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10% of NPP

10% of NPP. To herbivores. 90% of NPP. To detritivores. Outline. Effects of herbivores on NPP Effects of herbivores on nutrient cycling. How do herbivores influence NPP?. Direct effects are negative Damage! Select growing tissue rich in nutrients

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10% of NPP

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

  2. Outline • Effects of herbivores on NPP • Effects of herbivores on nutrient cycling

  3. How do herbivores influence NPP? • Direct effects are negative • Damage! • Select growing tissue rich in nutrients • Loss of C and nutrients from plant BUT • Indirect effects can sometimes be positive (??) • Herbivore optimization hypothesis (HOH)

  4. In 276 studies of grazed systems around the world,  17% showed a positive relationship between grazing and aboveground production Milchunas and Lauenroth 1993 Overcompensation: Increase in ANPP induced by herbivores is greater than ANPP consumed

  5. Williamson et al. 1989

  6. ANPP is maximized at moderate levels of herbivory Moderately grazed plants overcompensate Williamson et al. 1989

  7. Mechanisms driving HOH • Removal of old leaves and litter maximizes Ps of remaining leaves • Increased C and N allocation from below- to • aboveground increases LAI and [N] Remember: plants allocate to maximize gain of the most limiting resource

  8. Mechanisms driving HOH • Indirect effects • Increased light due to litter removal • Increased soil nutrient availability • But—herbivory may reduce plant demand for nutrients (smaller plants)

  9. What about roots?

  10. Root function is important • Positive response to herbivory is dependent upon belowground reserves

  11. Increased mowing frequency reduces belowground reserves, decreases ANPP Turner et al. 1993

  12. Effects of herbivores on nutrient cycling are complicated… Evidence for both positive and negative effects

  13. Positive effects of herbivores on nutrient cycling • “Short circuit” microbial decomposition • Urea hydrolyzes quickly to NH4+ • Feces have a low C:N • Decreased C allocation belowground reduces microbial N immobilization (more for plants!) • Damage may interfere with N retranslocation; hi N litter

  14. Negative effects of herbivores on nutrient cycling • Selective feeding alters plant spp. composition • Select spp. with hi N, lo lignin, lo defenses… • Leave behind poorly decomposable spp.

  15. Moose browsing increases spruce abundance relative to hardwoods Low spruce litter quality decreases decomposition and mineralization Pastor et al. 1993

  16. Negative effects of herbivores on nutrient cycling •  nutrient inputs and  losses

  17. Negative effects of herbivores on nutrient cycling •  nutrient inputs and  losses • Select N fixers, reduce inputs

  18. Knopps et al. 2000

  19. Herbivores “open up” ecosystem nutrient cycling Negative effects of herbivores on nutrient cycling •  nutrient inputs •  losses • Basic urea =  NH3+ flux •  microbial immobilization and plant N demand =  nitrification, leaching •  erosion due to stomping • Migration moves N away…

  20. Zimov et al. 1996

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