what controls the productivity and abundance of plants in this ecosystem?
DESCRIPTION
Nutrients. What controls the productivity and abundance of plants in this ecosystem?. Bottom-up Controls – refer to control of abundance or productivity of a species or functional group by supply of resources. and Benthic Faunal Biomass. mg/m 3. Global Ocean distribution of Chlorophyll. - PowerPoint PPT PresentationTRANSCRIPT
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 Chlorophylland Benthic Faunal Biomass
mg/m3
Nutrients
Bottom-up Controls – refer to control of abundance or productivity of a species or functional group by supply of resources.
Top-Down Controls – refer to control of abundance or productivity of prey species or functional group by the actions of its consumers
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
Top-Down Control & Inferences about Trophic Cascades
Hairston, Slobodikin and Smith 1960
“Predators and parasites in controlling herbivores … must be food limited.”
But, plants may become depleted whenever herbivores become numerous enough (insect outbreaks, e.g. adelgid)
The remaining general method of herbivore control is predation”
World is Green
Nutrients
Limited by prey availability
Limited by predation
Limited by nutrient availability, not herbivory
Green World Hypothesis
• HF receives sewage sludge based fertilizer• UP receives equivalent does of urea/phosphate• C are control plots
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
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 winter-spring period – why?
Foreman 1989
Seasonality of fish and macroinfaunal abundance
Mac
rofa
unal
Den
sity
(1
000’s
per
m-2)
Werme et al.
Caging Experiments in a Salt Marsh
Exclude fish and crabs with cage
Caging ExperimentBegin 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 algae in the summerForeman 1989
Effect of excluding fish and crabs using cages and fertilization on benthic productivity and respiration
From Giblin, Foreman & Banta, 1995
A “Trophic Cascade”
+ Bass
- Bass
Control
Food web manipulations inPeter, Paul and Tuesday Lakes
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 manipulation experiments of in 3 lakes (from Carpenter et al., 1987. Ecology 68:1863-1876).
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
ZOOPLANKTON PHYTOPLANKTON
Sea Otters and urchin grazers
And Killer Whales
TROPHIC CASCADE IN KELP ECOSYSTEMS
Effect of Killer whale predation on Sea Otters
From Estes et al. 1998 Science 282:473
No. p
er 0
.25
m2
10 8 6 4 2 0 1972 1985 1989 1993 1997
Year
Total Kelp Density
400300200100 0
gms
0.25
m
-2
Sea Urchin Biomass
Amchitka I.N. Adak I.Kagalaska I.L. Kiska I.#
Otte
rs (%
max
co
unt)
100
80
60
40
20
0
Sea Otter Abundance
Grazing Intensity
605040302010 0
% L
oss
24
hr -1
Are Trophic Cascades All Wet?
Comparative Strength of Trophic Cascades Across Ecosystems based on Manipulations of Predators
in Six Types of Ecosystems (102 studies)Plot loge (Abundance+predators/Abundance-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)
Ratio
B+p
reda
tors/B
-pre
dato
rs fo
r Pl
ants
0.7% 1.8% 5.0% 13.5% 36.8% 100%
20.0
7.4
2.7
1.0Ratio N+predators/N-predators for Herbivores as %
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)
Frequency distributions showing the proportion of NPP removed by herbivores in ecosystems with different primary producers
Median, 18%
Median, 30%
Median, 79%
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 GeeseR
e lat
ive
Am
o un t
Ea
ten
From Buchsbaum et al
Demonstrates that chemical composition of plants can affect feeding by herbivores
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;
HP-
C1
-+
C2
-+-
Trophic Cascades and Feedbacks
C3
-+-+
Links1 (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
CX But organisms feeding at multiple trophic levels can complicate picture
Nutrients
P
Both Top-down and bottom-up controls influence the abundance and productivity of popluations
Num
ber o
f tre
es
1750 1850 1950 2000
30
15
0150
75
0
QuickTime™ and a decompressor
are needed to see this picture.
ASPEN
COTTON-WOOD
Num
ber o
f wo
lves
in lo
wer
48 (1
,000
’s) 250
200
150
100
501750 1850 1950 2000
The Study of ECOLOGY
Ripple et al. 2005 BioScience
WOLVES IN NORTH AMERICA
Wolves present absent
WolvesRe-introduced
Ripple et al. 2006. For. Ecol & Mgt. 230:96
Per
cent
Bro
wsin
g
Willo
w He
ight
(cm
)100
50
0400
200
098 99 00 01 02 03 04 05
WOLVES AND WILLOWS
2-3 m3-4 m
<2 m
Behavior alters species roles in ecosystem
C1HP
-
Nutrients
P
MummichogInvertsAlgae
No Trophic Cascade : Creek Infauna Abundance was lower
Tota
l Ann
elid
s (#
/ m
2 x
10 3
)
0
20
40
60
Reference Nutrient-FishFish -FishFish
Expected
Observed
Mummichog Reduction
MummichogInvertsAlgae
6
8
10
12
Shr
imp
d 15
N
Fish -Fish Fish -FishReference Nutrient
enrichment
Removal of mummichogs allows shrimp to forage in more open areas and become more carnivorous.
Behavior alters species roles in ecosystem
David JohnsonKari GalvanLinda Deegan
Preisser et al. 2005. Scared to death? The effects of intimidation and consumption in predator prey interactions. Ecology 86:501
Non-Lethal EffectsAka Trait-mediated
Lethal EffectsAka:Density mediated
THE IMPORTANCE OF INDIRECT EFFECTS IN ECOSYSTEMS
63%
51%
Ratio of effect size to total predator effect
Num
ber o
f stu
dies
25
15
5
040
20
00 0.5 1.0 1.5 2.0
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.
The End
Top Down or Bottom-up??