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Chapter 16

Biology 2483A Chapter 16: The Nature of Communities

Course Code
BIOL 2483A
Hugh Henry

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Lecture 15 The Nature of Communities
Seaweed species discovered in water of Monaco but from Caribbean and found in warmer waters.
o Mystery as to why it was there an able to survive, but also it began to spread very rapidly.
o Found out it originated from strain tolerant of cold water developed in Germany and was accidently
introduced to Monaco.
o Classic example of an invasive species and it replaced sea grass communities that provide
structure, habitat and food for fish species.
o Sea weed species operated very differently in terms of time of the year it grew the most and how it
affected sedimentation. Huge shifts in the types of fish you’d see.
Question: how do you characterize these shifts? How can you compare one community to
another in some quantitative way?
Although so far we have considered species interactions in two-way relationships, in reality, species
experience multiple interactions that shape the communities in which they live.
What Are Communities?
Communities are groups of interacting species that occur together at the same place and time.
In practical terms, ecologists usually define communities based on common
o Physically defined community: might encompass all the species in a sand dune, a mountain
stream, or a desert.
o Biologically defined community: might include all the species associated with a kelp forest, a
freshwater bog, or a coral reef. This approach emphasizes the importance of an abundant species,
such as trees.
Huge task to study, if not impossible to characterize system all at once or even begin to
understand how they’re all interacting.
Ecologists often define a community somewhat arbitrarily, based on the questions they are posing.
o (Studying entire communities isn’t very practical, so ecologists often simplify the definition to subset
related to question of study.)
o Example: A study of marine invertebrates in seagrasses might restrict the definition of the
community to that interaction, and not include mussel-eating birds, etc.
Counting all the species in a community is difficult to impossible, especially if small or unknown species are
o Ecologists usually consider a subset of species when they define and study communities.
Subsets of species can be defined by:
o Taxonomic affinity e.g., all bird
in a community or fish community in a pond.
o Guildgroup of species that use the same
o Functional groupspecies that
in similar ways, but do not necessarily use the same
resources and might not be closely related (often used for studying plants).
A) bird communities
B) feeding guild all feed on pollen
C) Can feed in similar way, but very
different resources. Or you can similar
Often how we organize a community we’re interested in what-eats-what.
When you look in terms of species interactions, when you eat something you’re essentially gaining the
energy it has in its bonds.
Food webs organize species based on
or energetic interactions.
o Trophic levels:
Primary producers
(autotrophs)plants and algae.
Primary consumers
Secondary consumerscarnivores.
Tertiary consumerscarnivores.
Food webs can get complicated when you start thinking about real organisms, specifically we don’t know
anything about the interaction strength (ie. arrows). Food webs tend to just show who is eating who as
opposed to how much is going within a certain path.
Food webs tell little about the strength of interactions or their importance in the community.
o Some species span two trophic levels, and some species change feeding status as they
mature (small to large).
o Some species are omnivores, feeding on more than one trophic level (ie. the C1 in the figure).
Food webs do not include nontrophic interactions (horizontal interactions, such as competition).
o Interaction webs more accurately describe both the trophic (vertical) and non-trophic
(horizontal) interactions (interactions between species in the same trophic level ie. competition)
than a traditional food web.
- Similar to food web, but instead showing the energy
flow and takes into account the competitive interactions
ie. The herbivores aren’t eating each other but are
competing for the same food resources, likewise the
carnivores aren’t
feeding on one another
but might share similar resources.

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Community Structure
Community structure is the set of characteristics that shape communities:
Species richnessthe number of species in a community.
Species evennessrelative abundances compared with one another.
Species diversity combines species richness and species evenness.
- Two communities of mushrooms.
- Same number of species represented in each
one, but more even distribution of abundance in
bottom = higher evenness = higher diversity
(combines richness & evenness).
- Species richness is equal in both communities =
equal to 4.
- Community B has much higher evenness.
The most commonly used species diversity index is the Shannon index:
o Based on the proportion of abundance of each species (proportion of the number of individuals
within the entire community of individuals. If species 1 is one individual and there are 10 individuals
of all species in the population, then it’s proportional abundance is 0.1 or 10% of all individuals).
= proportion of individuals in the
i th
= number of species in the community
- Example from mushroom community
about how to calculate measure of
diversity using the Shannon index.
- First calculate the proportional
abundances, so looking at the top
community we’ll have 17 of one species
and 1 of each of the other.
Proportion (yellow) = 17/20 = 0.85
Proportion (other) = 1/20 = 0.05
Diversity index = pi ln(pi)
Shannon index (H) = 𝒑𝒊𝒍𝒏(𝒑𝒊)
- In community B total ends up being
higher than community A once multiplied
by (-).
Biodiversity (broader term) describes diversity at multiple spatial scales, from genes to species to
communities. Implicit is the interconnectedness of all the components.
o Biodiversity incorporates genetic diversity.
o Genetic diversity affects the viability of populations (ability to respond to environmental change),
which in turn affects species diversity in a community.
o The number of community types in an area is critical to diversity at larger regional and latitudinal
Problem w/ species diversity indices is that they’re boiling a whole bunch of information down to a single
number. So we can end up with very different arrangements of evenness and richness that will give us the
same diversity number. So there are ways of providing more information in terms of describing community
where you condense the information, but not as much. Most of these are called Graphical representations
of species diversity.
Graphical representations of species diversity can give an explicit view of commonness or rarity.
o Rank abundance curves (most common) plot the proportional abundance of each species
) relative to the others in rank order.
- Rank abundance curves for two
mushroom communities.
- One of them is 4 species w/ same
proportional abundance (blue).
- Other (red) one take the most
abundant and plot it first in terms of
- Proportional abundance is on a log
- tells us more about how the individual
species are contributing to community
and you can start to infer information
about the species interactions by
looking at this.
- Community A dominant species has a
strong effect of the others suppressing
or outcompeting them, whereas in
Community B we don’t have such
- Key thing about these rank
abundance curves is that they’re
describing community structure, but
not really describing mechanism.
- Fundamentally will have to do
experiment to get at what’s happening
in terms of species interactions.
Relative abundances can suggest what species interactions might be occurring.
o In Community A, the dominant species might have a strong negative effect on the three rare
o Experiments that
add or remove
species are used to explore these relationships.

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Ex: Species diversity and rank abundance curves were determined for two soil bacteria communities in
pastures. One pasture had been fertilized regularly.
o Bacteria species can be identified quickly using DNA sequencing of 16S ribosomal DNA. They are
then grouped using phylogenetic analysis.
o Both pastures had similar community structure. A few species were abundant; most species were
- undisturbed = red
- fertilized = blue
These are the rank abundance diagrams, and
you can see a very similar structure in these
dominant members followed by a lot of rare
ones. The species compositions were
different, but in terms of the rank abundance
diagram they had very similar pattern of
relative abundances amoung dominant
species vs. less dominant.
- Not only care about describing community
about abundance relationship, but also
matters what is there.
- Example has very similar communities
based on relative abundance but the big
player is being different.
Species compositionidentity of species in a community.
o Two communities could have identical species diversity values, but have completely different
o The identity of species is critical to understanding community structure.
Species accumulation curvesspecies richness is plotted as a function of the total number of
individuals that have been counted.
o These curves can help determine when most or all of the species in a community have been
- You’ll feel more confident when you
eventually level off in terms of the
number of species that can then
In this particular case, if you’re finding
a new species when you take these
initial samples, but when you get up to
later point and have a lot of sampling
effort you’ll find the same stuff that you
found in the earlier ones and when you
get to that point then you have some
confidence that you described the
o Hughes et al. (2001) compared species accumulation curves for five different communities.
Looked at different types of species.
In some of these we see it level
off, but in others we haven’t
even come close (ie. tropical soil
There’s a lot of variation in
communities in terms of the
sampling effort required to describe
the community. But you make these
curves to build confidence that you
sampled adequately.
The communities varied greatly in the amount of sampling effort necessary to determine species richness.
o The temperate forest and tropical bird community were adequately represented before half the
individuals were counted.
o For tropical soil bacteria, more effort was needed to sample this extremely diverse community.
Spatial scale is also important.
o If we sampled bacteria in tropical soils at the same scale as Costa Rican moths, the bacterial
diversity would be immense in comparison.
o The study highlights how little we know about community structure of rarely studied assemblages,
such as microbial communities.
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