Patterns in Species Diversity
Lecture 10
Principles of Ecology
College of Forestry, Guangxi University
Eben Goodale
Let’s discuss Terborgh et al. 2001
No class Saturday… change syllabus again!
MAY
APRIL
Date
Topic
Reading
March 17
March 21
Introduction
The Physical Environment and the World’s Biomes
1
2, 3
March 24
Coping with Water, Temperature and Energy
4, 5
March 28
March 31
April 4
April 7
Evolution and Ecology; Life Histories
Behavioral Ecology
Holiday
6, 7
8
Population Distribution, Abundance, Growth and Regulation
9, 10
April 11
April 14
April 18
Population Dynamics and Competition
Midterm Exam
Other species interactions
11, 12
April 21
April 25
April 28
May2
May 5
Holiday
The Nature and Change in Communities
Biogeography and Species Diversity
Holiday
Ecosystem Productivity and Energy Flow
13, 14,
15
16, 17
18, 19
20, 21
May 9
Food Webs and Nutrient Cycling and Conservation Biology
22, 23
May 12
May 16
Landscape and Global Ecology
Final Exam
24, 25
Review
• We defined a community as a group of species
living in the same area.
• We said a community has 3 characteristics:
• 1) # species
• 2) species have different relative abundances
• 1 and 2 can be measured together by species
diversity.
• Today we ask the question, Why?
• Why so many species?
Today
• Species diversity varies over big spaces:
– Species diversity is higher in the tropics.
– Species diversity on islands of different sizes.
• Species diversity within a region:
– Heterogenous community theory
– Intermediate disturbance hypothesis
• Does species diversity matter?
The Science of Biogeography
（生物地理学）
Alfred Wallace (1823-1913)
• Actually co-discover of natural
selection, but Darwin is given most
credit.
• Wallace better remembered for his
contributions to Biogeography.
• Biogeography is the study of the
variation in species composition
and diversity among geographic
locations,
• Like Darwin, Wallace traveled
widely, collecting animals as he
went.
The Science of Biogeography
“Wallace’s Line”:
•A line that runs through
Indonesia
•Wallace discovered that
the faunas on the two
sides of this line very
different.
•In fact fauna of Philippines
closer to Africa (5500 km)
than to New Guinea (750
km).
Just like Darwin discovered natural selection
But didn’t understand genetics, Wallace
discovered this line, but didn’t understand the
mechanism. What was it?
Plate tectonics（板块构造论）
Movie of changes in continents
Plate tectonics（板块构造论）
Plate tectonics: responsible for
difference in large regions of globe
• Explains Wallace’s line.
• Explains why different
lineages of plants,
animals in 6 regions of
the world proposed by
Wallace.
• Explains divergence like
that seen in flightless
birds.
Species diversity: a latitudinal pattern
（随纬度变化）
Why are the tropics so diverse?
Plants
From Willig et al. 2003
Birds
Species diversity: a latitudinal pattern
Why are the tropics so diverse?
They are
more
productive
（生产的）
They’ve been around
for longer.
They are bigger…
They are better
fitted to organisms’
biology
And more stable
In their climate
But this isn’t all: Roede (1997) lists 28
different hypotheses!
Bigger populations,
less extinction.
Species diversity: a latitudinal pattern
From Colwell and Lee 2000
The mid-domain effect … for anything that
affects organisms’ ranges, more organisms
will be found in the middle of the gradient…
And the equator is at the middle of the world.
Robert Colwell, UConn
Essentially we’re coming down to explaining these broad trends by geometry（几何学）.
Exceptions（例外） to species area
curves（种-面积曲线）
3 temperate forest zones about the same size …
But Asia has 6 X tree species of Europe and 3 X NAmerica. Why?
Exceptions to species area curves
Alps blocked the range movements of European trees!
In a way too this is a geometric solution…. Shows that length of time
Important.
Exceptions to species-area curves
• Another exception is seabirds,
which are more at high
latitudes.
• Indeed oceans are more
productive at high latitudes.
• Some evidence that
productivity matters.
• Overall summary of tropical
diversity: many theories that
are not-exclusive, and good
evidence for many of them.
Species diversity: other scales
•
•
•
•
•
Global diversity: all species
Regional diversity: plate tectonics.
Regional diversity: latitude
Regional diversity called γ
It includes two components:
– α Diversity: the # of species in any one community.
– Β Diversity: the change in what species you encounter
as you move from community to community in the
region.
Species-area curves
S  zA  c
General rule in ecology: the larger area you sample,
the more species you will find. Adding β diversity.
Species-area curve
is different on continents（大陆） vs.
islands
S  zA  c
Is Z higher or lower
for islands?
Z is higher because dispersal
Is limited to islands.
What are islands?
Lakes in a
terrestrial matrix
Islands in an aquatic matrix（基质）.
Mountains in an arid matrix
like Arizona
And what appears to be an island
changes based on organism
Dispersal ability
varies widely among
organisms
Spores / seeds of some organisms
everywhere
Very occasionally a lizard might raft（木筏） across ocean
Theory of island biogeography
.
Originally, a
theoretical model,
but has been used
for applications
for how to build
reserves – islands –
within areas of human
disturbance.
Robert MacArthur
1967
E.O. Wilson
How do new species get to islands?
• They immigrate（迁入）.
Immigration very high
when no species on
island.
Immigration
Rate
(Species
Per year)
Krakatau
Number of species on island
Important note: this theory doesn’t include
speciation on the islands.
Immigration rate hits 0
when all of the mainland
species have already arrived
What factors effect immigration?
Distance to (or isolation of) island
LARGE
Immigration
Rate
Size of island
Say this immigration line
is for a close island…
what would the immigration
line be for a far island?
Farther island, fewer immigrants
(Species
Per year)
FAR
Number of species on island
Say this immigration line
is for a small island…
what would the immigration
line be for a large island?
Larger island,
bigger target
What factors effect immigration?
One more question:
Should this be a straight line (linear（线性的）)?
Immigration
Rate
Species have different
dispersal abilities,
so ones that disperse
easily will come quickly.
(Species
Per year)
Number of species on island
How do species disappear from islands?
• Extinction（消失）
What does this
graph look like?
Extinction
Rate
Because the more
species there are,
the bigger the pool
of species to go
extinct.
(Species
Per year)
Number of species
Nothing can go extinct because there are no species!
What factors effect extinction?
Size of island
The larger the island,
the bigger population
a species has.
The bigger population
a species has, the less
chance it will go extinct.
Extinction
Rate
(Species
Per year)
LARGE
Number of species
Hence, bigger islands
have lower extinction rates.
OK, so let’s say that this
graph is for a small island.
What would the graph for
a large island look like?
What factors effect extinction?
• Distance of island
How about the distance
of an island?
FAR
Extinction
Rate
(Species
Per year)
If it’s close, more
immigrants will come
and there will be fewer
extinctions.
So if we plot a
farther island on this
plot it goes where?
Number of species
What factors effect extinction?
• Extinction
Now is this graph linear?
As there are more and
more species, interspecific
competition will increase,
so curve will look like…
Extinction
Rate
(Species
Per year)
Number of species
Theory of Island Biogeography
• Postulates（假设） that for every island, there will be
some number of species where immigration will be
balanced by extinction.
Equilibrium
Point（平衡点）
(Species
Per year)
extinction
immigration
Species on island
Level above X axis
Represents turnover（物种的数量） in species
Important point:
equilibrium does
not mean that the
same organisms
will be there year
after year…
identity of species
will change, but
number stay the
same
Theory of Island Biogeography
New larger
(more species)
equilibrium
(Species
Per year)
Species on island
Say this is the graph for a
small island
What’s the graph for a
large island?
Note that
the turnover
rate
may change
as well
Same result for a close
island
Exercise
The curves in blue shows a
island that is ______ than the
curves in black.
A)Farther or smaller
B)Farther or larger
C)Closer or smaller
D)Closer or larger
Extinction
Immigration
(Species
Per year)
Species on island
Theory of Island Biogeography
Go through this figure yourself, and make
sure you understand it…
Testing the theory:
observation
Plant species on Galapagos Islands
Teting the theory: experimentation
• Many observational studies show that island
area and isolation are major factors
influencing species number.
• But correlation does not prove causation.
• Best to do an experiment.
Experimental islands
Simberloff and Wilson (1969) fumigate（烟熏） islands and wipe out all
insects on them, then watch the number of insects that return…
Experimental islands
Close island
Far island
Experimental islands
# of species goes down
The Simberloff (1976)
took some islands
and made them
smaller…
Island-biogeography theory applied to
fragmentation and conservation
• Fragmentation（破碎化）
is a process of making
islands.
• People have used island
biogeography to develop
sets of reserves.
• Also used it to predict
how many species might
go extinct given a certain
amount of
fragmentation.
A really large scale experiment
• Biological Dynamics of
Forest Fragments
Project (BDFFP).
• Started in 1979 by
Thomas Lovejoy.
• Made experimental
fragments in Amazonian
Brazil.
As expected, size of island a big factor
• Smaller fragments
much more hit by
extinction.
• Also interesting findings
show that different
kinds of animals and
plants have very
different kinds of
responses.
From Stouffer et al. 2011
Today
• Species diversity varies over big spaces:
– Species diversity is higher in the tropics.
– Species diversity on islands of different sizes.
• Species diversity within a region:
– Heterogenous community theory
– Intermediate disturbance hypothesis
• Does species diversity matter?
OK, so communities differ in species
diversity … but why?
• “Heterogeneous environmental theory（环境
异质化理论）”.
• First developed by Robert MacArthur
• Came out of the idea of niches, and the
competitive exclusion theory
• MacArthur generally
worked with birds
Robert MacArthur
Student of Hutchinson
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” (Gause’s experiments)
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
Basically the idea is that the
More diverse an area is in vegetation,
The more niches can be fit into it =
The more species.
Heterogeneous environment theory: plants
vs. animals
• For animals, the
environment that’s
important is usually
structure of vegetation.
• What about for plants?
• Different plants may use
different types of soil
nutrients.
• And we know that soil
resources can be variable in
space, suggesting they can
help explain plant diversity.
Here one species of phytoplankton
dominants at high Si:P rations, and
one at low
Variable（变异的） conditions can also
retain diversity
• Under unvarying
conditions, competitive
exclusion occurs.
• But disruptive（破坏性）
processes such as
predation and
disturbance can alter the
course of competition.
• Remember a sea star
kept a dominant
competitor from getting
too abundant.
Variable conditions can also retain
diversity
• Or a sea palm on a rocky
tidal short that was a poor
competitor, but could
colonize areas of high
disturbance.
• Hence, variable
environments can maintain
species co-existence
Intermediate Disturbance Hypothesis
（中度干扰假说）
• This idea leads to another hypothesis explaining
diversity: the intermediate disturbance hypothesis.
• Very disturbed environments will have low diversity.
• So will very stable environments.
• Diversity will peak somewhere in the middle.
Our “K species”
will do really well
here…
But both
could exist
in the middle
r species
could do well
here
Intermediate Disturbance Hypothesis
Intermediate
Disturbance
Hypothesis tested by
Wayne Sousa
(1979), on rock tidal
communities.
Intermediate Disturbance Hypothesis
Disturbance in streams
Prairie dog disturbance
Frequent but small-scale
disturbances explain the diversity
of tropical rainforests with their
mosaic of patches at different
successional levels
Today
• Species diversity varies over big spaces:
– Species diversity is higher in the tropics.
– Species diversity on islands of different sizes.
• Species diversity within a region:
– Heterogenous community theory
– Intermediate disturbance hypothesis
• Does species diversity matter?
Stability（稳定性） and species
diversity
• Stable community = after a disturbance,
remains at, or returns to original function.
• Does species diversity influence stability?
Large experiments
of David Tilman and
colleagues in
Minnesota, USA
Stability and species diversity
First result:
Observational.
The plots with more
species were more
resistant to drought.
Stability and species diversity
Second result:
Experimentally
Manipulated species
Diversity. High species =
Overall biomass higher.
After a certain level,
Reaches a threshold
（临界值).
Species are redundant
（多余）
to each other.
Stability and species richness
• If species differ in their nutrient requirements
or niches.
• Then if disturbance comes and alters some
parts of the environment, the more variety of
species, the more likely some will survive and
do well.
• Stability is one way of measuring diversity’s
impact on community properties. We will
revisit this idea when talking about food webs.
Homework
• Chapters 20, 21 (Productivity and Energy Flow: Introduction to
Ecosystems).
• Jackson et al. 2001. This is another food web article, because
we will talk more about food webs next class (Ch 21). It talks
about how humans have impacted marine food webs.
• Questions:
– What kind of scientists wrote this article? What do they
study and what kind of data did they collect.
– What kind of marine habitats do they review?
– What’s their conclusion as to what has been the most
important human activity that has harmed these
ecosystems?
Key concepts
• Continental drift has
resulted in differences
between regions in their
species diversity.
• For many reasons, species
diversity is highest in the
tropics.
• Island biogeography is an
elegant theory that
explains why species vary
in relation to the size and
isolation of islands.
• Heterogeneous resource
theory predicts that one can
pack in more species (and
their niches) into a more
diverse environment.
• The intermediate
disturbance hypothesis
predicts low diversity at
high and low disturbance
levels.
• Diversity appears to have a
positive influence of
community stability.