Why are parts of the world green?Multiple factors control productivity and the

distribution of biomass

Gary A. Polis. OIKOS 86: 3-15. Copenhagen 1999

Topics on Ecosystem Ecology. Petra Bachmann and Luisa Ricaurte

What does Polis (1999) argue?“Cascading predation appears

to explain only a very small fraction of the variance in the temporal and spatial

distribution of plant biomass in land communities”

Example of a food chain in a Swedish lake. Wikipedia, 2009.

THERE ARE MORE FACTORS TO

EXPLAIN THE DISTIBUTION OF

BIOMASS

(determine the „greenness“)

6 hypothesis

1. Productivity and distribution of biomassis regulated by TOP DOWN forces

GREEN WORLD HYPOTESIS(GWH)

Hairston, Frederick Smith and Lawrence Slobodkin, 1960

EXPLOITATION ECOSYSTEM HYPOTESIS

(EEH)

Fretwell, 1977, 1987; Oksanen et al. 1981

GREEN WORLD HYPOTESIS (GWH)

EXPLOITATION ECOSYSTEM HYPOTESIS (EEH)

Predator Herbivore Biomass

Predator Herbivore BiomassPredator

2. Nutrients, limit herbivore number, and thus their effect

Nutrients limitation

Natural or Influenced by humans

3. Abiotic factors: climate and disturbance, limit herbivore numbers to observed levels

Sunlight

Climate

Natural catastrophes

Cyclical events

More factors……

• 4. Plants are not passive agents, waiting to be decimated by herbivores – resource defenses

• 5. Spatial and temporal heterogeneity – scales, season and periods

• 6. Herbivors limit their own numbers – self regulation

Estimates of ANPP

Only a fifth part of ANPP on land is eaten each year

Plant SB only represents an estimated 3-7 % of ANPP

When does trophic cascades affect biomass productivity?

o Appropriate environmental conditionso Characteristics of key consumers and

resourceso Multichannel omnivory and subsidies

When does trophic cascades affect biomass productivity?

Community cascades but not species cascades

Pathogens and parasites cascades

-> only community cascades explain sometimes GWH and EEH

More knowledge is needed in:

• How population dynamics affect ecosystem process and vice versa

• How do spatially variable productivity and flux of trophic entities affect populations and communities?

• How does past productivity, stored and used, affect current interactions?

• How age and stage structured processes affect food webs and communities?

• Is energy and nutrient internally recycled or linked to population dynamics of larger species?

Conclusions

• GWH and EEH are only in a very few spatial and temporal scale the reason for biomass productivity, universally abiotic factors, nutrient, self-regulatory, competition and so one control much more the green world than predator-herbivory-cascades

• GWH and EEH occur more in water than land cascades