chapter 1: introduction to the science of...
TRANSCRIPT
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Just what is ecology, anyway?
• Root from Greek “oikos”
• First used as a word by Henry David
Thoreau in 1858
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Just what is ecology, anyway?
• Definitions according to
– Ernst Haekel: “total relations of an animal to
both it organic and inorganic environment”
– Charles Elton: “scientific
natural history”
– H.G. Andrewartha: “scientific study of the
distribution and abundance of organisms”
– Eugene Odum: “structure and function of
nature”
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Just what is ecology, anyway?
• Definitions according to
– Charles Krebs: “scientific study of the
interactions that determine the distribution
and abundance of organisms”
• where organisms are found
• how many occur there
• why they live there
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History of ecology
• Hunter-gatherers
• Agricultural age
• Egyptians through Aristotle
– fear of plagues
– explanations relating to ecology
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History of ecology
• Advances beginning in the 17th century
– John Graunt (c. 1662)
• father of demography
• described human populations in
quantitative terms
– Antony van Leewenhoek (c. 1687)
• reproductive rates of grain beetles, carrion flies
(1 pair >740,000 in 3 months), human lice
• first attempts to calculate theoretical rates of
increase for animal species
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History of ecology
• Advances after the 17th century
– Buffon (c. 1756)
• Georges-Louis Leclerc, Comte de Buffon
• author of Natural History
• humans, plants and animals all regulated by the
same processes
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History of ecology
• Advances after the 17th century
– Thomas Malthus (c. 1798)
• author of Essay on Population
• numbers of organisms increase geometrically, but
their relative food supply may never increase more
than arithmetically
Generation
_0 1 2 3 4__
Population: 2 4 8 16 32
Food supply: 2 4 6 8 10
• concluded that food supply limits population size
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History of ecology
• Advances after the 17th century
– Adolphe Quetelet (c. 1835)
• potential ability of a population to
grow geometrically is balanced by a
resistance to growth
– Pierre-François Verhulst (c. 1838)
• derived equation describing growth of a
single population over time: logistic
• his work was overlooked for 100 years
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History of ecology
• Advances in the 18th and 19th centuries
– balance of nature versus natural selection
– recognition of interrelations of organisms
within communities
• Edward Forbes (c. 1844)
– communities in British coastal waters and Mediterranean
– zones of different depths with different communities
• Stephen Forbes (c. 1887)
– author of The Lake as a Microcosm
– affecting one species in a community can influence the
whole community
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Ecology versus environmental science
• Ecology: focuses on the natural world of
interactions of plants and animals
(including humans) with their natural
environments
• Environmental science: focuses on
human impacts on the Earth’s
environments (physical, chemical,
biological)
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Approaches to studying ecology
Descriptive ecology (what?)
Functional ecology (how?)
Evolutionary ecology (why?)
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• Species
– organism level of integration
– fundamental unit in ecology
– three-part definition
• group of actually or potentially interbreeding
organisms,
• that are reproductively isolated from other kinds
of organisms, and
• that produce viable offspring
Levels of integration in biology
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• Population
– group of organisms of the same species
occupying a given area at a given time
– unique property = density
– examples:
• humans
• bluebirds
• fox squirrels
Levels of integration in biology
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• Community
– group of interacting populations occupying a
given area
– unique property = species diversity
– examples:
• bottomland hardwood plant community
• coastal prairie plant community
• bay bottom benthic community
Levels of integration in biology
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• Ecosystem
– group of interacting communities, linked by
lines of energy transfer
– abiotic features
– biotic features
– examples:
• Galveston Bay
• East Texas piney woods
• pond
Levels of integration in biology
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Methods used to study ecology
• Theoretical (mathematical)
• Laboratory
• Field
• Plant ecology versus animal ecology
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Ecology and evolution
• Evolution (in ecological terms)
– change in traits of a population over time
– involves changes in the frequency of
individual genes in a population from one
generation to the next
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What mechanism drives evolution?
• Neutralists
– evolution occurs by chance (genetic drift)
– genetic mutations occur 1/1,000,000 DNA
replications
– some get fixed in the population change
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What mechanism drives evolution?
• Selectionists
– evolution is driven by adaptation and natural
selection (e.g., Darwin, Wallace in the 1850s)
– parameters
• genetic variation
• excess offspring
• competition
• fitness
• heritability of traits
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How natural selection works
Theory Example
The reproductive potential of
populations is great, but
Rabbits should be able to cover
the whole earth, but
populations tend to remain
constant in size, because
they don’t, because
populations suffer high mortality. many are caught by predators.
Individuals vary within
populations, leading to
Some rabbits run faster than
others
differential survival of individuals. and escape from predators.
Traits of individuals are inherited
by their offspring.
So do their offspring.
The composition of the population
changes by the elimination of unfit
individuals.
Populations of rabbits, as a
whole, tend to run raster than
their predecessors.
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Why natural selection occurs
• Genetic variability in individuals
• Heritability of genetic traits
• Influence of the environment on survival
and reproduction
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Processes of evolution
• Adaptation
– natural selection acts on phenotypes to cause
change in the genetic make-up of a
population over time
• Speciation
– members of a population become
reproductively isolated from each other,
leading in time to separate species
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Natural selection by adaptation
• Natural selection operates on phenotypes
• Changes in gene frequencies occur only
when there is a correlation between
phenotype (fitness) and genotype
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Fig. 2.3 (p. 21): Directional selection for beak size in the
Galapagos ground finch, Geospiza forza.
Fig. 2.6 (p. 23):
Disruptive selection
in the three-spine
stickleback in British
Columbia, (a)
smaller, limnetic
form, (b) larger,
benthic form.
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Natural selection by adaptation
• Determination of clutch size in birds
– David Lack (1947)
– balance between
• proximate factors (physiology of the birds that
control ovulation and egg-laying)
• ultimate factors (how many young can
successfully fledge, determined by genetic,
population and environmental factors)
Fig. 2.8, p. 25):
Production of
young blue tits
in relation to
clutch size,
Wytham Woods,
Oxford, England
– survival of
young birds in
manipulated
nests is poor.
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Fig. 2.9 (p. 25): Number of house wren chicks fledged
from manipulated broods – disagreement with Lack’s
hypothesis.
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• Industrial melanism in the common
peppered moth (Biston betularia)
Natural selection by adaptation
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• Early 1800s: mostly light form, few melanics (carbonaria) in England
• Over next 100 years, melanics increased in abundance
• In some industrialized areas, found only melanics
• Light or dark forms follow simple Mendelian genetics
• 1950s: HBD Kettlewell’s mark-recapture and ecology studies
Natural selection in B. betularia
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B. betularia mark-recapture study
Number of
moths
Industrial site Non-industrial
site
Light
form
Dark
form
Light
form
Dark
form
Marked and
released201 601 496 473
Recaptured 34 205 62 30
Percent
recaptured16 34 12 6
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• What is the specific agent of selection for fitness in the dark form moths?– industrial site: tree trunks
darker from soot, dark form camouflaged
– non-industrial site: dark forms stand out on lighter trunks
Natural selection in B. betularia
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Number of B. betularia taken by birds
Light
form
Dark
form
From trees at non-
industrial site 26 164
From trees at
industrial site 43 15
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Natural selection within a species
• Favors those individuals who can produce the most offspring
– number of offspring per mating• at 2 per mating: 24 8 16 32
• at 3 per mating: 3 9 27 81 243
– number of times individual can reproduce over its lifetime
• 1 time @ 2 young = 2
• 3 times @ 2 young = 6
• 10 times @ 2 young = 20
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Natural selection within a species
• Favors those who survive the given
environmental conditions (fitness)
• Offset by parental investment: parental
care decreases as number of offspring
increases
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Coevolution
• Specific trait of
Species A evolves in
response to a specific
trait of Species B,
which in turn evolves
in response to the trait
of Species A
SPECIES A
SPECIES B
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Coevolution
• Ehrlich and Raven from
studies on plants and
insects that eat them
• Specific and reciprocal
• Diffuse coevolution: >2
species involved
SPECIES A
SPECIES B
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Examples of coevolution
• Oryctolagus rabbits and Myxoma virus
• Tropical ant-plant relationships: melastomes and
Inga
• Cecropia trees and Azteca ants
• Oropendulas and wasps
• Amazon water lily (Victoria amazonica) and
scarab beetles (Cyclocephala sp)
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Myxoma virus and Oryctolagus
rabbits in Australia
• Rabbits introduced to Australia in late 1800s
• Rapidly increased in abundance severe problem, destroyed rangelands used for sheep
• Myxoma common parasite of new world rabbits
– causes smallpox
– in balance in S. American rabbit populations
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• Myxoma introduced as a potent killer of Australian
rabbits
– initially killed 99.8% of the rabbit population
– within three generations, virus 40-60% effective
– rabbit populations increased and reached an
equilibrium with the virus
Myxoma virus and Oryctolagus
rabbits in Australia
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Fig. 13.16a (p. 248): Population crash of European
rabbits in Australia following introduction of Myxoma
virus in 1951.
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Fig. 13.16b (p. 248): Decline in mortality rates of
European rabbits in Australia as a function of time after
introduction of Myxoma virus.
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Fig. 13.16c (p. 248): Effect of vaccination on numbers of
adult rabbits in fenced areas in SE Australia.
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• Apparent coevolution between rabbits and Myxoma virus
– rabbits that were fit (survived and reproduced) were resistant to lethal effects of virus and passed that trait to young
– simultaneously, Myxoma virus increased its fitness by becoming less virulent
• virus transmission requires mosquito vector
• best strategy is intermediate virulence: not kill host before mosquito can transmit
Myxoma virus and Oryctolagus
rabbits in Australia
Fig. 2.11 (p. 27): Evolution and “arms race” between
parasitic cowbird and parasitized species that try to
defend their nests by ejecting cowbird chicks.
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Fig. 2.12 (p. 28): Evolution and “arms races” between
rough-skinned newt (Taricha granulosa) from western
N. America and garter snake (Thamnophis sirtalis) that
preys on these newts.
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Evolution by speciation
• Basic requirement: reproductive isolation
between populations
• Reproductive isolation
– prevention of mating
– production of nonviable offspring
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Reproductive isolation
• Prezygotic (prevention of mating)
– environmental
– behavioral
– mechanical
– physiological
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Reproductive isolation
• Postzygotic (production of nonviable
offspring)
– offspring weak, do not survive to maturity
– developmental problems
• sterility
• abnormal gonads
– segregational
– F2 breakdown
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• Allopatric (geographic) speciation
– physical or geographical separation
– each population undergoes independent evolution,
adaptation to separate environment
• Parapatric speciation
– part of population enters new habitat
– no physical barrier
• Sympatric speciation
– occurs within population before any differences can
be seen
Three hypotheses for speciation