Download Community ecology – interactions between individuals of different

Community ecology – interactions between individuals
of different species.
Community ecology – interactions between individuals
of different species.
Competition – who does this and why?
Niche concept: if individuals of two species rely on the
same resources…
-- and we presume that the two species are not identical
in their ability to exploit those resources
-- then one species will out-compete the other
-- and the one who acquires resources less well will be
driven to extinction (competitive exclusion)
1. Competition leads to local extinction
Competitive exclusion in two species of Paramecium
Number of individuals
400
300
Paramecium
aurelia
200
100
0
Paramecium
caudatum
0
5
10
15
Time (days)
Figure 53-3a Biological Science 2/e ©2005 Pearson Prentice Hall, Inc.
20
25
Number of individuals
using resource
When niches overlap completely
Species 1: Strong competitor
Species 2: Weak
competitor, driven to
extinction
Niche range
Figure 53-3c Biological Science 2/e ©2005 Pearson Prentice Hall, Inc.
Caveats:
• What if one species is better at obtaining resource A and
the other at resource B?
• Of all the resources needed, for how many is there direct
competition?
• For a given species, the most critical resources for
population 1 may not be the most critical for population 2.
• What does it mean to rely on the same resources (how
similar must the resources be such that they are “the
same”)?
When there is partial niche overlap 
2. Competition leads to niche partitioning
Suppose the resource is a place to attach on an intertidal
zone rock. Who gets that resource?
Find two barnacle species, but distributed in distinct zones.
Chthamalus in upper intertidal
zone – it’s realized niche.
Mean tide level
Balanus in lower intertidal zone –
competitively excludes
Chthamalus
The lower intertidal zone is better. If you remove Balanus,
Chthamalus will occupy its fundamental niche.
Percent survival
Remove-Balanus results:
80
Chthamalus survival is
higher when Balanus
is absent
60
40
20
0
Competitor
absent
Competitor
present
Figure 53-6b part 2 Biological Science 2/e ©2005 Pearson Prentice Hall, Inc.
Number of individuals
using resource
If niches do not overlap completely, weaker competitors use
non-overlapping resources.
Species 1
(strong
competitor)
Species 2 (weak
competitor)
Fundamental
niche
Realized niche
Niche range
Figure 53-4a Biological Science 2/e ©2005 Pearson Prentice Hall, Inc.
3. Competition leads to evolutionary change
There is also the possibility of longer-term evolutionary
change in populations where there is competition and partial
niche overlap 
Sympatric vs allopatric populations of Darwin’s finches….
(Work by Grant & Grant)
Number of seeds
consumed
For any species, there is variability in each population.
In this case, beak size determines preferred seed size.
Some individuals in the population are born with larger
beaks, others with smaller beaks.
Seed size
Figure 53-2a Biological Science 2/e ©2005 Pearson Prentice Hall, Inc.
When there are sympatric populations of two different
species, and partial niche overlap for preferred seed size,
competition is most intense for seeds of intermediate
size.
Number of seeds
consumed
Species 2
Species 1
Seed size
Medium-beak birds of species 1 and medium-beak birds of species 2
have lowest fitness, and the distributions skew.
Over time, competition can cause niche differentiation,
or niche partitioning.
Species 1
Species 2
Number of
individuals using
resource
Time 1
Niche range
Natural selection favors individuals
Species 1
that do not compete.
Number of
individuals using
resource
Species 2
Time 2
Niche range
Figure 53-4b Biological Science 2/e ©2005 Pearson Prentice Hall, Inc.
Niche partitioning can also lead to speciation
What happens if
the 1b population
“knows” that it
should not mate
with an individual
from population A.
Why not?
Consider the
Wombo…
Species 1,
population A
Species 1,
population B
Niche partitioning can also lead to speciation
Geographic range
A
B
In sympatric zone,
pop A matings with
pop B – get some
offspring in pop B
with small wombos.
These offspring get
outcompeted.
Species 1,
population A
Size of wombos
Species 1,
population B
Size of wombos
So it is an advantage to pop A and pop B
individuals to recognize the difference and avoid
matings between populations.
To help with recognition, small differences (often
unrelated to function) can develop quickly into
large differences so each can tell who is from
which population.
This leads to “character displacement” and the
development of a “prezygotic isolating
mechanism”.
Speciation (i.e., evolution) has been observed over
a period of some years on the Galapagos islands.
Rapid development of species isolating
mechanisms occurs in zones of sympatry
where it is likely that intermediate offspring
can be produced.
An incubator for this sort of speciation occurs
on islands, where you have successive waves
(say once every 300 years!) of mainland
immigrants of the same species.
One reason islands have so many endemic species
Immigration 1
Population expands
Immigration 2
Character Displacement
& “Adaptive Radiation”
S. America:
Iguana fills large lizard niche
Anolis fills small lizard niche
BUT… Anolis is a much better colonizer!
What happens when colonists raft from South
American to lizard-unpopulated islands in the Antilles?
Only Anolis, the better colonizer, survives the raft trip.
The scenario...
1. Anolis immigrants arrive. After 100's of years,
small Anolis everywhere.
2. By genetic drift and adaptation, the Anolis
distribution becomes broader than that of their
friends left behind on the mainland. In particular,
some larger individuals do well (there is no
competition with iguanas).
The scenario...
3. New wave of small Anolis arrives and establishes a
population “beach head”. They compete with old
Anolis inhabitants. The larger members of the old
Anolis population have less competition, though,
because the new immigrants are all small.
4. The large members of the old Anolis population
undergo rapid character displacement and develop
species isolating mechanisms to not breed with the
newcomers.
And this is what you end up with…
Types of Competition
There are a number of variations on the competition
theme:
Consumptive – take all nutrient resources
Pre-emptive – get there first and occupy
Overgrowth – overwhelm other species with your
numbers
Chemical – make a space unsuitable for other species
Territorial – defend a space
Encounter – opportunistic defense of transient resource
Consumptive competition: Organisms consume the same resources.
These trees are competing for water and nutrients.
Preemptive competition: Individuals occupy space, preventing access to
resources.
Space preempted by these barnacles is unavailable to competitors.
Overgrowth competition: One organism grows over another.
The large fern overgrows other individuals and shades them
Chemical competition: One species produces toxins that
negatively affect another.
No grass next to or under the Salvia shrubs.
Territorial competition: Mobile organisms protect a feeding or breeding
territory.
Grizzly bears drive off black bears.
Encounter competition: Organisms interfere directly for access to specific
resources.
Spotted hyenas and vultures fight over a kill.
Other interactions…
1. Neutral
2. Competition✔
3. Predator-prey
4. Parasite-host
5. Commensalism
6. Mutualism
7. Symbiosis
Often evolution
proceeds in this
direction.
Neal Smith experiments in Panama:
Involves -- Parasitism, mutualism, predation
Cast of characters
•
•
•
•
oropendola
cow bird
bot fly
wasp
Oropendolas and their nests
The Cow bird is a brood parasite:
o Lays eggs in host nest
o Cow bird eggs develop faster
o Cow bird chicks are larger and more
aggressive, and outcompete Oropendola chicks
Natural Selection should favor Oropendolas that
recognize Cow bird eggs and throw them out of
the nest.
Cow Bird Eggs
Cow bird
Find shell fragments under Oropendola nests.
Some CB eggs are good Orop. mimics, others
not so much.
Q1: Why don’t all CBs make mimetic eggs?
Smith expts: Pole, ladder, clip – mess with Orop.
Nests.
Two more examples of mutualism (next two slides;
from your text)
Mutualism between ants and acacia trees
Plant makes goodies for ants; ants defend tree from other
insects and vertebrate herbivores.
Mutualism between cleaner shrimp and fish
Figure 53-15b Biological Science 2/e ©2005 Pearson Prentice Hall, Inc.
Mutualisms – Human skin microbiome
1. 1st line defense against some pathogens.
2. Many health effects.
Changes in floral composition associated with…
• Obesity
• Dermititis
• Periodontal disease
• Bacterial vagninosis
• IBS
• Autoimmune diseases
Microbiome seems to be personal:
Only 13% of bacterial species on the palm of
your hand are shared between individuals.
Community is variable over time. There are
more transient and more permanent species.
Compared to gut flora, skin is a more diverse
community with less long-term stability.
In “primitive” cultures (Amazonia) people
have 2x the variability re: N. Americans.
Ant – Tree mutualisms
About ants
13000 species!
15% of terrestrial
biomass!!
Ants provide defense
of trees.
Chafer beetle landed on a wild cotton
plant (U.S. Southwest). Mutualist ants
took care of herbivore.
Acacia trees have hollow thorns that provide
food and shelter for defensive ants.
Why don’t cheater ant species take Acacia tree food
and not bother providing defense?
Or what about herbivorous insects that evolve rapid
strike techniques to get food before the defensive
ants can mobilize?
Acacia food pods full of protease inhibitors.
Completely shuts down gut of herbiverous
insects.
The Acacia guard ants (Pseudomyrmex
ferruginea) make the protease chymotrypsin-1,
which is not affected by the protease inhibitors!
Mutualisms can change quickly
Long term study in Africa:
Put fence around trees to preclude
herbivore predation.
In 10 years the host trees stopped making
food resources for defensive ants.
Relative to predator-prey interactions…
How come herbivores or other predators don’t eat
everything?
1. Predators and prey keep each other in check
(muscles & crabs, lynx & hares, starfish &
mussels)
2. Plants are just not that nutritious.
3. Plant-defense hypothesis (beavers &
cottonwoods)
Correlation between predation rate and prey
defense: Mussels vs crabs keep each other in
check.
Mussel attachment
strength is higher when
predation is high
Shell Thickness (mm)
Attachment strength (N)
40
Mussel have thicker
shells when predation
is high
30
20
10
1.8
1.2
0.6
0.0
0
Low
High
predation
predation
Site type
Low
High
predation
predation
Site type
As before: The hare-lynx populations cycle every
11 years, on average.
The peak in the lynx population lags behind that of
the hares.
4000
25
2000
15
500
10
100
5
0
0
0
Hare
10
20
30
Years
Lynx per 100 km2
Hares per km2
20
40
Lynx
Experimental manipulations imply that ”release from
predation” is a major contributor to population control.
Results:
Hare density relative to
controls
15
Hare population
doubles when
predators are
excluded
10
Hare population
triples when food
is added
5
0
Control
Predators
excluded
Food
added
Predators excluded,
food added
Hare
population
increases
over 16 times
when
predation is
low and food
high
Conclusion: Hare populations are limited by both predation and food availability. When predation and food
limitation occur together, they have a greater effect than either factor does independently.
Pisaster clobbers muscles, but without the starfish,
muscles take over everything (& species diversity goes
down).
Number of species
present
20
Keystone predator present
15
10
5
Keystone predator
absent
0
1963 ’64 ’65 ’66 ’67 ’68 ’69 ’70 ’71 ’72 ’73
Year
Figure 53-17 Biological Science 2/e ©2005 Pearson Prentice Hall, Inc.
Adaptive responses to predation: Resprouted trees make more
defensive compounds. Increased toxicity to vertebrate herbivore
comes at an energetic cost to the plant, though.
20
Salicortin concentration
(mg/g dry mass)
15
10
5
0
Control trees
Cut and
resprouted
trees
BUT…
…herbivorous insect larvae actually like the toxins, and
survive longer on resprouted trees. So it is not so simple!
Larval survival time (sec)
400
300
200
100
0
Larvae from
control trees
Figure 53-12c Biological Science 2/e ©2005 Pearson Prentice Hall, Inc.
Larvae from cut
and resprouted
trees
Question: Do identical communities develop in
identical environments?
Clements: Identical environments = identical communities
Gleason: Identical environments +/- identical
communities
Start with 12 sterile
ponds
At end, same
communities of
plankton in each?
Outcome: more Gleason than Clements
Figure 53-18 part 1 Biological Science 2/e ©2005 Pearson Prentice Hall, Inc.
The composition of communities can, to some
extent, be predicted based on abiotic factors.
BUT…. there are equally powerful factors
making things more random:
Who are the first colonists to arrive after a
disturbance?
Has some unpredictable event influenced a
keystone species?
Historical record shows that different species
compositions occur over time in the same habitat.
So “the” community is not the same over time.
Of course, not just any group of organisms can fit
together!
Often there are predictable succession stages
leading to a climax community.
plowed field – no
plants
Weeds colonize 1st
Weeds 
Pioneering species
Bigger weeds replace
Bigger weeds 
Early successional
community
Shrubs replace
Shrubs 
Figure 53-20 part 1 Biological Science 2/e ©2005 Pearson Prentice Hall, Inc.
Mid-successional
community
Shrubs 
Mid-successional
community
Small tree invasion
Short lifespan trees 
Latesuccessional
community
Bigger trees invade
Cedar/hemlock
in the PNW.
Figure 53-20 part 2 Biological Science 2/e ©2005 Pearson Prentice Hall, Inc.
Climax
community
Primary succession: major disturbance wipes out
everything in a region, including soil.
Secondary succession: at least soil is left, so seeds
blown into the area can germinate right away.
35 years ago last week!
Mt. St. Helens succession
1980 sterile pumice plain
1982 first lupine plant
1986 16,000 lupine plants
1992 first insect lupine herbivores
2001 first alders
2005 all small mammals originally present
have returned.
Some small mammals like deer mice
survived eruption in burrows, and 12/14
species of amphibians survived at
bottom of Spirit Lake & stream bottoms.
Succession has not been exactly the same
everywhere.
The lupine predators lead to a dynamic change in
the lupine population, but this is different for upland
and wetter regions.
At the outset, how sterile was it?
1 billion cubic meters of material ejected, plus
avalanche (the largest recorded in human history)
covered 13 miles to a depth of 300 feet.
On top of this, pyroclastic flow at 800 °C.
Not only completely sterile, but NO SOIL!
After a few years wind and run-off born dirt 
enough soil for the 1st lupine plant.
Why a lupine?
They are among the very best
colonizers.
Dispersal filter: Who
can travel & survive
the trip?
Environmental filter: Who
can tolerate primitive
conditions at new spot?
Stable (+/-)
population
Community
interaction filter:
Who can resist
predators?
Plants with wind-born seeds are great at dispersal,
but lupines unique in passing the environmental
filter.
1. Very drought resistant: They
make extremely long tap roots.
2. Need almost no soil nutrients:
a) They have nitrogen-fixing
root nodules.
b) The roots secrete carboxylic
acids which erode elemental
phosphorous from rock.
The only plants to start growing after the lupines
were some grasses that could grow on dead lupine
plants!
The lupines provided the only carbon source for
other plants.
BUT…
9 years after the 1st lupine plant establishes,
the population starts going through crashrebound cycles every 3-5 years.
Before crash
After crash
Crash due to lupine-specific insect herbivores.
Largest crashes are in lowest-density lupine
patches!
High-density lupines compete with each other, so
any one plant has fewer nutrients.
Insects pick
sparse plants
with higher
concentration of
nutrients.
When lupines have made enough soil, willows can
start.
Willows critical
in succession –
they add a
vertical
component to
habitat.
Vertical habitat:
cover, shade,
bird perches,
nest sites.
… but willows attract a host-specific weevil,
which kills willows in upland habitat, where the
trees are more water-stressed. (Much less
mortality in wetter areas).
surprisingly…
More rapid succession where willows are the most
impacted:
Weevil attack  willow drops stems  more
carbon goes into the soil!
In a primary succession environment, with a small
number of species, the community interactions are
few and direct.
Interaction Strength (I.S.) = per capita effect of one
species on another.
Primary succession: high I.S.  unstable
interactions.
Succession greatly slowed with low-density highcrash lupine sites, and increased in drier willow
sites.
Bottom line: Community interactions (consumer
effects) affect succession.
To what extent is succession predictable?
1. Some species are better colonizers.
a) Fewer nutrient/shade requirements
b) Faster growing
c) Drought tolerant
d) r-selected
1. After the pioneer species live for some generations, the
physical environment is changed by those species.
a) Soil stabilization
b) Nutrients added
c) Habitat suitable for different sorts of decomposers,
pollinators
1. Climax cedar/hemlock: most shade tolerant, life longer &
grow taller than Doug fir!
Natural selection has produced some species who are
specifically adapted to be colonizers!
Such species are experts at dealing with “disturbed”
habitats.
Chaparral – seeds
only germinate after
a fire, and fire is part
of the natural
ecosystem in the SW
deserts. (fire scars
the seed coat and
allows water to enter)
Reconstructing history from fire scars
Number of fires per
century
50
40
30
20
10
0
0
400
800
1200
1600
2000
Years A.D.
Natural selection can operate on regular, predictable abiotic
events. Note that predictability not on a human life-span scale,
is often overlooked.
So…. Clements and Gleason are both right!
Community composition, given abiotic factors, is
somewhat predictable and stable – but there are
many factors that influence the details thus there
is no one community that is guaranteed.
Three distinct successional pathways occur in Glacier Bay.
PATHWAY 1 (Lower Bay)
Alder patches; extensive
spruce
Early-mid successional
PATHWAY 2 (Upper Bay)
Extensive alder; some spruce,
cottonwood
Mature hemlock, spruce
Spruce
Hemlock arrives
Cottonwood
Alder
Mid-late successional
Climax
Mature spruce; some alder
Alder dies back
No hemlock?
?
Early successional
Mid-successional
PATHWAY 3 (Uppermost Bay)
Extensive alder; scattered
spruce
Alder dies back
Late successional
Mature spruce, cottonwood; some
alder
Climax
No hemlock?
?
Early successional
Mid-successional
Late successional
Figure 53-21b Biological Science 2/e ©2005 Pearson Prentice Hall, Inc.
Climax
Hemlock
Some communities are more diverse than
others.
large number of species
High diversity = (richness), but not a great
deal of uneveness.
Diversity highly correlated with stability
(relative to factors that lead to disturbance).
Agricultural monocultures are the most vulnerable!
Desert & high latitude communities are very fragile.
Number of vascular plant
species per 10,000 km2
Diversity declines with latitude
10,000
8000
6000
!
4000
2000
0
0
10
20
30
40
50
Latitude (degrees North or South)
Figure 53-23 Biological Science 2/e ©2005 Pearson Prentice Hall, Inc.
60