Community Ecology

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Community Ecology
The level above populations is communities. A
community is the assemblage of all the
populations of organisms of different species
living close enough together that they can
potentially interact. Communities have a number
of aspects of their structure of interest to
ecology, among them a. a diversity b. a
trophic structure
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Diversity may be measured in a number of
ways. The objective is to find a measure that
assesses both species richness (the number of
species comprising the community) and relative
abundance (the number of individuals in each of
those species). For practical reasons, the most
common measure is species richness. Counting the
number of different species in a sample area is
generally straightforward. There are two commonly
used measures that include a relative abundance
component 1. Simpsons index 2. Information
theory (Shannon-Wiener) index
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Simpsons index In Simpsons index you count the
number of individuals in each species, then
calculate the proportion of the total number of
individuals in the sample that are in each
species. The index comes from the sum of squares
of those proportions.
i designates the species The index increases as
more species (and concomitantly smaller
individual pis are squared and summed.
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Shannon-Wiener Information theory diversity Again
you use the proportion of the total in each
individual species. This time you use the
information theory index that was developed
during WWII to break German codes. The formula is
The formula is the equivalent of asking how many
questions would it take to determine what number
you had chosen (say over the range 1-16). The
answer 4 questions. The sign is there because
pis are all lt1, and each log2pi is negative.
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Trophic levels and Ecosystem energetics This
view considers who eats whom. Trophic levels
(from the Greek trophe feed) represent the
different strategies for what types of food
to eat. The basic trophic levels are autotrophs
- self-feeding organisms, making food by
photosynthesis herbivores - or primary
consumers, eat plants carnivores - secondary
consumers, eat herbivores top predators -
tertiary consumers, eat carnivores decomposers -
get their food from the dead bodies of
those at all other levels
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When you consider a single sequence of
organisms that eat and are eaten in turn, you are
looking at a food chain.
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Consider all the different species in each
trophic level of an ecosystem. When you consider
how feeding interactions among them work, you
are considering a food web.
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What limits the length of a food chain (or food
web)? Energy. Why? Transfers of energy from one
trophic level to the next are not very efficient.
The usual approximation is that 10 of the energy
in one level is converted to mass at the next
level. We need 2800 kcal/day. If we were
predators, how much energy would have to be
produced by photo- synthesis to support a human?
28,000 kcal of herbivore and 280,000 kcal of
plants. That is the productivity of 36,500 m2 of
grassland. To support one person thus takes about
4.5 hectares (9 acres) for minimum food needs
alone.
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If the people of New York State lived on
hamburgers alone (MacDonalds would get really
rich), it would take the cows produced on
250,000 square miles of land, which is most of
the east coast of the U.S. That wouldnt work on
a larger scale.
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What happens to the other 90 of energy at any
trophic level? It is lost as (waste) heat in
metabolism. In a short phrase Energy is
constantly dissipated, it cannot be recycled. The
10 law explains why food chains are limited to 4
levels in real ecosystems. A few aquatic chains
may reach 5. Think how much area it would take
to support a 5th or 6th level. As it is, top
predators (eagles, some wolves and large cats)
move over large areas to find enough food. The
movement costs energy, so that the transfer is
even less efficient than the 10 rule suggests.
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What is the effect of energy efficiency on
trophic structure? Pyramids of energy, numbers
and biomass
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A pyramid of numbers for a Michigan bluegrass
field 3 TERTIARY CONSUMERS
354,904 SECONDARY CONSUMERS 708,624 PRIMARY
CONSUMERS 5,842,424 PRODUCERS
and in an aquatic community
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Now that we know something about the structure of
communities, we need to consider how the observed
structure is achieved. We consider the other part
of the definition species living close enough
together that they can potentially
interact. There are many kinds of interactions.
The most important ones can be separated by
assessing the effects of the interaction on the
two species interacting. means the interaction
benefits the species - means the interaction
is detrimental.
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Species
A
- 0 - 0 Species
B - - -- -0
mutualism - predation,
parasitism, or herbivory -- competition
0 commensalism -0 amensalism
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Competition Competition occurs if and only if two
(or more) species use a resource that is present
in insufficient quantity to meet the needs of the
species. We can tell that the species are both
using a resource if their niches overlap. A niche
is a species role in a community (awfully hazy)
or the sum total of its use of the biotic and
abiotic resources of its habitat (much easier to
visualize).
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The niche of a species in the absence of
interactions is set by its tolerance range. Its
called the fundamental niche. The tolerance range
is the range of some variable (say temperature)
over which a species can survive and reproduce.
It will have both a low and a high limit
Performance
Low Temperature High
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In measuring a species niche, we dont worry
about relative performance, so we end up not with
a curve, but a line reaching from the low to the
high limit
low
high
Temperature
This is the niche in 1-dimension. There are other
important environmental variables. What happens
if we consider humidity, as well?
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We then have a 2-dimensional niche
Humidity
Temperature
We can expand that to 3-dimensional or even more,
even if those more complicated niches, called a
niche hypervolume, cant be plotted on paper.
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A niche in 3 dimensions
Humidity y
x
Temperature
z Nutrient
3 axes ( 3 variables)
There are an infinite number of possible
dimensions
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Now you need to remember that species do not live
alone in communities they interact. Those
interactions may limit a species to a narrower
range than is suggested by the fundamental niche
Fundamental Niche (entire hypervolume no
competitors)
Realized Niche (where a species occurs with
competitors)
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Now we return to competition When species
compete, their niches overlap, and the
competition occurs where (to the extent that)
their niches are overlapping.

species 1
species 2
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A Russian ecologist named Gause studied
competition between different species of
Paramecium and found that two species competing
for the same limiting resources cannot coexist in
the same place. An American ecologist, Garrett
Hardin, re-stated that observation as the
competitive exclusion principle When two species
have identical niches (with respect to a limiting
resource), one will use the resource more
efficiently and drive the other locally extinct.
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If niche overlap is not complete, then the two
species may be able to coexist by resource
partitioning. Beak size differences among
Darwins finches on the Galapagos permit multiple
species to coexist on islands by feeding on seeds
of differing size. The partitioning of space (and
differences in tolerance to exposure when the
tide is out) permits two species of barnacles to
coexist on the rocky shoreline of Scotland. Here,
one species (Balanus) outcompetes the other
(Chthamalus) in the lower part of the range,
where it is rarely exposed. Chthamalus persists
above because it can tolerate exposure.
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Balanus Chthamalus