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10% of NPP

10% of NPP. To herbivores. 90% of NPP. To detritivores. Whittaker 1961. Trophic levels : groupings of species with similar feeding relationships Useful oversimplification Complicated by omnivory. Food webs in real ecosystems are complex. 10% of NPP. To herbivores. 90% of

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10% of NPP

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  1. 10% of NPP To herbivores 90% of NPP To detritivores

  2. Whittaker 1961 • Trophic levels: groupings of species with similar feeding • relationships • Useful oversimplification • Complicated by omnivory

  3. Food webs in real ecosystems are complex

  4. 10% of NPP To herbivores 90% of NPP To detritivores

  5. What controls amount of NPP transferred to herbivores versus detrivores? • NPP provides upper limit to energy flow through food webs • Plant tissue quality is a major control over transfer to herbivores • -- Animals rely on microbial symbionts to digest lignin and • cellulose • -- Plant defense compounds are toxic to herbivores Remember: microbes’ feeding styles are much more biochemically diverse than animals

  6. Plant Defensive compounds Quantitative • -- Reduce palatability and digestibility • -- Present in high concentrations • -- Tannins, lignin… • Productivity =  quantitative defenses  Productivity =  quantitative defenses Qualitative • -- Toxic: damage herbivore’s cells • -- Present in low concentrations • -- Alkaloids, anthocyanins…

  7. Plant structure Structural materials are similar to quantitative defenses • -- Reduce palatability and digestibility • -- Present in high concentrations • -- Lignin, cellulose… Allocation to structure varies among ecosystems • -- Grasslands (no wood):  transfer of NPP to herbivores • -- Forests (lots of wood):  transfer of NPP to herbivores • -- In aquatic ecosystems, plants allocate very little to • structure and >50% NPP can be transferred

  8. Whittaker 1961 Transfers of energy among trophic levels are not very efficient

  9. Transfers of energy among trophic levels are not efficient Trophic efficiency The proportion of prey production that is transferred to the next trophic level • = consumption efficiency x assimilation efficiency x production efficiency

  10. What controls the abundance and productivity of organisms at any given trophic level? Top-down control: • -- Biomass at any trophic level is controlled by consumers Bottom-up control • -- Biomass at any trophic level is controlled by resource • availability

  11. 2 1 Plants Resources 3 2 1 Plants Resources Brown world Top-down control Even # trophic levels Green world Bottom-up control Odd # trophic levels

  12. Trophic cascades Strong top-down forces (brown world) control plants • Example: removal of top predator increases abundance of prey, which reduces abundance of its food. When are they likely? • -- Simple food chains (linear) • -- Low diversity systems • -- Highly edible plant biomass • -- Constant environment

  13. A classic example of an aquatic trophic cascade

  14. A rare terrestrial example of a trophic cascade: simple system of wolves, moose and spruce

  15. Wolves Reduction in wolves leads to increase in moose Moose Moose eat “spruce” Fir NPP responds to resources (H2O, soil nutrients) only when wolves eat lots of moose Fir AET

  16. Eretmocerus sp. whitefly Trophic cascade in an agroecosystem Cotton Resources Nitrification?

  17. Trophic cascades Not common (the world is primarily green) Why? • -- Complex food webs (non-linear) • -- Long-lived plants, well-defended with lots of structure • -- Heterogeneous environment • -- Relatively high diversity

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