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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
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. Simpson’s index
2. Information theory (Shannon-Wiener) index
Simpson’s index
In Simpson’s 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.
S  1
s
p
i 1
2
i
i designates the species
The index increases as more species (and
concomitantly smaller individual pis are squared and
summed.
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:
s
H '    pi log 2 pi
i 1
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 <1, and each log2pi is negative.
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
When you consider a single sequence of organisms
that eat and are eaten in turn, you are looking at a
food chain.
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.
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 photosynthesis 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.
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 wouldn’t work on a
larger scale.
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.
What is the effect of energy efficiency on trophic
structure?
Pyramids of energy, numbers and biomass:
A pyramid of numbers for a Michigan bluegrass field:
3
354,904
708,624
5,842,424
TERTIARY CONSUMERS
SECONDARY CONSUMERS
PRIMARY CONSUMERS
PRODUCERS
and in an aquatic community:
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.
+
+
++
-
+-
Species A
+-
0
+0
Species B
--
++ = mutualism
+- = predation, parasitism, or herbivory
-- = competition
+0 = commensalism
-0 = amensalism
-0
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).
The niche of a species in the absence of interactions
is set by its tolerance range. It’s 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:
Low
Temperature
High
In measuring a species’ niche, we don’t 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?
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, can’t be plotted on paper.
Humidity
y
A niche in 3 dimensions
Temperature
x
3 axes (= 3 variables)
There are an infinite number of possible dimensions
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)
Numbers
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
Gradient
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.
If niche overlap is not complete, then the two species
may be able to coexist by resource partitioning.
Beak size differences among Darwin’s 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.
Balanus
Chthamalus