Lecture 27 Populations, Communities,

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Lecture 27Populations, Communities, Ecosystems
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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
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5 Levels of Ecological Organization

• Populations
• Individuals of the same species living together
• Communities
• Populations of different species living together
• Ecosystems
• Combination of communities and associated
  non-living factors
• Biomes
• Major terrestrial assemblages of organisms that
  occur over wide geographical areas
• The Biosphere
• All biomes together with marine and freshwater
  assemblages

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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 populations
  range
• Population growth
• How populations grow and the factors affecting
  growth

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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

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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
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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

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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

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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

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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

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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 populations 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

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Population Pyramids

• A populations age structure and sex ratio can be
  used to assess its demographic trends

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Human Populations

• Throughout most of our history, human populations
  have been regulated by
• Food availability
• Disease
• Predators
• Two thousand years ago, the human population was
  130 million
• It took one thousand years for it to double
• And another 650 years for it to double again
• Starting in the 1700s, technological changes gave
  humans more control over their environment
• 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

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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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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

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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

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Resource Partitioning

• Gause formulated the principle of competitive
  exclusion
• No two species with the same niche can coexist
• Gauses 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

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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

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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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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

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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

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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 anemones tentacles
• They also glean scraps from the anemones 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

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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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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

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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

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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

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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

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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

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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

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Nutrients Chemicals Cycle

• Raw materials are not used up when organisms die
• They are recycled back into the ecosystem for use
  by other organisms

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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

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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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The Process of Succession

• Succession involves three dynamic critical
  concepts
• Tolerance
• First to come are weedy r-selected species that
  are tolerant of the harsh abiotic conditions
• Facilitation
• Habitat changes are introduced that favor other,
  less weedy species
• 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

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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 ecosystems 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

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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 size
• Larger islands have more species than smaller
  ones
• Distance from mainland
• Distant islands have less species than those near
  the mainland