why are parts of the world green? multiple factors control productivity and the distribution of...
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![Page 1: 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](https://reader036.vdocuments.us/reader036/viewer/2022062322/5697bfca1a28abf838ca9544/html5/thumbnails/1.jpg)
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
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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.
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THERE ARE MORE FACTORS TO
EXPLAIN THE DISTIBUTION OF
BIOMASS
(determine the „greenness“)
6 hypothesis
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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
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GREEN WORLD HYPOTESIS (GWH)
EXPLOITATION ECOSYSTEM HYPOTESIS (EEH)
Predator Herbivore Biomass
Predator Herbivore BiomassPredator
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2. Nutrients, limit herbivore number, and thus their effect
Nutrients limitation
Natural or Influenced by humans
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3. Abiotic factors: climate and disturbance, limit herbivore numbers to observed levels
Sunlight
Climate
Natural catastrophes
Cyclical events
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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
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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
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When does trophic cascades affect biomass productivity?
o Appropriate environmental conditionso Characteristics of key consumers and
resourceso Multichannel omnivory and subsidies
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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
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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?
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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