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 Chlorophylland Benthic Faunal Biomass mg/m3

“Trickle Up” effectIncreases in primary production propagate

through the food web

What controls production in an ecosystem?

The “bottom up” view of Ecosystem

production

The “top down” view of Ecosystem production

Nutrients

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.”

World is Green

But, plants may become depleted whenever herbivores become numerous enough (insect outbreaks, e.g. adelgid)

The remaining general method of herbivore control is predation”

Nutrients

“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.

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

Large zooplankton out compete small zooplankton. Populations of small zooplankton increase in

abundance when large zooplankton are depleted.

Size-efficiency Hypothesis. Larger zooplankton more effectively graze phytoplankton than smaller zooplankton. Therefore if zooplanktivorous fish are absent, large herbivorous zooplankton will increase and phytoplankton abundance will be reduced.

Zooplankton Size Distribution in Lakes with:

SMALLBosmina Tropocyclops Cyclops

DOMINANT ZOOPLANKTERS

CeriodaphniaAsplanchna

Alewife

LARGE

DOMINANT ZOOPLANKTERS

Dioplomus CyclopsDaphnia Mesocyclops

Lepiodore5 mm

Epischura

No Alewife

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

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 webBoth Top-down and Bottom-Up controls influence

abundance and productivity of populations

CX But organisms feeding at multiple trophic levels can complicate picture

Nutrients

P

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)

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)

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)

(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

o 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 ⇧

Reasons why we might not ‘see’ top down cascades in land

ecosystemsMany 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

Re l

ativ

e A

mo u

n t E

aten

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

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

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

No Trophic Cascade : Creek Infauna Abundance was lower

Fleeger et al. Accepted JEMBE

Tota

l Ann

elid

s (#

/ m

2 x

10 3

)

0

20

40

60

Reference Nutrient-FishFish -FishFish

Expected

Observed

Mummichog Reduction

MummichogInvertsAlgae

Behavior alters species roles in ecosystem

C1HP

-

Nutrients

P

MummichogInvertsAlgae(Shrimp)

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

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.