Transcript Top-Down Controls

What controls the productivity and abundance of
plants in this ecosystem?
Nutrients
Bottom-up Controls – refer to control of abundance or productivity of
a species or functional group by supply of resources.
Global Ocean distribution of Chlorophyll
and Benthic Faunal Biomass
mg/m3
Top-Down Controls – refer to control of abundance or productivity of
prey species or functional group by the actions of its consumers
Nutrients
Bottom-up Controls – refer to control of abundance or productivity of
a species or functional group by supply of resources.
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.”
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)
 World is Green
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
Limited by nutrient availability, not
herbivory
Nutrients
Controlled manipulation of resources by
ecosystem fertilization at Great Sippewissett
Salt Marsh (Valiela, Teal, et al. 1970-1990)
False color aerial infra-red photo of
fertilized plots at Great Sippewissett Salt
Marsh
• HF receives sewage sludge based fertilizer
• UP receives equivalent does of urea/phosphate
• C are control plots
Above ground biomass of salt marsh
grasses in fertilized vs. control
plots
Despite naturally high porewater N levels,
further N-additions increase productivity of marsh
grasses
Annual Cycle of Benthic Chlorophyll in
fertilized and control creeks at Great
Sippewissett Salt Marsh
• Lowest biomass of benthic microphytes in
summer
• Fertilization effect only occurs in winterspring period – why?
Foreman 1989
Seasonality of fish and macroinfaunal
abundance
Werme et al.
Caging Experiments in a Salt Marsh
Exclude fish and crabs with cage
Caging Experiment
Begin
Caging
Open one
Cage
-In cages, algae biomass remains high in cages as grazing pressure by
fish and crabs is reduced and fertilization effect persists
-Biomass converges on ambient (low) levels when cages are opened and
grazers can gain access; diminishes fertilization effect
-Suggests consumer control overwhelms resource control of benthic
Foreman 1989
Effect of excluding fish and crabs using cages and
fertilization on benthic productivity and respiration
From Giblin, Foreman & Banta, 1995
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
Are
Trophic
Cascades
All Wet?
Plot loge (Abundance+predators/Abundance-predators)
20.0
7.4
2.7
Ratio
for Plants
B+predators/B-predators
Comparative Strength of Trophic Cascades Across Ecosystems based
on Manipulations of Predators
in Six Types of Ecosystems (102 studies)
0.7%
1.0
1.8% 5.0% 13.5% 36.8% 100%
Ratio N+predators/N-predators for Herbivores as %
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 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
 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)
Aquatic
Terrestrial
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
Are Top Down Controls and Trophic Cascades All
Wet?
Perhaps grazing in terrestrial
ecosystems is inhibited
by Antiherbivore Compounds/Lignin Derivitives
Feeding Experiments with Marsh Invertebrates on
Agar Plates
Mix Spartina detritus or grass
with agar, spike with different
concentrations of ferulic acid
and allow invertebrates to feed.
Count bite marks.
Effect of Tannins on palatability of grass to Geese
Demonstrates that chemical composition of plants can affect feeding by
herbivores
From Buchsbaum et al
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;
Trophic Cascades and Feedbacks
CX
Nutrients
H
PP
+
+
For simple food chains:
But organisms feeding at multiple
trophic levels can complicate picture
C1
+
-
C2
+
C3
Links
1 (odd)
2 (even)
-
3 (odd)
4 (even)
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 the abundance and
productivity of popluations
Number of
wolves in lower
48 (1,000’s)
WOLVES IN NORTH AMERICA
The Study of
ECOLOGY
250
200
150
100
1750 1850 1950
Wolves present
Number of
trees
50 30
2000
absent
15
0
150
QuickTime™ and a
decompr essor
ar e needed to see this pic tur e.
75
0
1750
ASPEN
1850
1950
Ripple et al. 2005 BioScience
2000
COTTONWOOD
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
Behavior alters species roles in ecosystem
Algae
Inverts
Mummichog
Nutrients
P P
H
-
C1
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
Fish
-Fish
Reference
Fish
-Fish
Nutrient
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
THE IMPORTANCE OF INDIRECT
EFFECTS IN ECOSYSTEMS
63%
Number of studies
25
Non-Lethal Effects
Aka Trait-mediated
15
5
40
0
51%
Lethal Effects
Aka:Density mediated
20
0 0.5 1.0 1.5 2.0
Ratio of effect size to total predator effect
0
Preisser et al. 2005. Scared to death? The effects of intimidation and consumption in
predator prey interactions. Ecology 86:501
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
• Trait mediated effects may be as
important as consumption effects
• Stronger in Aquatic or Terrestrial?
No consensus just yet.
Top Down or
Bottom-up??
The End