Download Lect 8_Mechanisms of coexistence

LV Competition
- Only 1 (in 4) solutions yields coexistence – Is coexistence the
unexpected outcome?
- Is competitive exclusion random or does the theory indicate
some property (not identity) of the competing species that can
guide us?
 Yes, Intraspecific competition > Interspecific competition
 Two species that occupy the same niche (they are identical) cannot
coexist = Competitive Exclusion Principle
Competition as a structuring force in biological communities
Medium seed
eater
100% Overlap
Frequency
In the diet
small
large
Seed Size
Medium seed
eater
Large seed
specialist
Small seed
specialist
Frequency
In the diet
X
small
10% Overlap
weak competition
large
Seed Size
Large seed
specialist
Small seed
specialist
Frequency
In the diet
small
large
Seed Size
Niche partitioning – there is a limit on the amount of similarity
between adjacent species that results in a regular spacing
of species’ morphology
Min spacing
Frequency
In the diet
small
large
Seed Size
Competition works as a biological filter to
remove or prevent the invasion of species
into biological communities
Niche partitioning in two
desert rodent communities
Wildcat
These coexisting
Felids in Israel
Caracal
Diameters of canines for 3 species of coexisting cats in Israel
(after Dayan et al. 1990)
Wildcat, fem.
Wildcat, male
Jungle cat, fem.
Caracal, fem.
Jungle cat, male
Caracal, male
5mm
6
7
8
9
Niche partitioning in the feeding appendages of Felids
Ghost of Competition Past – Differences in species ecology that reduces
competition is due to the action of competitive effects in their evolutionary
Past
Medium seed
eater
Large seed
specialist
Small seed
specialist
Frequency
In the diet
small
Seed Size
large
Medium seed eater invades of community
w/o competing species
Niche Expansion – Over time it evolves to
fill vacant niches
Frequency
In the diet
small
large
Medium seed eater invades of community
w/ smaller beaked species
Frequency
In the diet
small
large
Medium seed eater invades of community
w/ smaller beaked species
Character Displacement– Over time it evolves to
avoid competition
Frequency
In the diet
small
large
A characteristic of Character Displacement
Differences in species’ morphology will be
greater in sympatry than in allopatry
sympatry = occur together
allopatry = occur separately
ALLOPTARY
Small seed
absent
SYMPATRY
Small seed
present
Frequency
In the diet
small
large
A characteristic of Character Displacement
Differences in species’ morphology will be
greater in sympatry than in allopatry
sympatry = occur together
allopatry = occur separately
Character Displacement
In
Darwin’s Finches
Conclusions:
Competition is a structuring force – dissimilar species can coexist
more easily
(1) Niche partitioning – Ecology
(2) Niche expansion when there is release from competition - Ecology
(3) Ghost of Comp Past & Character Displacement - Evolution
A. In Ecological Time competition is a Species Taker
B. But in Evolutionary Time it is a Species Maker
Insufficiencies of LV Competition and the rise of Behavioral Ecology
LV is phenomenological
LV takes the mass-action approach, i.e., organisms are a bunch of molecules
that diffuse through space occasionally bumping into one another and when
they do, an interaction occurs (whether competition, predation, facilitation).
K (carrying capacity) and α’s (interaction coefficients) are not
explicitly defined
Basically, there is a conspicuous absence of REAL
BIOLOGY
Heuristically, coexistence requires
Axis of heterogeneity – some “resource” that occurs in variable
amounts or states
Tradeoffs, such a species being efficient at utilizing one end of
the axis necessitates being poor at utilizing the other end.
These two components make up a Mechanism of Coexistence
Mechanisms of Coexistence between forest tree species:
American beech (Fagus grandifolia) and Sugar maple (Acer saccharum)
What’s our axis of heterogeneity??  Sunlight (or shade tolerance)
beech
maple
oak
cherry
very
shade
tolerant
ash
tulip
Sun
loving
What’s the Tradeoff ??  Allocation of Energy
Energy devoted to vertical growth cannot be used for horizontal growth
Mechanisms of Coexistence between forest tree species:
American beech (Fagus grandifolia) and Sugar maple (Acer saccharum)
Beech’s Strategy:
Allocate growth plenty of
energy to horizontal growth
(i.e., specialize in lightcapturing in the shade)
Large collection surface
Long-horizontal branches
seek out light flecks
Maple’s Strategy:
Allocate growth plenty of
energy to vertical growth
(i.e., specialize in fast growth
and filling canopy gaps).
S. Maple
Vertical growth into the canopy
Beech
Smaller collection surface
Who wins ?? Beech or maple, or can they coexist ??
Beech can persist for a very
long time in understory,
whereas maple cannot
When a canopy tree eventually falls
creating a light gap it is filled by beech
Who wins ?? Beech or maple, or can they coexist ??
However, if gaps open up
frequently,then small, fast
growing maples can still beat
beech to the canopy
Although beech will likely
continue to persist
Who wins ?? Beech or maple, or can they coexist ??
Because beech grows better than maple in shade
but
Maple grows better in the sun (light gaps)
Canopy dominance depends on the frequency of light gap formation.
1960’s: Tip-overs rare,
beech dominated
1980’s: Frequent storms and
tornadoes, tip-overs common,
maple began to dominate
e.g., Warren Woods, MI
(Poulson and Platt 1996)
beech
very
shade
tolerant
maple
oak
cherry
ash
tulip
And similarly with the other tree species along
the axis, with the more sun loving species
increasingly dependent on gaps
seedlings
saplings
poles
Sun
loving
canopy
Black cherry
Red oak
Beech
353
44
21
0
0
16
0
6
2
13
4
0
Rye field
Abandoned in ~1930
Black cherry
Red oak
Beech
32
3
116
0
0
19
0
0
28
0
1
16
Bienke stand
(old growth)
Data: Ecology class 2001, Hopkins Memorial Forest, Williamstown, MA
Canopy (light) gaps provide sites
for local recruitment of sun-loving
species so that coexistence and
high diversity of tree species can
be maintained in the forest
Aerial view of a forest
In Panama, 2 species of Kingfishers coexist
Green Kingfisher (38g)
Ringed Kingfisher (300g)
Diet is big and small fish,
but larger size requires
more energy. Perches
high to scan wider area for
big fish but then cannot
see small fish.
Diet is small fish, which
it searches for by
perching near water
Tradeoff between prey size
and encounter rate
What are the primary mechanisms of coexistence??
(1) diet choice
(2) habitat selection
(3) cream skimmers versus crumb pickers
(4) competition versus predator avoidance
(5) regional (i.e., metapopulation)
(6) Non-EQ processes
How common are the various mechanisms??
The real answer we probably don’t know, but David
Lack attempted to answer the question for birds in
1944 (before many mechanisms were proposed)
3
> 18
2
•
5
Geographical separation
Separation by habitat
Different winter ranges (temporal habitat selection)
Separated by feeding habits
Size differences (feeding??)
5-7 Apparent overlap
Optimal Foraging Ecology and Diet Choice
-- The need to acquire energy to maintain homeostasis is
a universal property of life
-- Natural selection
We expect to see adaptive behaviors that permit individuals
to efficiently and effectively acquire and utilize energy
Assumptions
Feeding behavior requires several activities. Consider:
-- search and encounter food, S
-- pursuit and capture of food, P
-- Net energy received, E
}
Costs of
foraging
Benefit of
foraging
(minus energetic cost to search and pursue it)
Furthermore, adaptive feeding requires an economic goal:
Assumptions
Feeding behavior requires several activities. Consider:
-- search and encounter food, S
-- pursuit and capture of food, P
-- Net energy received, E
(minus energetic cost to search and pursue it)
}
Costs of
foraging
Benefit of
foraging
Furthermore, adaptive feeding requires an economic goal:
Maximize the rate of energy acquisition:
Energy
Time
Develop a scenario - On locating a food item, a forager has 2 choices:
(1) To pursue its prey
or
(2) To forgo pursuit and instead search again for a better item
and it pursue once encountered
Develop a scenario - On locating a food item, a forager has 2 choices:
(1) To pursue its prey
or
(2) To forgo pursuit and instead search again for a better item
and it pursue once encountered
The better option is that which yields the greater E/T
Develop a scenario - On locating a food item, a forager has 2 choices:
(1) To pursue its prey
or
(2) To forgo pursuit and instead search again for a better item
and it pursue once encountered
The better option is that which yields the greater E/T
In other words: a forager should pursue an encountered food item
if and only if it could not BOTH locate and catch something better
Let’s decide which is better …
(1) To pursue prey (that was already encountered):
rate of energy gain =
E
P
Let’s decide which is better …
(1) To pursue prey (that was already encountered):
rate of energy gain =
E
P
(2) To search for and pursue another prey:
rate of energy gain =
E
S+ P
Let’s decide which is better …
(1) To pursue prey (that was already encountered):
rate of energy gain =
E
P
(2) To search for and pursue another prey:
rate of energy gain =
E
S+ P
If there is one a single food type – then it never pays to refuse a prey
item and search for another – you always pay the extra cost of search
But what if there are two prey (i.e., an axis of heterogeneity) ??
Prey #1 is preferred – by that we mean that
E(1)
P(1)
>
E(2)
P(2)
If the forager had unfettered access to either prey, prey #1 offers
the higher energetic reward per time spent capturing/consuming it
Now what does our optimal forager decide ??
To pursue or continue searching ??
Scenario (#1): Forager has encountered its preferred prey, Prey 1.
Reward if
pursues
E(1)
E
P(1)
S+ P
Reward if
Searches/pursues
What is the better option ????
Scenario (#1): Forager has encountered its preferred prey, Prey 1.
Reward if
pursues
E(1)
E
P(1)
S+ P
Reward if
Searches/pursues
What is the better option ????
E(1)
Since you can NEVER do better than
E(1)
P(1)
Is ALWAYS
>
E
S+ P
P(1)
Conclusion #1: A forager should always
consume its preferred prey
Scenario (#2): Forager has encountered its non-preferred prey, Prey 2.
Reward if
pursues
E(2)
E
P(2)
S+ P
Reward if
Searches/pursues
What is the better option ????
Scenario (#2): Forager has encountered its non-preferred prey, Prey 2.
E(2)
E
P(2)
S+ P
Reward if
pursues
Reward if
Searches/pursues
What is the better option ????
IT DEPENDS
If S is small:
E
S+ P
>
E(2)
P(2)
Keep searching for preferred prey,
do not consume non-preferred prey
Selective Diet
Otherwise: be a generalist and consume all encountered prey
Conclusion #2: A forager is either
selective on its preferred prey
or it always consumes both its
preferred and non-preferred prey
i.e., opportunist
How does diet selection provide a mechanism of coexistence??
Tradeoffs between E’s, S’s or P’s such that
2 (or more) species have distinct prey
preferences and therefore do not share resources
i.e., no resource competition
Caracal
Serval
Cheetah
5 coexisting cats in the Serengeti:
Species
mass
preferred prey
prey mass
Lion
190 kg
wildebeest, zebra
270-690 kg
Leopard
65 kg
Cheetah
50 kg
Caracal
Serval
Impala
behavior
coop. hunting
60 kg
stealth
Thompson gazelles
25 kg
speed
20 kg
Hyrax, steenbok
5 kg
???
13 kg
birds
100’s g
leaping