Why are parts of the world green? Multiple factors control productivity and the distribution of biomass Gary A. Polis. OIKOS 86: 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) EXPLOITATION ECOSYSTEM HYPOTESIS (EEH) Predator Herbivore Biomass Predator Herbivore Biomass Predator

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