proportion of males and fames
in a population
 the number of births is usually directly related to the
number of females
• Age distribution is the proportion of individuals
in different age categories
 when a population lives in a constant environment for
a few environments, its population size remains fairly
constant and is said to be a stable population
38.4 Population Demography
• A survivorship curve is one way to express the
age distribution characteristics of a population
 survivorship is defined as the percentage of an
original population that survives to a given age
 there are three types of survivorship curves
• type I has highest mortality for the youngest individuals
• type II has relatively the same mortality risk for all ages
• type III has highest mortality for the oldest individuals
Figure 38.7 Survivorship curves
38.5 Communities
• Community refers to the species that occur at
any given locality
 communities can be characterized by either their
constituent species (a list of all species present in the
community) or by their properties, such as primary
productivity
 interactions among community members govern
many ecological and evolutionary processes
• for example, predation, competition, and mutualism affect the
population biology of a particular species, as well as the way
in which energy and nutrients cycle through the ecosystem
Figure 38.8 A Tanzanian savanna
community
38.5 Communities
• Two views exist on the makeup and
functioning of communities
 individualistic concept
• advanced by H. A. Gleason, this concept holds that
a community is nothing more than an aggregation
of species that happen to co-occur at one place
 holistic concept
• views communities as an integrated unit and this
concept was first proposed by F. E. Clements
• the community is viewed as a “superorganism”
38.6 The Niche and Competition
• The niche an organism occupies is the sum total of all the ways it
utilizes the resources of its environment
 sometimes species are not able to occupy their entire niche
because of the presence or absence of other species
 competition describes the interaction when two organisms
attempt to use the same resource when there is not enough of
the resource to satisfy both
– interspecific competition occurs between individuals of
different species
– intraspecific competition occurs between individuals of the
same species
38.6 The Niche and Competition
• Fundamental niche
is the entire niche that
an organism may
theoretically occupy
• Realized niche is the
actual niche that the
organism is able to
occupy because of
competition
Figure 38.9 Competition among two
species of barnacles limits niche use
38.6 The Niche and Competition
• G.F. Gause demonstrated the principle of
competitive exclusion
 if two species are competing for a resource,
the species that uses the resource more
efficiently will eventually eliminate the other
locally
 in other words, no two species with the same
niche can coexist
Figure 38.10 (a) Competitive exclusion
among three species of Paramecium
Figure 38.10 (b) Competitive exclusion
among three species of Paramecium
Figure 38.10 (c) Competitive exclusion
among three species of Paramecium
38.6 The Niche and Competition
• Species in communities act to avoid competition
whenever possible
 when their niches overlap, two outcomes are possible
• competitive exclusion (i.e., winner takes all)
• resource partitioning, which divides up resources to create
two niches
 thus, persistent competition between two species is
rare in natural communities
 either one species drives the other to extinction or
natural selection reduces the competition between the
them
Figure 38.11 Resource partitioning
in warblers
38.6 The Niche and Competition
• Resource partitioning can often be seen in
similar species that occupy the same
geographical area
 such species are sympatric
 when a pair of species occupy the same habitat (i.e.,
when they are sympatric), they tend to exhibit greater
differences in morphology and behavior then the
same two species do when living in different habitats
(i.e., when they are allopatric)
• the evident differences are called character displacement
and are favored by natural selection to facilitate habitat
partitioning and reduce competition
38.7 Coevolution and Symbiosis
• Coevolution is the adaptation of a
species not only to its physical
environment but also to the other
organisms that share it
 examples of coevolution include
• plants and animal pollinators
• predator-prey interactions
• symbiotic relationships
Figure 38.13 Pollination by bat
38.7 Coevolution and Symbiosis
• In a symbiotic relationship, two or more
kinds of organisms live together in often
elaborate or more of less permanent
relationships
 there are three major kinds of symbiotic
relationships
• mutualism
• parasitism
• commensalism
38.7 Coevolution and Symbiosis
• Mutualism is a
symbiotic relationship
in which both species
benefit
Figure 38.14 Mutualism: ants and aphids
Figure 38.15 Mutualism: ants and acacias
38.7 Coevolution and Symbiosis
• Parasitism is a symbiotic relationship in
which one species benefits while the other
is harmed
 this interaction is really a form of
predator/prey relationship but a parasite
usually does not kill its host
 the parasite is much smaller than the host and
remains closely associated with it
38.7 Coevolution and Symbiosis
• There are many forms of parasitism in nature
 external parasites
• also known as ectoparasites, these parasites feed on the
exterior surface of a host
• parasitoids are insects that lay eggs on living hosts
 internal parasites
• also known as endoparasites, these parasites feed
internally in their hosts
 brood parasitism is a form of parasitism in which the
parasite does not consume the body of its host
• brood parasites are birds, such as cowbirds and cuckoos,
that lay their eggs in the nest of other species for the host to
raise
Figure 38.16 Parasitism
38.7 Coevolution and Symbiosis
• Commensalism is a symbiotic
relationship that benefits one species but
neither hurts nor helps the other
 there is no clear-cut boundary between
commensalism and mutualism
Examples of Commensalism
Figure 38.17 Commensalism in the
sea
Figure 38.18 Commensalism between
oxpeckers and African cape buffalo
38.8 Predator-Prey Interactions
• Predation is the consuming of one organism by
another
 in nature, predators often have large effects on prey
populations
• population cycles may be, in some situations, stimulated by
predators
• a classic example is the “10-year cycle” of the snowshoe
hare, Lepus americanus, that appears to be under the
influence of food plants and predators
 under laboratory conditions, predators may exhaust
their prey species and then starve
Figure 38.21 A predator-prey cycle
Figure 38.20 Predator-prey in the
microscopic world
38.8 Predator-Prey Interactions
• Predator-prey interactions are an essential
factor in the maintenance of communities
that are rich and diverse in species
 predators prevent or greatly reduce
competitive exclusion by reducing the number
of individuals of competing species
 predators that reduce competition and
increase community diversity are known as
keystone species
Figure 38.22 Predation reduces
competition
38.9 Plant and Animal Defenses
• Pressures from predation have driven the
evolution of mechanisms that defend organisms
from predation
 plants produce chemical defenses that make them
toxic to herbivores
• some herbivores have, as a result, evolved a tolerance to
these chemicals and may use them for their own defense
 many animals have defensive coloration
• aposematic coloration is a warning coloration that is
characteristic of animals that use poisons
• cryptic coloration is color that blends in with surroundings
Figure 38.23 Insect herbivores are
well suited to their hosts
Figure 38.24 A blue jay learns that
monarch butterflies taste bad
Defensive Coloration
Figure 38.25 Cryptic coloration
Figure 38.26 Vertebrate chemical
defense
38.9 Plant and Animal Defenses
• Predation can exert strong selective
pressures on prey populations
 a coevolutionary arms race between
predators and prey is likely because
• any feature that acts to decrease the probability of
capture should thus be strongly favored by
selection in prey
• natural selection would also favor counteradaptations in predators
38.10 Mimicry
• Batesian mimicry is when a palatable species
resembles a poisonous one
 there may also be nonvisual cues, such as olfaction,
involved
• Müllerian mimicry is when several unrelated,
but protected, species come to resemble one
another
 for example, the colors black, yellow, and red are
used often in aposematic coloration
Figure 38.27 A Batesian mimic
Figure 38.28 Müllerian mimics
38.10 Mimicry
• Self mimicry is a special case of mimicry
in which one animal body part comes to
resemble another body part
 this occurs in both prey and predators
• prey might use this form of mimicry to startle a
predator or to provide a false target for attack
• predators might use this mimicry to simulate bait to
lure prey in
Figure 38.29 Self mimicry
38.11 Ecological Succession
• Succession is the orderly replacement of
one community with another
 primary succession
• occurs on bare, lifeless substrates, such as those
left behind when a glacier retreats or when a
volcanic island emerges
• pioneering community is the first to become
established
 secondary succession
• occurs after an already established community has
been disturbed
Figure 38.30 Plant succession produces
progressive changes in the soil
38.11 Ecological Succession
• Succession happens because species alter the habitat
and the resources available in it, often in ways that favor
other species
• Three dynamic concepts are of critical importance
 tolerance
• early succession stages are characterized by weedy r-selected
species that tolerate harsh conditions but do not compete well
 facilitation
• the weedy species introduce local changes in the habitat that favor
non-weedy species
 Inhibition
• sometimes the changes in habitat caused by one species may
inhibit the growth of the species that caused them
Figure 38.31 Primary succession at
Alaska’s Glacier Bay
Inquiry & Analysis
• Are the populations with
lower juvenile mortality
bigger or smaller than the
populations with higher
juvenile mortality?
• Do the population sizes of
these song sparrows
appear to exhibit density
dependence?
Graph of Effects of Population
Size on Songbird Success