Transcript Document

Ecology Review
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.
Ecology deals with three levels of concern:
1. 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
Approaches to Ecology:
A number of different approaches have developed:
a. Physiological.
b. Population.
c. Community.
d. Behavioral
e. Ecosystem.
f. Landscape.
Effects on Individuals
The distribution
of black and
white spruce in
Canada,
showing the
northern
distributional
limit of these
trees.
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.
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.
Temperature and Performance of Organisms
• Most species perform best within a narrow range of
temperatures
• Begins at enzyme level
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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
Homeostasis – an organism’s 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.
Life Histories
Life History: refers to any aspect of
the developmental pattern and mode
of reproduction of an organism.
We can consider five fundamental
aspects of life history:
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Size
Metamorphosis
Diapause
Senescence
Reproductive Patterns
Frequency
distribution with
respect to log body
mass for North
American mammals,
birds, and
freshwater fish.
The curve of
reproductive
power as a
function of log
body mass, as
predicted. Closely
follows observed
distribution.
Metamorphosis – Organisms that
metamorphose undergo radical changes
during development.
What sort of fitness advantage could outweigh the
complications of such a strategy?
1. Exploitation of habitats with high, but transient,
productivity?
2. Dispersal?
3. Reduction of competition?
Diapause
A stage in the life cycle
characterized by a cessation of
development and a protein
synthesis, and by suppression of the
metabolic rate.
Red kangaroo reproductive cycle
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.
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.
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.
Methuselah Grove in
the Ancient
Bristlecone Pine
Forest of Inyo
National Forest in
eastern California.
The world’s 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?
Currently, there are two main hypotheses to
explain the process of senescence
1. Mutation accumulation hypothesis.
2. Evolutionary senescence hypothesis.
Reproductive Strategies
Organisms have only a certain amount of energy
available to them for reproduction.
1. 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.
2. 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.
Now consider the trade-offs.
1. Clutch size.
2. Present versus future reproduction.
3. Age at sexual maturity.
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.
Populations
Population: a group of individuals of
a single species inhabiting a
specific area.
Usually implies interaction, and
often implies a shared gene pool
Spatial structure of populations has
three main attributes…..
• Distribution
• Dispersion
• Density
Distribution: What controls where
organisms are found?
The physical
environment limits the
geographic distribution
of species.
Plants in the genus Encelia show
distributions along a moisturetemperature gradient from the
California coast eastward.
Dispersion: Where are organisms
found in relation to one another?
We often recognize three patterns of dispersion, which
have different indications for the biology of the species.
Distribution patterns of the creosote bush of the
American southwest are well studied.
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.
Whale population sizes have
been estimated using mark-andrecapture techniques for years.
Classification of Commonness and Rarity
Deborah Rabinowitz devised a system for
classifying commonness and rarity based on
combinations of three factors:
1. Geographic range – extensive versus restricted.
2. Habitat tolerance – broad versus narrow.
3. 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.
Population
Dynamics
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
Some Definitions
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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.
Life table and
survivorship curve
Mortality
rate among
perennial
plant
population
Three types of
survivorship
curves
Age distribution
of white oak
population
Age distribution
of Rio Grande
cottonwoods
population
Size and
generation time
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
Quaternary
Sex ratio can change over the life history of an
organism.
Population Growth
When resources are unlimited, populations may be
expected to grow at an exponential rate.
In rare circumstances, we may see this in nature.
Nt  N 0 e
rt
Equation describing exponential population
growth. In derivative form, may be written as:
dN
dt
 rN
Typically, however, we expect population growth to slow at
increasing population size, resulting in this “sigmoid” growth
pattern.
Some organisms, like
these English herons,
show small-magnitude
irregular fluctuations.
Others, such as the moth
Dendrolimus, show large-scale
fluctuations on an irregular timescale.
Collared lemmings in the arctic show
tremendous variation on a very
predictable four-year cycle.
We may also see regular cycles in abundance.
House mice may show
“irruptions”, in which they live at
low population densities for long
periods, then explode.
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.
We divide competition into two classes:
Intraspecific competition – between
members of the same species.
Interspecific competition – between
members of different species.
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-independent effects: situation occurring
when population regulation occurs in a manner
that is unrelated to the density of the population.
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 densitydependent factors have to play a role. In fact,
such things as density-dependent effects on
fecundity and survival have come to define
population regulation.
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.
Extrinsic Factors:
Food supply
Extrinsic Factors:
Predation
Densities of wolves and moose on Isle Royale are related
in a rather complicated manner.
Extrinsic Effects:
Parasitism and Disease
Prevalence of
brucellosis in
Yellowstone bison
increases with
population size.
For density-dependent
factors, the birth rate and
the mortality rate tend to
change with population
density…..
….. while for densityindependent factors these
rates are unrelated to
population size.
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?
Several types of possible intrinsic mechanisms have
been suggested:
Social Stress Hypothesis
Territoriality
Genetic Polymorphism
Dispersal
Nonequilibrium
Ideas
Early in the debate, two Australian entomologists,
H.G. Andrewartha and L.C. Birch, were
emphasizing the importance of densityindependent 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.
Abiotic Extrinsic
Regulation:
Many factors could be
involved. Rainfall for
instance.
Density of Larrea at
11 sites in the Mojave
Desert
Doesn’t seem to be
related to food
limitation, because
the density doesn’t
decline as the density
of competing shrubs
increases.
Rainfall flucuates
greatly in the
Galapagos. The
density of two species
of Darwin’s finches
seems to respond.
Community
Structure
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.
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.
A combination of the number of species
and their relative abundance defines
species diversity
A commonly applied measure of species
diversity is the Shannon-Wiener index:
s
H    pi log pi
i 1
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.
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.
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.
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.
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.
The birds are very
similar in size and
in bill shape. All
are insectivores.
MacArthur showed that the
five species are actually
foraging at slightly different
locations within the tree.
They are not sharing the
same niche.
2. Patterns of Species Distribution
We saw earlier the results of Jared
Diamond’s 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.
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.
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.
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.
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.
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.
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.