Bioe 147/247
Lecture Notes
Community Ecology: Basic Concepts and Approaches
Community Ecology = The Ecology of Biodiversity
Community ecology and biodiversity deal with difficult issues:
 Many species: different evolutionary histories & different roles

Community patterns and processes  vary in details
Type of system (aquatic v terrestrial; altitude; latitude, etc.)
Stage of community development (e.g., time since disturbances)
Adjacent/nearby communities: Regional vs Local effects
Abiotic factors (e.g., climate (always changes); nutrients, disturbances)
 BUT: there are many similarities of processes & principles across different communities,
and I’ll stress them in this course.
PROBLEM: What is a community?
Many ways to define community  reflect different approaches & types of questions
Morin & most other texts: define community as a collection of spp coexisting together in space & time.
This is:
 very weak, too general and misleading.
 ignores what species do.
This definition is also confusing and ambiguous because it leaves open these questions:
1. What is coexistence?
-
Just being there?
-
Interaction?
-
How long?
-
Species evolve – so what units are coexisting?
 Do spp do more than just ‘coexist’?
2. What spatial scales?
-
grassland plants & how many neighbors? Forests? Open ocean?
-
Some species only use the same space infrequently: don’t co-occur most of the time, but
interact in important ways (e.g., plant/rabbit/coyote) DRAW
 Species can affect others even if they don’t share the same space most of the time.
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3. What temporal (time) scales?
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ecological events among interacting spp can differ.
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trees, seeds, insect herbivores DRAW
 Spp can affect others even if their time frames of existence are very different.
There are 3 different approaches: (see Table 1 – pgs 3-4) to studying communities that reflect different
ways to address those problems in defining coexistence and scale. These approaches also address
different questions.
For this class: we use all approaches as we address different aspects of communities.
Definition:
For this class, we will use the following definition (after TFH Allen) to avoid some of the usual problems.
An ecological community is
a) an assemblage of species that occurs together because of interactions that have led to
ecological or evolutionary accommodations among the species; and
b) an assemblage of species that may also reflect regional processes (e.g., dispersal,
disturbance and evolution beyond a local community).
Main words: interactions, accommodation, regional
Why accommodation? (presence  affects others)
Why evolution?
e.g. Competition
NOW
Exclusion of weak competitors
Changes in competitive relationships
(‘The Ghost of
Competition Past’)
PAST
Extinctions (reduce# of spp)
Character displacement  interactions  use different
resources (note: this can be very fast- in only a few
generations)
Why regional?
Dispersal different effects? not evolutionary?
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With this new definition, there are no problems viewing the coyote-rabbit-plant interactions as in the
same community  they form a chain of interactions even though they use both space and time
differently.
Major Themes of Community Ecology:
- Mechanisms/processes that determine membership (especially interactions)
- Patterns: arising from mechanisms
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TABLE 1: NOTE: I’ve circled the important features of each approach that you should know and think about.
Assemblage Approach
Functional Group Approach
Interactions Approach
Groups:
Main Qs relate to:
Combines elements of assemblage &
Interactions among species
Broad groups:
species approaches to:
Taxa (birds, butterflies…)
Trophic level (plants,
Uses: subsets of spp (fn gps = guilds) based
carnivores…)
on roles in communities or by morphology,
OR
physiology, etc.
Particular taxa (eg, hawks) w/in
groups
e.g., N-fixing plants, encrusting algae, sapsucking herbivores, parasites on flies….
Focus
Compare taxa (groups of spp)
among samples
Assess
- Group traits: S, biomass…
- Species ID &/or abundance
- Compare different fn gps (properties,
interactions, responses, etc.)
- Responses and roles of the fn group(s) and
NOT the component species
Mechanisms (= processes) that allow
membership in local community
- Species’ responses to other spp
(over time &/or space)
General methods
Field (tactics)
Analyses
Observations (mostly)
Experiments (some)
between
groups
Statistics - quantify differences &
similarities of samples:
classification, ordination; some tests
of hypotheses
Observations (some)
Experiments (some)
between fn gps
Statistics a) classification/ordination
methods to help id the fn groups and
compare samples/communities; b) test
hypotheses about differences betw/ fn gps.
Experiments (mostly)
Observations (some)
among
species
Statistics - test hypotheses re
interactions and processes
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Strengths
- ID broad patterns between
- Can ID either patterns and mechanisms
different communities or samples
(depending on the specifics of a study)
- May ID some mechanisms (if do
expts or id key features of the
environment that result in
particular communities patterns)
What do we learn?
Limitations
- Emphasizes that spp play active
role in community
- Spp don’t occur in rigidly
defined groups
- Spp may belong to several
communities
- Group (taxon) patterns vary in
space & time
- At any time/place – spp present
are subsets of potential members
 many more spp can live there
than do.
- Properties of the group as a whole can
swamp those of individual/particular
species.
- Largely descriptive
- Groups can be defined by different
functions and this could lead to different
groupings.
- All spp are assumed equal
- Ignores what spp do
- Cannot id mechanisms w/in
community
- Can ID mechanisms of
membership in community (not
just pattern)
e.g.: Properties of groups (such as
biomass) may be constant even though
the particular species may vary
considerably [see reading by Hay about
species as ‘noise’ for wk 2]
The composition of groups may change
if different criteria are used (e.g.,
defining spp according to resource use
yields a different grouping than if use
physiological criteria). Which is right???
Both, but that leads to some
complications.
- membership in a community may
be active, & due to:
a) population processes (incl some
behaviors) of species that affect
b) interactions among species, &
c) evolution in response to other
species.
OR smts:
- d) dispersal
- What is an interaction?
- How strong an interaction is
needed to affect other spp or
community properties?
- Reductionist  Detailed expts on
only a few spp (usu the most
common)
 ‘Cartoon” of communities
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(See - pg 11 of supplement).
In addition: there is often a big difference between ‘textbook’ science and ‘frontier’ science. In
community ecology, I do as much ‘frontier’ science as possible – which means that ideas are
changing/evolving as we get more information.
FOR this class: we use examples from terrestrial, marine and freshwater communities. I’ve selected the
best examples to make each point, regardless of the system.
FINALLY: I have a political motive here because everyone needs to be concerned about biodiversity
and because biodiversity and community ecology are closely linked.
I want to make sure that you (and I) work hard so that you are scientifically literate re community
ecology. Some comments [from an Op-Ed piece by Brian Greene in the New York Times (6/1/08)] about
science in your life and about science and social responsibility:
- It is crucial to cultivate a “general public that can engage with scientific issues; there’s simply no other
way that as a society we will be prepared to make informed decisions on a range of issues that will shape
the future.”
- Too often “our educational system fails to teach science in a way that allows students to integrate it into
their lives”.
- “…science can play” a powerful role “in giving life context and meaning”.
- “Science is a way of life. Science is a perspective. Science is the process that takes us from confusion to
understanding in a manner that’s precise, predictive and reliable — a transformation,”
- “…science is still widely viewed as merely a subject one studies in the classroom or an isolated body of
largely esoteric knowledge that sometimes shows up in the “real” world in the form of technological or
medical advances.” Or else people “feel that the science itself is just not relevant to their lives.”
Knowing about science is a social responsibility.
 This course deals with material critical to major current issues in your lives and our society,
especially:
Biodiversity (by definition of community ecology)
Conservation
Climate Change
 We don’t discuss solutions, but you need to know the deep issues behind the problems.
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Basic Concepts in Community Ecology & about Biodiversity
Basic questions in Community Ecology/ Biological Diversity (mechanisms  membership of species
in a community):
1. Community Boundaries
2. Do communities recur in space and/or time? How similar are they?
3. Why do different communities have different numbers of species?
4. What are the patterns? What causes them?
5. What is the role of history vs. current interactions?
6. What is the role of regional vs. local processes?
7. What are ‘emergent’ properties of communities (e.g., stability, productivity, succession) and how
are they affected by species diversity, or by particular species?
Major Themes:
1. Species diversity (richness)
2. Interactions
3. Change
4. Scale: space, time (implicit)
Tactics – how the course is structured:
1. Community Patterns = assemblages, S, similarities, diversity, food webs, roles
2. Short-term Dynamics = multi-species interactions
3. Emergent Properties (linking structure   dynamics): stability, coevolution
4. Long-term Dynamics = change, evolutionary patterns, history (succession, evolution, climate
change, site change [e.g., disturbances], future changes
Finding Community-level Generalizations is hard:
o Spp. have different evolutionary histories
o Spp. have different ecological roles
o Community patterns and processes may vary with
- Type of system (terrestrial or aquatic; abiotic properties (e.g. altitude, climate))
- Stage of community development (time since disturbance).
- Nearby communities  regional vs local impacts

In addition: there is often a big difference between ‘textbook’ science and ‘frontier’ science. In
community ecology, I do as much ‘frontier’ science as possible – which means that ideas are
changing/evolving as we get more information.
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