Transcript What controls production in an ecosystem?

Resource & Predation Controls on Ecosystem
Production
Bottom-up Controls – refer to control of abundance or productivity of
a species or functional group by supply of resources.
What controls production in an ecosystem?
The “bottom up” view of
Ecosystem production
Nutrients
Global Ocean distribution of Chlorophyll
and Benthic Faunal Biomass
mg/m3
“Trickle Up” effect
Increases in primary production propagate through the
food web
What controls production in an ecosystem?
The “bottom up” view of
Ecosystem production
Nutrients
The “top down” view of
Ecosystem production
Top-Down Control & Inferences about
Trophic Cascades
Hairston, Slobodikin and Smith 1960
Basic Premise:
“Any population which is not resource limited must, of course,
be limited to a level below that set by its resources.”
 World is Green
Therefore the “usual condition is for
populations of herbivores not to be
limited by food supply….” and producers
are limited by resources, not herbivores
 But, plants may become depleted
whenever herbivores become numerous
enough (insect outbreaks, e.g. adelgid)
The remaining general method of herbivore
control is predation”
“Predators and parasites in controlling
herbivores … must be food limited.”
“Green World Hypothesis”
Limited by prey availability
Limited by predation and parasitism
Limited by nutrient availability, not
herbivory
Nutrients
“Green World Hypothesis”
Although the authors did not
explicitly call this series of
interactions a “trophic cascade” , that
was the idea that was implicit in the
Green World Hypothesis.
Nutrients
Systems with and without predators
Brooks and Dodson (1965) examined 9 lakes:
•Four lacked Alewifes
•Four had natural Alewife populations
•One had introduced Alewifes
•
sampled before and after the introduction
Zooplankton Size Distribution in Lakes with:
No Alewife
Epischura
Lepiodore
5 mm
DOMINANT
ZOOPLANKTERS
LARGE
Asplanchna
Dioplomus
Cyclops
Daphnia
Mesocyclops
Ceriodaphnia
Alewife
Bosmina
Tropocyclops
Cyclops
DOMINANT
ZOOPLANKTERS
SMALL
Size-efficiency Hypothesis.
Larger
zooplankton
moreout
effectively
than smaller
Large
zooplankton
competegraze
small phytoplankton
zooplankton. Populations
zooplankton.
of small zooplankton increase in abundance when large
Therefore if zooplanktivorous
fish are
large herbivorous
zooplankton
areabsent,
depleted.
zooplankton will increase and phytoplankton abundance will be reduced.
A “Trophic Cascade”

If piscivores added to lake, they will eat and deplete
zooplanktivorous fish.

Populations of larger herbivorous zooplankton will
grow.

Phytoplankton populations will be reduced.
Food web manipulations in
Peter, Paul and Tuesday Lakes
- Bass
Control
+ Bass
Food web manipulation experiments of in 3 lakes (from Carpenter et al.,
1987. Ecology 68:1863-1876).
ZOOPLANKTON
PHYTOPLANKTON
Control
+Bass
-Bass
Lakes with bass (piscivore added), zooplanktivorous fish depleted, large
zooplankton increase, chl a is lower
Lakes with piscivore removed have higher chl a
TROPHIC CASCADE IN KELP ECOSYSTEMS
Sea Otters
and urchin
grazers
And Killer Whales
Effect of Killer whale predation on Sea Otters
Sea Otter Abundance
80
60
40
Sea Urchin
Biomass
200
100
060
50
40
30
20
10
10
0
8
6
4
2
1972
0
% Loss 24
hr -1
-2
20
400
0
300
Amchitka I.
N. Adak I.
Kagalaska I.
L. Kiska I.
Grazing Intensity
No. per 0.25
m2
gms 0.25 m
# Otters (% max
count)
100
Total Kelp
Density
1985
1989
1993
1997
Year
From Estes et al. 1998 Science 282:473
Trophic Cascades and Feedbacks
CX
Nutrients
H
PP
+
+
But organisms feeding at multiple
trophic levels can complicate picture
C1
+
-
C2
+
C3
Links
1 (odd)
2 (even)
-
3 (odd)
4 (even)
For simple food chains:
An odd number of trophic links results in control of primary producers by
grazing (top-down)
An even number of trophic links results in control of primary producers by
resources (bottom-up)
And nutrients or other factors limiting producers can still increase
producer biomass and have effects that propagate up the food web
Both Top-down and Bottom-Up controls influence abundance and
productivity of populations
Are Trophic Cascades All
Wet?
Reasons Why Trophic Cascades Might be Stronger in
Aquatic Ecosystems than in Terrestrial Ecosystems
 Herbivores are bigger relative to plants in aquatic
ecosystems (e.g. zooplankton vs. phytoplankton
compared with insects vs. trees)
 Aquatic primary producers are more nutritious
and have element composition more like their
herbivores
 Terrestrial plants have lower P:B ratios than
aquatic plants (is this a cause or a consequence??)
 Aquatic herbivores consume about 3X more
autotrophic production than terrestrial herbivores
(but why??)
Comparison of Herbivory in Aquatic and Terrestrial
Ecosystems
(Cyr and Pace, 1993 Nature 361:148)
Median, 79%
Median, 30%
Median, 18%
Frequency
distributions
showing the
proportion of
NPP removed
by herbivores
in ecosystems
with different
primary
producers
Regardless of amount of NPP, rates of Herbivory are on
average about 3X higher in Aquatic than Terrestrial
Ecosystems (note: log scale)
Comparative Strength of Trophic Cascades Across Ecosystems based
on Manipulations of Predators
in Six Types of Ecosystems (102 studies)
Ratio
for Plants
B+predators/B-predators
Plot loge (Abundance+predators/Abundance-predators)
o
20.0
7.4
2.7
0.7% 1.8% 5.0% 13.5% 36.8% 100%
Herbivore response greatest in lentic & marine systems (e.g. density with predators ~6%
to13% of that without predators) and least in terrestrial (density + predators ~71% of density
- predators)
o
Plant response greatest in marine benthos (biomass 4.7X > in systems with predators) vs.
terrestrial (1.1X > with predators)
o
Across systems, as Herbivores ⇩ plant biomass response ⇧
(modified from Shurin et al. 2002, Ecology Letters 5:785)
Reasons why we might not ‘see’ top
down cascades in land ecosystems
Many terrestrial plants have complex structural tissue that is harder
to digest and have evolved ‘anti-herbivore’ compounds
Perhaps grazing in terrestrial ecosystems is inhibited by
Antiherbivore Compounds/Lignin Derivitives
Effect of Tannins on palatability of grass to Geese
From Buchsbaum et al
Reasons why we might not ‘see’ top
down cascades in land ecosystems
• Many terrestrial apex predators have been
hunted to near or local extinction
Willow Height (cm) Percent Browsing
WOLVES AND WILLOWS
100
<2 m
50
2-3 m
3-4 m
4000
200
98 99
0
00
01 02 03 04 05
Wolves
Re-introduced
Ripple et al. 2006. For. Ecol & Mgt. 230:96
Reasons why we might not ‘see’ top
down cascades in terrestrial ecosystems
• Plants have complex tissues and anti-herbivore compounds
• Terrestrial may have more complex and more detritus based
food webs, less direct grazing.
• Many terrestrial apex predators have been hunted to near or
local extinction
• Longevity of the plant community (decades to centuries for
mature plants) makes it hard to measure the results
• Terrestrial ecosystems are less experimentally tractable than
their aquatic counterparts, in part because of extreme longevity
of the plant community
• Many of the more charismatic species now enjoy stringent legal
protection, which hampers manipulation;
"The ecology of fear"
• Defensive strategies to avoid predation
• Reduced energy intake
• Alternate food sources
• Increased susceptibility to other
predators
• Lower mating success
• Emigration
Mummichog Reduction
No Trophic Cascade :
Creek Infauna Abundance was lower
Algae
Inverts
Mummichog
Total Annelids
(# / m 2 x 10 3)
Expected
60
40
Observed
20
0
Fleeger et al.
Accepted JEMBE
Fish
-Fish
Reference
Fish
-Fish
Nutrient
Behavior alters species roles in ecosystem
Algae
Inverts
(Shrimp)
Mummichog
Nutrients
P P
H
-
C1
Behavior alters species roles in ecosystem
Removal of mummichogs allows shrimp to forage in
more open areas and become more carnivorous.
Shrimp d 15N
12
10
8
6
David Johnson
Kari Galvan
Linda Deegan
Fish -Fish
Reference
Fish -Fish
Nutrient
enrichment
SUMMARY
• Top down controls can be important in
ecosystem dynamics
• Can affect more than just the level below
(cascade), including effects on adjoining
ecosystems
• Interact with nutrient level
• Indirect trait mediated effects may be as
important as direct consumption effects
• Stronger in Aquatic or Terrestrial?
No consensus just yet.