Ecology Review

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Ecology Review
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What is ecology? Word is derived from the green
Oikos, meaning home. Coined by the German
biologist Ernst Haeckel in the 19th Century.
There are a number of definitions. Most focus
on the interactions between organisms and their
environment. Study of the "household". More
informative definition - Krebs (1972) Ecology is
the scientific study of the interactions that
determine the distribution and abundance of
organisms. Environment occupies central role.
Environment includes both biotic and abiotic
factors.
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• Ecology deals with three levels of concern
• The individual organism
• How does the environment (biotic and abiotic)
  affect the individual organism?
• 2. The population, consisting of individuals of
  the same species.
• Deals with the presence or absence of particular
  species, their relative abundance, and trends or
  fluctuations in their numbers.
• 3. The community, consisting of a number of
  interacting populations
• Deals with the composition and structure of
  communities, and with the functioning of
  communities, and with the movement of energy and
  mater through them

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• Approaches to Ecology
• A number of different approaches have developed
• Physiological.
• Population.
• Community.
• Behavioral
• Ecosystem.
• Landscape.

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Effects on Individuals
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The distribution of black and white spruce in
Canada, showing the northern distributional limit
of these trees.
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The northern and eastern boundaries of the range
of the saguaro cactus in Arizona. Dots represent
locations where there are no records of periods
longer than 36 hours without a thaw. Crosses
represent locations where such periods have been
recorded.
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Even mammal distributions are controlled by
temperature. The northern limit of the
distribution of the flying squirrel is related to
temperature. In the northern extremes of their
range, they use huddling behavior to stay warm.
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Temperature and Performance of Organisms

• Most species perform best within a narrow range
  of temperatures
• Begins at enzyme level
• Rigid shape at low temperature
• Excessively high temperatures destroy their shape
• Work best at some intermediate temperature
• Can measure enzyme effectiveness by determining
  substrate concentration required for enzyme
  function at a particular temperature. Low
  substrate concentration high enzyme affinity

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Homeostasis an organisms physiological
mechanisms that work to maintain a constant
internal environment in the face of varying
external factors. Homeostatic mechanisms
typically require the expenditure of energy, and
can typically operate only within a narrow range
of a factor.
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Life Histories
Life History refers to any aspect of the
developmental pattern and mode of reproduction of
an organism.
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We can consider five fundamental aspects of life
history

• Size
• Metamorphosis
• Diapause
• Senescence
• Reproductive Patterns

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Frequency distribution with respect to log body
mass for North American mammals, birds, and
freshwater fish.
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The curve of reproductive power as a function of
log body mass, as predicted. Closely follows
observed distribution.
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Metamorphosis Organisms that metamorphose
undergo radical changes during development.

• What sort of fitness advantage could outweigh the
  complications of such a strategy?
• Exploitation of habitats with high, but
  transient, productivity?
• Dispersal?
• Reduction of competition?

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Diapause A stage in the life cycle characterized
by a cessation of development and a protein
synthesis, and by suppression of the metabolic
rate.
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Red kangaroo reproductive cycle
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Senescence the timing of aging and death. Why
must organisms die? Could the timing of death
have evolved? Why do some organisms live longer
than others? Some recent findings suggest that
life span has some genetic basis.
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Many insects have very short life spans. The
adult mayfly may live only a few hours.
Annual plants have a lifespan of less than a year.
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Galapagos tortoises live at least 150 years, and
perhaps as much as 200 years. If this is true,
there may be tortoises in the Galapagos that were
youngsters when Darwin was there.
Atlantic sturgeon may live 150 years.
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Methuselah Grove in the Ancient Bristlecone Pine
Forest of Inyo National Forest in eastern
California. The worlds oldest living thing
lives here, a bristlecone pine 4,723 years old.
It is not identified for its own protection.
How does life span evolve?
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• Currently, there are two main hypotheses to
  explain the process of senescence
• Mutation accumulation hypothesis.
• Evolutionary senescence hypothesis.

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Reproductive Strategies

• Organisms have only a certain amount of energy
  available to them for reproduction.
• Species must make an evolutionary decision on
  how to apportion that energy. Clutch size,
  parental care, age at reproduction, etc. There
  are a series of tradeoffs.
• A relationship exists between the demography of
  the species and its reproductive pattern.
  Reproduction and mortality interact. Each
  reproductive effort may be expected to increase
  the mortality rate.

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• Now consider the trade-offs.
• Clutch size.
• Present versus future reproduction.
• Age at sexual maturity.

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The theory of r and K selection. Suggests that
organisms can be placed into two fundamental
groups on the basis of their position on the
sigmoid growth curve and the resulting life
histories.
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Populations
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Population a group of individuals of a single
species inhabiting a specific area. Usually
implies interaction, and often implies a shared
gene pool
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Spatial structure of populations has three main
attributes..

• Distribution
• Dispersion
• Density

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Distribution What controls where organisms are
found?
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The physical environment limits the geographic
distribution of species.
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Plants in the genus Encelia show distributions
along a moisture-temperature gradient from the
California coast eastward.
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Dispersion Where are organisms found in relation
to one another?
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We often recognize three patterns of dispersion,
which have different indications for the biology
of the species.
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Distribution patterns of the creosote bush of the
American southwest are well studied.
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Density the number of individuals per unit area
or volume. Estimating population size is critical
to the study of population dynamics. It is
rarely possible to count all members of a
population.
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Whale population sizes have been estimated using
mark-and-recapture techniques for years.
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Classification of Commonness and Rarity

• Deborah Rabinowitz devised a system for
  classifying commonness and rarity based on
  combinations of three factors
• Geographic range extensive versus restricted.
• Habitat tolerance broad versus narrow.
• Local population size large versus small.
• We can use this system of classification to come
  up with eight possible combinations of these
  factors, which give us seven forms of rarity, and
  one of abundance.

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Population Dynamics
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Population dynamics involves the behavior of a
population. What happens to it over time.
At the core of population dynamics are life and
death.
Nnow Nthen B D I - E
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Some Definitions

• Demographic process a process capable of
  changing the size or composition of a population
• Life table a summary of the age- or
  stage-related survivorship of individuals in a
  population
• Mortality death rate of individuals in a
  population. The probability that a
  representative newly-born individual will die
  before reaching a certain age.
• Survivorship the complement of mortality. The
  probability of an individual surviving to a given
  age.
• Fecundity the number of eggs, seeds, or
  offspring produced by an individual.
• Fecuncity schedule a data table displaying
  lifetime birth patterns among individuals of
  different ages in a population.

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Life table and survivorship curve
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Mortality rate among perennial plant population
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Three types of survivorship curves
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Age distribution of white oak population
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Age distribution of Rio Grande cottonwoods
population
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Size and generation time
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Sex Ratios another way in which we can examine
population structure
Defined as the ratio of males to females in the
population. We can recognize sex ratios at four
different stages Primary Secondary Tertiary Quate
rnary
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Sex ratio can change over the life history of an
organism.
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Population Growth
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When resources are unlimited, populations may be
expected to grow at an exponential rate.
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In rare circumstances, we may see this in nature.
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Equation describing exponential population
growth. In derivative form, may be written as
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Typically, however, we expect population growth
to slow at increasing population size, resulting
in this sigmoid growth pattern.
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Some organisms, like these English herons, show
small-magnitude irregular fluctuations.
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Others, such as the moth Dendrolimus, show
large-scale fluctuations on an irregular
time-scale.
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Collared lemmings in the arctic show tremendous
variation on a very predictable four-year cycle.
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We may also see regular cycles in abundance.
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House mice may show irruptions, in which they
live at low population densities for long
periods, then explode.
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Time now to introduce the concept of
competition an interaction between individuals
or species over a limiting resource that
negatively affects the fitness of one or both.
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We divide competition into two classes Intraspeci
fic competition between members of the same
species. Interspecific competition between
members of different species.
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Must define a couple of other terms as
well Density-dependent effects situation
occurring when population regulation is related
to the density (size) of the population. Density-i
ndependent effects situation occurring when
population regulation occurs in a manner that is
unrelated to the density of the population.
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For several decades, ecologists have debated the
relative significance of different factors on the
regulation of natural populations. The main
dichotomy centers on the relative importance of
density-dependent versus density-independent
factors. In general, we now believe that
density-dependent factors have to play a role.
In fact, such things as density-dependent effects
on fecundity and survival have come to define
population regulation.
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We believe that a number of factors can regulate
population size in a density-dependent fashion.
We further believe that we can group those
factors into extrinsic and intrinsic factors.
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Extrinsic Factors Food supply
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Extrinsic Factors Predation
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Densities of wolves and moose on Isle Royale are
related in a rather complicated manner.
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Extrinsic Effects Parasitism and Disease
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Prevalence of brucellosis in Yellowstone bison
increases with population size.
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For density-dependent factors, the birth rate and
the mortality rate tend to change with population
density.. .. while for density-independent
factors these rates are unrelated to population
size.
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Intrinsic Mechanisms? Is it possible that
populations could be self-regulating, with
intrinsic factors such as stress, territoriality,
and dispersal playing a primary role in
controlling population size?
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Several types of possible intrinsic mechanisms
have been suggested Social Stress
Hypothesis Territoriality Genetic
Polymorphism Dispersal
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Nonequilibrium Ideas
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Early in the debate, two Australian
entomologists, H.G. Andrewartha and L.C. Birch,
were emphasizing the importance of
density-independent factors like weather on
population density. There is currently a lot of
interests in these ideas, stemming in part from
the availability of long-term data sets in stable
environments. There seems to be good evidence
that many populations do not behave in an
equilibrium-type fashion.
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Abiotic Extrinsic Regulation Many factors could
be involved. Rainfall for instance.
Density of Larrea at 11 sites in the Mojave Desert
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Doesnt seem to be related to food limitation,
because the density doesnt decline as the
density of competing shrubs increases.
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Rainfall flucuates greatly in the Galapagos. The
density of two species of Darwins finches seems
to respond.
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Community Structure
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Community The entire assemblage of interacting
species in a given area. We must determine the
extent of the interactions that we want to use to
define the community. Must also distinguish
communities from taxonomic associations and
guilds.
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Consider two communities The great lakes of the
African Rift Valley are inhabited by a large
number of fish belonging to the family Cichlidae.
More than 250 species are found in Lake Victoria
alone. They have undergone an adaptive radiation
that has allowed them to diverge into different
niches including at least ten different trophic
styles. There is apparently a great deal of
niche overlap.
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A combination of the number of species and their
relative abundance defines species diversity
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A commonly applied measure of species diversity
is the Shannon-Wiener index
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Plots of species number versus sampling effort
typically only show part of this normal curve.
Much sampling effort is needed to find the rare
species.
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Rank abundance curves show the proportional
abudance of a given species plotted versus their
rank abundance. This shows the evenness of the
community at a glance.
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Two major groups of ideas regarding community
structure have been developed. They are
typically categorized as equilibrium and
nonequilibrium explanations. Do communities
reach equilibrium, with the structure at
equilibrium being determined by biological
processes occuring within it? Or do external
disturbances prevent equilibrium from being
established.
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Equilibrium Approaches to Community
Structure The Effect of Interspecific
Competition How are the coexisting species in a
community related to one another. How are the
niches arranged in highly diverse communities.
Two phenomena can be viewed suggesting that
interspecific competition plays a real role in
community structure.
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1. Changes in Niche Dimensions Recall the idea of
the realized versus the fundamental niche.
Robert MacArthur studied the way in which
resources are divided among five similar species
of warblers in coniferous trees in northern
forests.
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The birds are very similar in size and in bill
shape. All are insectivores.
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MacArthur showed that the five species are
actually foraging at slightly different locations
within the tree. They are not sharing the same
niche.
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2. Patterns of Species Distribution
We saw earlier the results of Jared Diamonds
study on the distribution of cuckoo doves in the
Bismarck Archipelago.
This seems to be strong evidence that the bird
communities on these islands are determined at
least in part by competition through competitive
exclusion.
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Equilibrium Approaches to Community
Structure The Effect of Predation Predation is
also known to play a major role in organizing
communities. Interesting, the mechanism by which
predation is thought to structure communities
includes a major role for competition.
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One of the classic studies leading toward an
understanding of the role of predation in
structuring communities was conducted by Robert
Paine in the intertidal zone of Neah Bay,
Washington.
Paine found that when he removed the sea star
Pisaster from experimental plots, species
diversity was significantly lower. Only 8 of 15
species remained in communities protected from
Pisaster predation.
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Paine believed this was related to competition.
In the absence of sea stars, the mussel Mytilus
came to dominate the community and eliminated
several species. Competitive exclusion by a few
dominant species led to decreased community
diversity.
This phenomenon gave rise to the keystone
predator hypothesis. A keystone predator is one
whose presence is central to the organization of
the community.
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Nonequilibrium Approaches The Role of
Disturbance Many abiotic factors (fire,
eruptions, floods, storms, etc.) disturb
communities in ways that affect the coexisting
species. These factors have different impacts on
the various species and thus affect community
organization.
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Joe Connell introduced the intermediate
disturbance hypothesis. He felt that the
frequency and intensity of disturbance determined
the importance of processes such as competition
and predation.
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At high levels of disturbance, species diversity
is low because few species can tolerate the
disturbance. At low levels of disturbance,
dominant competitors may drive other species to
extinction. Diversity is highest at intermediate
levels of disturbance.