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Community ecology
Lecture 9
Principles of Ecology
College of Forestry, Guangxi University
Eben Goodale
Let’s discuss the midterm exam
• Average grade was 80
•
•
•
•
•
•
•
•
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3 A (93+)
1 A- (90-93)
7 B+ (85-90)
3 B (80-85)
4 C+ (75-80)
2 C (70-75)
1 D+ (65-70)
3 D (60-65)
1 F (< 60)
Discussion about Isack and Reyes
(1989)
• This is a fascinating（迷人的） mutualism
（共栖）.
• What data do they show to establish that both
sides (humans, honeyguides) benefit?
• What data do they show to establish that
honeyguides communicate（联系）
information about the hive（蜂巢） to
humans?
Back to our
levels slide
Characteristics of
Individuals?
Cold resistant, drought resistant, etc.
Also, life history strategies (long life,
many offspring) apply to individual.
Back to our
levels slide
Characteristics of
populations?
Abundance, distribution, demography.
Life history traits also apply (the average
values of the individuals in the population)
Back to our
levels slide
Community
Species interactions:
-/ - Examples?
+ / - Examples?
+ / + Examples?
Today’s lecture
• What is a community?
– How do we know how many species?
– Species relative abundance and diversity
– Interactions of more then two species
– Special species
• How do communities change over time?
– What is a community: a debate
A community is an association（群组） of
interacting species inhabiting（栖息） some
defined area.
A coral reef community
What is a community?
• Is a community a
group of species
working together, a
“super-organism”
（超有机体）?
• Are community types
very distinct（不同）
from each other?
• This view held by the
ecologist Clements.
Frederick Clements
(1874-1945)
Community
Type A
Type B
Type C
#s
of
individuals
Environmental gradient（环境梯度）
What is a community?
• Or do communities grade
into each other（平缓
地）, without obvious
types.
• In this view, the members
of a community do not
work together.
• It just happens that they
live together at a certain
time and place.
• This view held by the
ecologist Gleason.
Henry Gleason
(1882-1975)
No community types
#s
of
individuals
Environmental gradient
Generally, ecologists today are more on the side of Gleason, although
there are some cases of discrete community types.
Back to our
levels slide
Moving on to the community:
Characteristics of the community:
1) Number of species
2) Species relative abundances
（相对多度）
3) Species diversity（多样性） (1 + 2)
Species number
G. Evelyn Hutchinson
Yale scientist
In 1958, write “A Homage to Santa Rosalia,
Or Why are There So Many Kinds of Animals?”
In limestone caves below a reliquary (church) in Palermo, Italy, Hutchinson
found 2 species of waterbugs. Why weren’t there 20 species, or 200?
Species number
• How many species are there?
To determine species number, need to
sample
These kind of curves
Called species accumulation（积累）
curves.
Species
number
Idea is that after a certain
Amount of time sampling,
you get less and less return
in your effort (fewer new species),
so should stop.
We want to see the line come
to an asymptote（渐近线） (level off).
Sampling effort
To determine species number, need to
sample
Which group of species here has the largest number?
Relative abundance of species
Community a
Species uneven in
abundance
Community b
This relationship can be shown
on a rank-abundance curve
Species even in
abundance
Curves for
Actual communities
Proportional
abundance
Abundance rank (#1, #2, #3…)
Relative abundance of species
Generally,
in any typical
community
there are some
very rare species
some very abundant
species, and most
species are average
Note that this axis is logarithmic（对数）
and thus this pattern is called the
“log-normal” distribution
Frank Preston 1896-1989
Spent his life as a engineer for glass manufacture
But also wrote 4 very influential scientific papers
Relative abundance of species
Lognormal distributions
of forest birds:
real data
Frank Preston 1896-1989
Spent his life as a engineer for glass manufacture
But also wrote 4 very influential scientific papers
Species Diversity Indexes
B.
Shannon-Wiener Index（多样性指数）
H  Proportion of the
Number of species
in the community
ith species
s
H     p loge p
i
i
i1
Claude Shannon
1916-2001.
Mathematician
famous for information
theory
natural
logarithm
s
H     p loge p
i
i1 i
speci
es
#
%
log
(%)
%
log
(%)
1
21
0.84
-0.17
-0.15
2
1
0.04
-3.22
-0.13
3
1
0.04
-3.22
-0.13
4
1
0.04
-3.22
-0.13
5
1
0.04
-3.22
-0.13
25
speci
es
-0.66
H' =
0.661
H' =
1.609
#
1
5
0.2
-1.61
-0.32
2
5
0.2
-1.61
-0.32
3
5
0.2
-1.61
-0.32
4
5
0.2
-1.61
-0.32
5
5
0.2
-1.61
-0.32
25
I expect you to be able to calculate this index.
-1.61
Dealing with more than 2 species
(linear)
Interactions can be linear…
or circular.
(circular)
Dealing with more than 2 species
There can be indirect interactions.
Trophic cascade（营养级）
Dealing with more than 2 species
Food webs（食物网） and interaction webs show interactions between many species.
Food webs and trophic cascades
Primary reading for
Next lecture:
“Ecological meltdown
（危机）
in predator free
forest fragments”
Can get very complicated! Estes et al. 1998
What happens to
communities
when big cats
removed?
Special Species（特有种）
• Species that have a major role in a community
– Dominant species.
– Keystone species.
– Ecosystem engineers.
In network（网状） structure,
some species may be
more important than
others.
Special Species
• “Dominant” species
– Are those species in a
community that are most
abundant or have the
highest biomass（生物量）
– Exert powerful control over
the occurrence and
distribution of other
species
A salt
marsh
Redwood
forest
A mangrove
forest
Special Species
• Keystone species（关键种）: one that exercises a
large amount of influence on the community
compared to its biomass.
Sea otters: top
of the trophic cascade
Fig trees:
fruit for animals
thoughout the year.
Pisaster starfish:
By predation keeps
diversity in system
(lowers numbers of
good competitors)
Special Species
8
Number of plant
species
• Foundation species act as
facilitators that have
positive effects on the
survival and reproduction
of some of the other
species in the community.
• Ecological engineers （工
程师）exert their
influence by causing
physical changes in the
environment that affect
community structure.
6
4
2
0
Salt marsh with Juncus
(foreground)
With
Juncus
Without
Juncus
Special Species
The beaver as
a ecosystem
engineer
Today’s lecture
• What is a community?
– How do we know how many species?
– Species relative abundance and diversity
– Interactions of more then two species
– Special species
• How do communities change over time?
Community Dynamics（群落动态）
Are the physical and biological characteristics
of a community static?
• Famous example of
dunes near a lake.
• Sand continually blown
ashore and gradually
dunes get bigger.
• Walking away from the
coast one encounters
older and older
communities.
Community Dynamics
Are the physical and biological characteristics
of a community static?
• Or, glacier（冰川）
retreats over time.
• Walking away from
glacier one encounters
older and older
communities.
• How do communities
respond to disturbance?
What is a disturbance（干扰）?
Mt St. Helens 1980
Where do human
activities fit?
What is a disturbance?
Primary vs. secondary succession
（初级和次级演替）
• Primary succession:
Colonization of habitats
devoid of life (e.g.,
volcanic rock).
• Secondary succession:
Reestablishment of a
community in which
some, but not all,
organisms have been
destroyed.
After a glacier: primary
An old field: secondary succession
What does succession lead to?
• Clements: succession is
like the development of
an organism with a birth,
a middle age and an end
stage.
• The end stage is a “climax
community（顶级群
落）”.
Recently retreated glacier
Dryas mat (~ 30 years)
Frederick Clements
(1874-1945)
Alder thicket (50 yrs)
Spruce forest (~ 100 yrs)
What does succession lead to?
But actually not so
simple: there can be
multiple different kinds
of climax communities
depending on
environmental
conditions
Muskeg (~ 200 yrs)
Climax community in
wetlands
Mature
hemlock forest
(~ 200 yrs)
Climax
community on
slopes
And how does succession progress?
• Clements: the different
species help each other.
• For example, the first
“pioneer（先锋种）”
plants help make soil
richer for what comes
later.
Frederick Clements
(1874-1945)
But again perhaps more complicated
Connell and Slatyer 1977
• Facilitation（简易化）
can occur: early
organisms help later
ones.
• Tolerance （忍受力）
can occur: early
organisms don’t have
much of an effect on
later ones, but later
ones live longer.
• Early organisms might
even inhibit（抑制）
later ones under some
circumstances
But again perhaps more complicated
In general, facultative interactions dominate early in succession,
And inhibitive late in succession.
Sometimes succession gets
“stuck（困住)”
This is when typical succession is altered, so that never get to
climax community. Some parts of community act to inhibit
establishment of others.
Fernlands in
Sri Lanka
stop succession
Coral reef
overgrown by algae
This particularly
happens due to
human disturbance
and is called a
“regime shift”.
Without intervention
these regime shifts
can be permanent
（永恒）.
Over time,
community grews more complex
More species
More nutrients
Disturbances range in
Frequency（频率), intensity（强度）
and extent（范围）
• So far we have talked
about large
disturbances.
• But small disturbances,
like large trees falling
over in the rainforest
are also important.
Disturbances range in
frequency, intensity and extent
• My wife Uromi studies the pioneer
trees that live in tree fall gaps.
• Just like weeds, these trees like
light, grow fast, and have many
small seeds.
• In contrast, “late-successional（推
迟演替）” species are more
tolerant（忍耐） of shade.
• Tree fall gaps create a mosaic（镶
嵌） of places in the forest of
different successional ages.
Disturbances also
different time scales
Glacier bay
~ 1500 years
Inter-tidal
succession
1.5 years
Old temperate field
~ 150 yrs
Sycamore Creek
60 days
Where we are going
• Today we talked about how to
measure species diversity, and how
it changes in one place over time.
• On Tuesday Apr 28 we talk about
how species diversity varies across
big regions (high near equator) and
in islands of different sizes (a famous
theory called “island biogeography”).
• On Saturday May 2 we talk about
how species diversity varies within a
region. Why is it higher in some
communities than others?
Island Biogeography Theory
Heterogeneous Habitat Theory
Homework
• Chapter 18 summary
• Primary reading: Terborgh et al. 2001. Prepare
to discuss in class next lecture.
Key concepts
• An old viewpoint is that a
• We use species
community is a super-organism
accumulation curves to
made up of species that work
estimate how many species.
together.
• Species differ in their
• It is true that a community has a
abundances; species
predictable succession after
diversity indices combine
disturbance, and that facilitation
the number of species with
(species helping each other) is
their abundance.
common early in this process.
• Some species may
particularly important role • But it also true that there are
multiple endpoints possible, and
in a community.
that species can also not help
(tolerate) or even inhibit each
other.
Tips for listening, reading
Listening:
http://www.artsci.utoront
o.ca/current/advising/ell/
pdfs/ELL_Strengthening_Li
stening_Comprehension.p
df
Reading to write:
http://www.artsci.utoront
o.ca/current/advising/ell/
pdfs/Reading_to_Write_P
reviewing.pdf
OK, so communities differ in species
diversity … but why?
• We will come back to this question, but today
just introduce an idea called “heterogeneous
environmental theory”.
• First developed by Robert MacArthur
• Came out of the idea of niches, and the
competitive exclusion theory
• MacArthur generally
Robert MacArthur
Student of Hutchinson
worked with birds
Died at age of 42 in 1972
Heterogeneous Environments:
Opportunities for Specialization
Remember that each
species has a niche –
Its occupation.
In these birds that
MacArthur studied
each species uses different
part of the tree.
And no two species can have the same niche
“competitive exclusion principle”.
We talked about this when talking about competition
So it makes sense that
the higher the canopy
of forests, the more
species there will be.
This was MacArthur’s
first result (1958)
Further experiments demonstrate that
vertical heterogeneity increases species
diversity
MacArthur hung a rope
vertically from the top
of the canopy and measured
how many times vegetation
hit the rope.
From this he calculated
vertical profiles of different
forests, and compared them
in their diversity (# of
different layers and evenness
of layers).
MacArthur and MacArthur 1961
Further experiments demonstrate that
vertical heterogeneity increases species
diversity
He found that
the more diverse
the vertical vegetation
the more species
of birds were present
MacArthur and MacArthur 1961
Species diversity
Is a fascinating subject
and we will look at it
In more detail later
In the course.