Download Ecology Part 2

Ecology Part 2
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
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.
-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.
It is possible for
populations to be
regulated by both
density-dependent and
density independent
factors.
Population may display short-term fluctuations due to densityindependent 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.
e.g.Bobwhite quail (southern Wisconsin) survival
varies with the depth of snow cover (densityindependent 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.
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,
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.
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.
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
-Population
boom-and-bust
logistics
growth curve
curve
-Death rate in young high
low
-Investment of
some
time in offspring
none
Characteristic
r-selected
K-selected
populations
populations
-Number of offspring many
few
per reductive episode
-# reproductions
one
several
early in
late in
life
life
unstable
stable
per lifetime
-Timing of first
reproduction
-Habitat`
Interspecific interaction can also act as a densitydependent 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.
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
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.
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.
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.
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.
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.
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.
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.
Competitive exclusion principle- No two species can occupy
exactly the same niche at the same time
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.
These warblers are
specialists and the
common crow is a
generalist living from
the farms to urban
areas.
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. Malecontest sexual selection has a profound effect on
gene frequency, since certain phenotype are more
likely to succeed than others.
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.
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.
Symbiosis are two organisms living and interacting
together.
Types
1. Mutualism-Both species benefit .
2. Commensalism-One species benefits and the other is
neither harmed or benefits.
3. Parasitism-One species benefits and the other is
harmed.
4. Enslavement-One species controls the reproduction
and destination of another for its own benefit.
Forming communities from populationsThe 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.
Type
Locality
# of species
Seed
Costa Rica
8,000Plants
18,400 sq. mi.
Coastal
California
3,050
24,520 sq. mi.
Baja California 1,500
British Isles 1,600
group
24,100 sq. mi.