Transcript Document

To this point we’ve been thinking about specialist predators
Predator
_
+
Prey 1
However, predators may be generalists (recall “searchers” do not
forage selectively but rather consume all encountered prey)
+
Predator
_
_
Prey 1
+
Prey 2
A critical difference:
Predator
_
+
Prey 1
+
Predator
_
_
Prey 1
+
Prey 2
A critical difference:
Predator
_
+
Is prey #1 necessary for the
predator’s persistence?
Prey 1
+
Predator
_
_
Prey 1
+
Prey 2
+
Predator
_
_
Prey 1
+
Prey 2
In the 2-prey example, the predator-prey interaction
is partially decoupled – the predator can over exploit
one or more prey to local extinction without itself going
extinct
This is extremely important, because it suggests that
decoupled pred-prey interactions can lead to the loss
of prey species and ultimately a collapse of species
diversity
Example: Spatial subsidies – the movement of prey
or predators between habitats
Habitat or
Community A
Habitat or
Community B
Predator
regulating
mechanism
Subsidized prey
Predator
Focal prey
Alternative prey
Predator
Alternative prey
Canis latrans
The beneficiaries of
resource subsidies are
often generalist mediumsized omnivores
Coastal sites
Inland sites
Coyotes drive down
the number of cats in
coastal CA
After Polis et al. (1996)
Linear flow of
species interactions:
Trophic cascade
Horizontal flow of
species interactions:
Shared natural enemies
Predator
?
prey 2
prey 1
-
in Southern California
+
Coyotes have positive
effects on songbirds by
eliminating cats and other
medium-size predators
But within small urban lots
of Southern , CA – coyotes
are lacking
Summary of correlations
between predators and
avian diversity
Fox
House cat-0.635
Opossum -0.464
Skunk
Raccoon
-0.361
Total
-0.539
-0.112
-0.487
How many songbirds do
house cats kill each year??
after Crooks and Soule 1999
Example # 2:
Parakeets, cats, and rabbits on Macquarie island (1800-1920)
Cats and parakeets coexisted until ....
....introduced rabbits greatly boosted the number of cats
(1)
(2) (3)
X
I don’t
deserve this
Shared Natural
Enemy
Rabbits are a parakeet’s
worst friend
Cat
+
_
_
Parakeet
+
rabbit
_
+
In this example, rabbits
have a negative effect
on parakeets through
the action of cats
And coyotes have a positive
effect on songbirds by
eliminating cats and other
medium-size predators (e.g.,
raccoons, skunks)
 INDIRECT EFFECTS
Indirect effects (> 3 spp)
Trait-mediated
Density-mediated
(via changes in spp traits)
(via changes in spp abundance)
-e.g., predator facilitation
- ecology of FEAR
- Resource competition
(via shared resources)
- Apparent competition
(via shared predators)
- Indirect mutualism
- Trophic cascades
- Resource competition
Cooper’s Hawk
(via shared resources)
Weasel
WF mouse
Chipmunk
Cooper’s Hawk
Weasel
Veery
Chipmunk
Acorns
(-) effect
(+) effect
Cooper’s Hawk- Chipmunk (-)
Chipmunk- Weasel
(+)
(-) × (+) = (-)
- Apparent competition
Cooper’s Hawk
(via shared prey)
Weasel
Cooper’s Hawk
WF mouse
Chipmunk
WF mouse
Chipmunk
Veery
Acorns
(-) effect
(+) effect
Chipmunk – Cooper’s Hawk (-)
Cooper’s Hawk- mouse
(+)
(-) × (+) = (-)
Indirect mutualism ??
Cooper’s Hawk
Weasel
Cooper’s Hawk
WF mouse
Weasel
Chipmunk
WF mouse
Chipmunk
Veery
Acorns
Weasel-Chipmunk-mouse-copper’s hawk
(-)
(-)
(+)
(-) × (-) × (+) = (+)
Community Ecology
 Functional
 Behavioral
 Population
Community
A simple definition – Ecological communities are sets of species
that occur together in a particular space at a particular time
Goal – to understand the origin, (dis)assembly, maintenance, and
consequences of biological diversity within local communities
Terminology - 2 ways of viewing local communities
Membership
Defined by members, specifically
their distribution and abundance
patterns
Trophic relationships
Defined by webs of interactions,
energy flow, or feeding relationships
food web
Descriptors:
- Spp richness = # spp
- Spp diversity – diversity index
H’= -  pi (ln pi)
Descriptors:
- # links
- # trophic levels
- Mean food chain length
- Connectedness – measure of linkage
relative to maximum linkage
Two opposing views of communities:
Clementsian (1916, 1936)
Gleasonian (1926, 1939)
- Communities are “closed”
- Communities are “open”
- Occur as discrete units w/sharp
boundaries
- Community structure and function expresses
the interactions of the individual spp
comprising the local assemblage and does
not reflect any organization above the spp
level
- Ecological limits of its members
coincide w/the distribution of the
community
“A fortuitous association of organisms whose
adaptations enable them to live together under
the particular physical and biological conditions
that characterize a particular place”
A plant association is not an “organism” but
“merely a coincidence”
Test via Gradient Analysis – Robert Whittaker, 1967
Montane plant distributions indicate an
open community structure
None of the 12 co-occuring tree species
in KY have similar distributions
Gleasonian Structure
- Ecological limits of its members coincide w/the distribution
of the community
Terminology - 2 ways of viewing local communities
Membership
Defined by members, specifically
their distribution and abundance
patterns
Trophic relationships
Defined by webs of interactions,
energy flow, or feeding relationships
food web
Descriptors:
- Spp richness = # spp
- Spp diversity – diversity index
H’= -  pi (ln pi)
Descriptors:
- # links
- # trophic levels
- Mean food chain length
- Connectedness – measure of linkage
relative to maximum linkage
(1) A “simple” and simplified
food web in the Antarctic
Owls on Kangaroo rats:
(1) Direct: owls consume K-rats
(2) Density-mediated: owls kill snakes
that kill K-rats
(3) Trait-mediated: owls scare k-rats
into the fangs of snakes
+
-
Total = ???
Have we forgotten any???
(4) Rattlesnake reduce their activity in
the presence of owls
(5) Owls reduce the abundance of K-mice,
a competitor
Time lags
A release from shortterm competition gives
way to release from a
long-term mutualism