Transcript Document

- 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
(+)
(-) × (+) = (-)
(1) A “simple” and simplified
food web in the Antarctic
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
Indeterminacy in food webs or why we often cannot predict
the effects of changes in the abundance of a species on other
members of the food web – 3 different sources
- Complexity, there are many, many links
oh, and did I say many links?
- Time lags – effects don’t show up until the interactions
flow through many intermediate species
- Unpredictable behavior – Trait-mediated interactions,
ecology of fear, ecosystem engineering
(1) A “simple” and simplified
food web in the Antarctic
From your text, a more realistic food web based on 10
fish species and their foods
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
Review Papers:
Sih et al. (1985) reviewed >100 predator removal studies
- most effects were large … > 2 fold change in prey abund.
- 22-46% of studies showed unexpected effects, i.e.,
prey declined in response to predator removal
Mauer (1999) – Few studies have shown significant effects due
to competition when analyzed after 3 years.
So what can we do????
Perhaps we cannot determine with confidence what the
long-term effect will be from variation in the abundance of
community members…so ….
(1) We can determine with much greater confidence what
the effects will be to trophic levels rather than individual spp.
(2) Ignore the weak interactions and concentrate on strongly
interacting species that have disproportionate effects
KEYSTONES
Trophic cascades model:
Alternating (-) and (+) effects
at each lower trophic level
+
+
_
Caterpillars
_
Oak trees
= the top trophic level limits the
abundance of the level below
itself,which in turn allows 2
levels below to flourish and so
on…..
Trophic cascades model:
Alternating (-) and (+) effects
at each lower trophic level
+
+
= the top trophic level limits the
abundance of the level below
itself,which in turn allows 2
levels below to flourish and so
on…..
_
Caterpillars
_
Oak trees
“Coyotes reduce cat populations that
allow birds to flourish in turning
keeping insects to a minimum so
that trees flourish” - i.e., the world is
GREEN
X
“Cat reduce the numbers of birds
allowing insects to flourish and
defoliate the forest” - i.e., the world is
BROWN
+
_
Caterpillars
_
Oak trees
X
“Cat reduce the numbers of birds
allowing insects to flourish and
defoliate the forest” - i.e., the world is
BROWN
+
_
Caterpillars
_
Oak trees
The difference is in the number
of trophic levels:
Odd # - GREEN World
Even # - BROWN World
But what if Predators don’t rule the world?
Bottom-up control
rather than
Top-down control
+
+
+
+
+
+
_
Caterpillars
_
+
Oak trees
Bottom-up control: GREEN BROWN
Response to an
increase in
productivity
Zooplankton Biomass
(1) An increase in Productivity
causes an increase in consumers
Plant Biomass
(chlorophyll)
After Leibold et al. 1997
Zooplankton Biomass
(2) Adding fish causes a decrease in
consumers and an increase in plants
Plant Biomass
(chlorophyll)
Ecological Meltdown in
Predator-Free Forest Fragments
• 4300-km2 hydroelectric impoundment in
the Caroni valley of the state of Bolivar,
Venezuela
• Isolated by rising water in 1986, these
islands range from 0.2 to 4.9 km from the
nearest point on the mainland, and all are
situated at least 100 m across open water
from any other landmass.
• The vegetation is semideciduous,
tropical dry forest.
Area (ha)
No. trees > 10 cm DBH
No. tree species
No. stems > 1 m tall
0.25
203
32
9
0.7
490
40
84
0.3
290
33
147
0.6
381
47
90
0.5
301
54
156
0.9
403
55
301
No. howler monkeys
Howler monkeys per ha
2
4.0
6
8.6
3
10.0
6
0.0
-
-
No. leaf-cutter colonies
No. leaf-cutter colonies/ha
1
4.0
3
4.3
2
6.7
4
6.7
2
4.0
1
1.1
Area (ha)
No. tree species per
stems > 10 cm DBH
11
49
375
12
51
236
350
50
304
350
55
-
150
46
379
ML
51
340
ML
50
-
No. leaf-cutter colonies
No. leaf-cutter colonies/ha
2
0.18
2
0.17
54
0.01
4
0.01
2
0.01
0.04
0.04
Jaguar
Harpy Eagle
Agouti
Howler Monkey
Leaf cutter ants
Coati
PLANTS