Lecture 27
Populations, Communities, & Ecosystems
What is Ecology?
 Evolution and ecology are two key concepts
 Evolution: Changes that occur in organisms’ traits over time
 Ecology: How organisms live in their environment
 The great diversity of life on earth is the result of evolution
 And evolution can be said to be the consequence of ecology
over time
 The term was coined by Ernst Haeckel (1866)
Thus, ecology is the study of how organisms interact with their
environment
5 Levels of Ecological Organization
1. Populations

Individuals of the same species living together
2. Communities

Populations of different species living together
3. Ecosystems

Combination of communities and associated non-living factors
4. Biomes

Major terrestrial assemblages of organisms that occur over wide
geographical areas
5. The Biosphere

All biomes together with marine and freshwater assemblages
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
The Exponential Growth Model
 Assumes a population is growing without limits at its maximal
rate
 Rate is symbolized by r and called the biotic potential
 The actual rate of population increase is
The Logistics Growth Model
 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
 As a population approaches its carrying
capacity, the growth rate slows because of
limiting resources
 The logistic growth equation accounts for
this
 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
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
 Density-dependent effects
 Effects that are dependent on
population size and act to regulate
growth
 These effects have an increasing
effect as population size
increases
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
or optimal yield lies partly up the
sigmoid curve
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
 Followed by rapid decrease
 K-selected adaptations
 Populations experience
competitive logistic growth
 Favor reproduction near
carrying capacity
 Most natural populations exhibit a combination of the r/k adaptations
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
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
Age structure
 Cohort = A group of individuals of the same age with 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
Population Pyramids
 A population’s age structure and sex ratio can be used to
assess its demographic trends
Human Populations

Throughout most of our history, human populations have been regulated by

Two thousand years ago, the human population was ~ 130 million

Starting in the 1700s, technological changes gave humans more control
over their environment
 Food availability
 Disease
 Predators
 It took one thousand years for it to double
 And another 650 years for it to double again
 These changes allowed humans to expand the carrying capacity of their habitats


Currently, the human population is growing at a rate of ~ 1.3% annually
 Doubling time at this rate is only 54 years!
Human population growth is not uniform
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
The Niche and Competition
 A niche is the particular biological
role of an organism in a community
 Habitat  place
 Niche  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
 Because of competition, organisms may not be able to occupy their
fundamental (theoretical) niche
 Instead, they occupy their realized (actual) niche
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
 However, P. caudatum declined to extinction
when grown with P. aurelia
 The two shared the same realized niche and the
latter was better!
 P. caudatum and P. bursaria were able to coexist
 The two have different realized niches and thus
avoid competition
Resource Partitioning
 Gause formulated the principle of
competitive exclusion
 No two species with the same
niche can 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
 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
 Sympatric species occupy
same geographical area
 Avoid competition by
partitioning resources
 Sympatric species tend to
exhibit greater differences
than allopatric species do
 Character displacement
facilitates habitat
partitioning and thus
reduces competition
 Allopatric species do not live in the same geographical area and thus
are not in competition
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
Mutualism
 Symbiotic relationship in which both species benefit
 Ants and Aphids
 Aphids provide the ants with food
in the form of continuously
excreted “honeydew”
 Ants transport the aphids and
protect them from predators
 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
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 (wasps) that lay eggs on
living hosts
 Endoparasites live within the bodies of
vertebrates and invertebrates
 Marked by much more extreme specialization than
external parasites
 Brood parasites (birds) lay their eggs in the nests
of other species
 Brood parasites reduce the reproductive success
of the foster parent hosts
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
 Cattle egrets and African cape buffalo
 Egrets eat insects off of the buffalo
 Note: There is no clear distinction
between commensalism and mutualism
 Difficult to determine if second partner
benefits at all
 Indeed, the relationship maybe even
parasitic
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!
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
 Food plants: Willow and birch twigs
 Predators: Canada lynx (Lynx
canadensis)


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
Plant 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)
 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
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
 Cryptic coloration: Color that blends
with surrounding
 Aposematic coloration: Showy color
advertising poisonous nature
 Chemical defenses
 Stings – Bees and wasps
 Toxic alkaloids – Dendrobatid frogs
Mimicry
 Many non-poisonous species have evolved to resemble poisonous
ones with aposematic coloration
 Batesian mimicry
 A harmless unprotected species
(mimic) resembles a poisonous
model that exhibits aposematic
coloration
 If the mimics are relatively scarce,
they will be avoided by predators
 Müllerian mimicry
 Two or more unrelated but protected
(toxic) species come to resemble one
another
 Thus a group defense is achieved
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
A Closer Look at Ecosystems
 Ecosystems: the fundamental units of
ecology
 All organisms in an ecosystem require
energy
 Almost all energy comes from the sun
 Energy flows
 Energy is lost at each step of the
food chain
 This limits the number of steps
Nutrients & Chemicals Cycle
 Raw materials are not used up
when organisms die
 They are recycled back into the
ecosystem for use by other
organisms
Biomes
 Rainfall and temperature are
the two most important
factors limiting species
distribution
 These physical conditions
with their sets of similar
plants and animals are called
biomes
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
The Process of Succession

Succession involves 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

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

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
 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

Latitude
 The number of species in the
tropics is far more than that in
the arctic region
 Two principal reasons
 Length of growing season
 Climatic stability
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