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 Biomassmg/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

(100

0’s

per m

-2)

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 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.

per

0.2

5 m

2

10 8 6 4 2 0 1972 1985 1989 1993 1997

Year

Total Kelp Density

400

300

200

100

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)

Rat

io B

+pre

dato

rs/B

-pre

dato

rs fo

r

Plan

ts

0.7% 1.8% 5.0% 13.5% 36.8% 100%

20.0

7.4

2.7

1.0

Ratio 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

elat

ive

Am

ou

nt

Eat

en

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

Nu

mb

er

of

trees

1750 1850 1950 2000

30

15

0150

75

0

QuickTime™ and a decompressor

are needed to see this picture.

ASPEN

COTTON-WOOD

Nu

mb

er

of

wolv

es

in low

er

48 (

1,0

00

’s)

250

200

150

100

50

1750 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

Pe

rcen

t B

row

sin

g

W

illow

Heig

ht

(cm

)100

50

0400

200

0

98 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 1

0 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 -Fish

Reference 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

Nu

mb

er

of

stud

ies

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??