Ecology Part 2

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Ecology Part 2
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II. Density-dependent-limiting factors
(intra-specific) regulate population growth
because increasing population reduces resource
availability and resource limitations ultimately
limits population growth. Intra-specific
competition the reliance of two or more
individuals of the same species on the same
limited resource. Competition between
individuals of the same species increases as
population size increases and r or (b-d) is
reduced in proportion to the intensity of the
competition. -Population growth declines because
death rate increases, birth rate decreases or
both. -Reduction in food may limit reproductive
output. e.g. eggs or seeds
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e.g. The number of seeds produce by plantain
decrease with increase sowing density. The
average clutch size of a sparrow decreases with
increase in population.
-Territoriality is a behavioral mechanism that
may reduce intraspecific competition since each
individual protects its own area. -Health and
survivorship decreases as crowding results in
smaller less robust individuals. -Predators when
prey population is high, takes greater percentage
of prey.
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-The accumulation of toxic metabolic wastes.
Intrinsic factors may also play a role in
limiting populations -High densities may cause
stress syndrome resulting in changes hormones
that delay sexual maturation, inhibiting
reproduction -High densities can also suppress
the immune system with stress
-High densities can reduce birth rates and
increase death rates. e.g. The percentage of
flour beetles (Tribolium confusum) surviving from
egg to adult decrease at moderate to high
populations densities, reducing the adults in the
next generation.
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It is possible for populations to be regulated by
both density-dependent and density independent
factors.
Population may display short-term fluctuations
due to density-independent factors. The
population Dungeness Crab fluctuates due to
density-dependent factors e.g. intra-specific
competition and cannibalism and
density-independent factors e.g. changes in water
temperature due to ocean currents.
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e.g.Bobwhite quail (southern Wisconsin) survival
varies with the depth of snow cover
(density-independent factor), but if the snow
depth is shallow then factors like reduction in
fecundity due to limited resources
(density-dependent factor) regulates the
population. Together these interact to keep the
population stable.
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Some populations show regular fluctuations in
density in a cyclic way. Mammals -Small
herbivores (lemmings) 3-5 year -Large
herbivores (snowshoe hares)9-11 year
cycle Hypotheses for fluctuations -Stress from
high densities may alter hormone balance and
reduce fertility or may cause physiological
changes -High density may cause changes in the
immune system,
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Oscillation in the population of the hare
preceded changes in the lynx density. This was
once considered to be interrelated but the hare
shows such
oscillations on islands where the lynx is absent.
The lynx population oscillation may be the
result of hare population but not the other way
around.
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The emigration of a species to reduce populations
can be caused by crowding which can cause
physiological changes.
e.g. Aphid population is mostly wingless
females, when the population increases, winged
females appear. Locusts when crowded will grow
longer wings, increase fat content, become darker
and emigrate very readily.
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Generalizations-Populations can be K-selected
(equilibrium populations) or r-selected
(opportunistic populations) Characteristic
r-selected K-selected
populations populations -Maturation
time short long -Lifespan short long -Populat
ion boom-and-bust logistics growth
curve curve -Death rate in young
high low -Investment of none some time in
offspring
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Characteristic r-selected K-selected
populations
populations -Number of offspring many few
per reductive episode - reproductions one sever
al per lifetime -Timing of first early in late
in reproduction life life -Habitat unstable
stable
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Interspecific interaction can also act as a
density-dependent limitation. Predation and
parasitism relationships can regulate
populations. Predator-kills and eats the
prey Parasite-live on or in they hosts deriving
nutrients and energy. If the parasite kills the
host, then it is called a parasitoid. Usually the
predator and parasite population fluctuates in
direct proportion to changes in the density of
the prey.
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The population on mites (above) that is preyed
upon by another species of mites
is regulated by the interaction of the two. The
predator population mirrors the population of the
prey with a lag time. As the prey numbers
increase, there is more food available for the
predator. So there will be a corresponding in
increase in the predator. This had the effect of
reducing the number of prey available. Thus with
less food available for the predator, there
ensued a corresponding decrease in predator
population. This is also true for the population
of moose and wolves found on Isle Royale
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Herbivory is also considered a type of predation.
The interactions of the herbivore and the plant
can be complex. e.g. Passion flower vines
produce toxic chemical that help protect leaves
from herbivorous insects. A counter adaptation
has evolved in the butterfly, Heliconius Its
larvae can feed on the leaves because they have
digestive enzymes the break down the toxic
chemicals. The females of these butterflies
avoid laying eggs on passionflowers leaves that
already carry bright yellow egg clusters reducing
intraspecific competition. The leaves of the
passionflower has responded by growing yellow
nectaries that resemble eggs. The butterflies
avoid laying eggs on these leaves. These
nectaries attract ants and other insects that
prey on butterfly eggs.
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There are times when the relationship can be
mutualistic (mutualism). e.g. Ants live in
acacia trees. The ants harvest the trees' nectar
and leaves, and they also burrow into the thorns
and stems to make their nests. The acacia trees
benefit by fact that the ants keep the trees
relatively free from plant-eating insects and
attack any mammals that are eat the tree. The
ants also prevent vines from growing on the
trees. The acacia provides hollow thorns for the
ants to live in and Beltian bodies found at the
tips of the leaves. These Beltian bodies provide
a source of protein for the ants.
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Interspecific competition can b a
density-dependent limitation. The more two
species go for the same limited resource, the
greater the competition.
Niche-refers the to functional role and position
of an organism in an ecosystem. Every aspect of
an organism's existence helps define that
organism's niche. A niche, theoretically, can be
quantified by graphing two or more resources.
For instance with a bird, the density of the
branches for nesting, versus, temperature
tolerance, versus food size. Where these
intersect would define the hypervolume of the
niche. Many more factors could be added to the
graph, giving more multidimensional enclosed
space for the species.
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When 2 species are competing for the same limited
resource their niche's are said to overlap.
The more similar two niches are, the more likely
that both species will compete for at least one
limited resource. There is a limit on the amount
of niche overlap compatible with coexistence.
Competition for one most limited resource
therefore usually leads to one or two of four
possible outcomes 1. One species will become
extinct. 2. One species may superior in some
regions and the second in other regions.
Sympatry will disappear. 3. One species may be
superior under normal conditions but at a strong
disadvantage under periodic crises, which will
reduce the population size of the superior
species. Competition will then begin anew as
long as generations overlap and the periodic
crises are frequent enough to prevent the
extinction of the inferior species.
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4. Given enough time and slight differences in
niche, selection may act to produce character
displacement, which reduces competition. e.g.
Paramecium cultures of 2 different species
demonstrate 1. The graphs shows the population
growth of P. aurelia and P. caudatum by
themselves in a culture. They both exhibit
typical logistics curves, but when both are put
together in a culture, P. aurelia outcompetes P.
caudatum. The growth of P. caudatum actually
decreases.
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e.g.2 Joseph Connell carried out studies using
barnacles. He cleared a rocky area and observed
repopulation patterns of 2 species. Semibalanus
by itself lived in the lower regions of the tidal
zone. It can not survive the periodic drying out
in the upper part of the tidal zone. When
Chthamalus lived by itself it could exploit both
the upper and lower tidal zones however when both
species were living together, whenever the
Chthamalus ventured in the lower tidal zone of
the Semibalanus, it was crushed by the
Chthamalus. So that being the case the Chthamalus
restricted itself to the upper tidal zone and the
Semibalanus was found in the lower tidal zone.
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This leads to the concept of a fundamental niche
vs. a realized niche. A fundamental niche is the
full environmental range that a species can
occupy if there is no direct competition. A
realized niche is a niche that is narrowed from
the fundamental niche due to competition. Competit
ive exclusion principle- No two species can
occupy exactly the same niche at the same time
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Communities where competition occurs between
species are characterized by resource
partitioning, meaning that every species uses a
different aspect of the resource. Three
different species of warbler occupies the same
spruce tree but they are found in different areas
of the tree. The Cape May warbler feed on the
tips of the highest branches the yellow-rumped
warbler feeds among the lower branches and the
bay-breasted warbler feed in the middle part of
the tree. Species with narrow niches (they can
only use one or a few types of food or growth
sites) are termed specialist and those species
adapted to a wide range of conditions are termed
generalist.
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These warblers are specialists and the common
crow is a generalist living from the farms to
urban areas.
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Social organization often plays an important role
in limiting the size of populations. Sociality
carries the cost of increase competition and the
benefit of increase food supply through
cooperative hunting, cooperative defense of food
territory against predators and sharing of
information about food availability. Dominance
pays an important role in access to resources,
particularly resources critical to breeding.
Male-contest sexual selection has a profound
effect on gene frequency, since certain phenotype
are more likely to succeed than others.
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selection has a profound effect on gene
frequency, since certain phenotype are more
likely to succeed than others. By hunting
together, these African wild dogs can capture a
much larger prey (wildebeest) than an individual
could take on its own.
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These bees cooperate to regulate the internal
temperature of the hive. They fan air out of the
hive entrance, drawing cooler air in else-where.
When the internal temperature is even higher, bee
collect water and spread it on the comb then fan
it to cool the hive evaporatively.
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• Symbiosis are two organisms living and
  interacting together.
• Types
• Mutualism-Both species benefit .
• Commensalism-One species benefits and the other
  is neither harmed or benefits.
• Parasitism-One species benefits and the other is
  harmed.
• Enslavement-One species controls the reproduction
  and destination of another for its own benefit.

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Forming communities from populations- The species
of a community interacts with each other and with
the physical environment. The biotic community
they form can be considered a unit of life, with
its own characteristic structure and functional
interrelationships. Species diversity and
complexity of interaction influence community
stability. A simple community may respond more
violently to a disturbance but often recovers
quickly. The complex community may sometimes
respond less dramatically but may continue to
show effects over a longer period. A diverse
physical environment seems to favor community
stability. The number of species in a particular
area varies. The rule of thumb, the closer the
environment is to the equator, there is an
increase in species diversity.
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Type Locality of species group Seed
Costa Rica 8,000Plants 18,400 sq.
mi. Coastal California 3,050 24,520
sq. mi. Baja California
1,500 24,100 sq. mi. British
Isles 1,600