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 biomass
is 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)
Predator
Herbivore
Biomass
EXPLOITATION ECOSYSTEM HYPOTESIS
(EEH)
Predator
Predator
Herbivore
Biomass
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 conditions
o Characteristics of key consumers and
resources
o 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