Transcript Document

Month Day
Oct.
28
30
Nov.
4
6
11
13
18
20
25
Topic
Plants and Herbivores
Trophic dynamics
Diversity and ecosystem
function
Exam
No class
Climate change
The global carbon cycle
Effects of elevated CO2
The global nitrogen cycle
C,M&M
11
11
12
8,9,11,12
15
15
15
15
10% of NPP
To herbivores
90%
of
NPP
To detritivores
Outline
1.
2.
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
• Loss of C and nutrients from plant
BUT
• Indirect effects can sometimes be positive (??)
• Herbivore optimization hypothesis (HOH)
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
Williamson et al. 1989
ANPP is
maximized at
moderate levels
of herbivory
Moderately
grazed plants
overcompensate
Williamson et al. 1989
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
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)
What about roots?
Root function is important
• Positive response to herbivory is dependent upon
belowground reserves
Increased
mowing
frequency
reduces
belowground
reserves,
decreases ANPP
Turner et al. 1993
Effects of herbivores on nutrient cycling are
complicated…
Evidence for both positive and negative effects
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
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.
Moose browsing
increases spruce
abundance relative
to hardwoods
Low spruce litter
quality decreases
decomposition and
mineralization
Pastor et al. 1993
Negative effects of herbivores on nutrient
cycling
•  nutrient inputs and  losses
Negative effects of herbivores on nutrient
cycling
•  nutrient inputs and  losses
• Select N fixers, reduce inputs
Knopps et al. 2000
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…
Herbivores “open up” ecosystem nutrient cycling
Zimov et al. 1996