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Chapter 13

BIOLOGY 2F03 Chapter Notes - Chapter 13: Intraspecific Competition, Interspecific Competition, The Floaters

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D R.Kajura

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Chapter 13 Competition
Most communities are dominated by their primary producers, and thus they
are the obvious living organisms of most ecological systems
Less solar light reaches the plants near the soil compared to those above the
tallest plants. AS the tallest plants cast shadows on the ones below
The plant roots are all tangles with no distinction between which belongs to
which plant
Mushrooms are the reproductive structures of fungi that form associations
with the roots of surrounding trees. The tree provides carbon to the
mushroom in form of sugar and receives mineral resources from the
mushroom. This association is not always even
Everywhere you look you see one organism somehow influencing the lives of
The organisms in a community are related through ecological interactions
that can be arranged into a grid
Competition is the most obvious interaction seen as plants shade each other
and take nutrients from one another
Commensalism is when trees provide a home to birds with no cost to them (+
Amensalism is when a tree shades other trees and do not receive any harm
or help from them below (_-, 0).
Neutralisms are when species come into contact but do not interact (0,0)
13.1 Forms of Competition
Individuals can compete with other individuals, or their own and of different
species, in a number of different ways
Intraspecific competition occurs between members of the same species and
interspecific competition occurs between individuals of different species
A good example is in coral reefs when damselfish compete with one another
for a territory by slapping each other with they tails. These damselfish
compete by intraspecific competition, interspecific competition, resource
limitations and interference competition
Competitive effects can by asymmetric with some individuals harmed and
other not
Research was conducted on pine forests in New Hampshire to see if the root
systems were in competition with one another. Researchers cut a trench in
the middle of the forest, which cut a lot of the root systems in that area. The
roots showed that vegetative cover on the floor that was released of roots
was 10 times that present on control plots. Suggesting interspecific
competition between the pine trees that suppressed vegetation growth.
The damselfish demonstrated interference competition by actively attacking
other individuals to prevent them from getting the desired resource. The
pine trees show exploitative competition as trees would take the resources
before others could
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Competition for resources occurs In diverse communities, with often
predictable consequences
13.2 Evidence of Competition in natural systems
Field and mesocosm studies show that resource limitations and competitions
are widespread
Intraspecific competition for limited resources slows population growth at
high densities
Intraspecific Competition in plant populations
Self-thinning occurs in plants as at first there are many seedlings in high
density, but as the plants begin to grow their density decreases, as the
resources are limited. Self-thinning states that even in the absence of
outside agent, the number if individuals found in a stand decreases as
average size of plant increase. Driving this process is competition
Self-thinning arises by intraspecific competition for limited resources. AS
the plants grow, they compete for nutrients and exploit them am different
rates causing some to die and decreasing the density as the plants grow
One way to plot self-thinning is to show total plant biomass against
population density. IF we plot the log of both the resulting slope is usually
-1/2. One unit increases in plant biomass for 2 units loss in density.
Figure 13.6. Population A begins at low density and grows on biomass, but
since the density is low there is no intraspecific competition (no change in
density). Population B starts at slightly high density, and as they grow they
hit the self thinning line, at this point there’s increase in biomass but
decrease in density. Population C and D occur at higher density, so self
thinning occurs quicker
Another way to show self-thinning is to produce a plot of average weight of
individual plant against density. This slope is normally -3/2
Self thinning occurs and appears to be consequence of intraspecific
competition for limited resources
Intraspecific competition among plant hoppers
Ecologists have failed to show that insects, herbivores necessarily compete
for limiting resources
One group of insects that due competes for limiting resources leafhoppers
and plant hoppers. Interaction of leafhoppers attributes the prevalence of
competition among Homptera to their habit of aggregation, rapid
population growth and mobile nature of food supply
Researchers were able to demonstrate intraspecific completion of these
plant hoppers in salt marshes of USA. Enclosing the insects with Spartine
seedlings controlled densities. At the highest density, the insects showed
reduced survivorship and increased developmental time, which are signs of
intraspecific competition de to low food quality as density increased. Pants
heavily occupied by plant hoppers showed reduced concentrations of
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proteins and chlorophyll. So competition between leafhoppers is from
limited resource supply, however interference competition can arise in
absence of obvious resource limitation
Interference Competition among Song Sparrows
Sparrows are predominantly monogamous. At age of 1 male take up
territories to sing and attract mates. The space for territory is limited so
some sparrows don’t get established territories
The males known as floaters live within territories and hide out until the
predominant male dies. If the predominant male sees the floater in its
territory, it leads to a chase, which is an example of interference competition.
Of interest to the researchers were intrusions by floaters and if some males
were at higher risk of being intruded. Prior study suggests territory size;
male age and health are likelihood of floater intrusion.
There was a strong relation of age of territory owner and intrusion pressure.
The younger and oldest territory males held onto their land the shortest
compare to 2 and 3 year old males. The males that lost their territory to the
floaters, likely broke a limb and show reduced body weight
Interference competition can occur in a natural setting. Like exploitive
competition, a variety of factors can influence the strength of competition
experienced by different individuals in a population. The floaters attacked
the weak and inexperience’s territory owners
Is competition a common ecological interaction?
Many phases of ecological study have been conducted from theoretical phase,
laboratory work, observation in field
In 1980’s very basic questions were unanswered in ecology: IS competition
natural in natural populations?
First test of this question by Joseph Connell and Thomas Schoeneer provided
field experimental evidence. The researchers studies many interactions and
obtained different amounts of intraspecific competition because they
analyzed different groups
Jessica Gurevitch stated that although competition has negative impacts on
biomass in general, there were variation amoung taxa. Latest research shoes
that competition is important force in lives of many organism, Competition is
just one factor that influences population dynamics
Animals are generally rare on landscape compared to plants. Since plants
grow in close approximant, there roots will likely interact. For plant
ecologists, the big question about competition is where its is strangest
According to Phil Gtime, plant competition is unimportant in unproductive
areas. However, David ilman disagree with this statement and said
competition is important between high and low resource availability.
Unproductive competition will be belowground, while productive completion
will occur above ground
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