An Overview of Herbivory as an Ecological Process.
A. percent of NPP removed by herbivores
B. Species-level effects
1. Tissue removal
a. direct effects "bad"; consequences, however, vary.
b. herbivores after "high quality" materials
(roots, sapwood, foliage, flowers, fruits, seeds)
2. Natural selection favors mechanisms to reduce herbivory
a. review of plant allocation figure (Aber, 11.2)
b. types of inhibitors
1. quantitative inhibitors: esp. lignins
(the dilution solution?)
2. qualitative inhibitors: carbon-based (isoprene,
terpenes) nitrogen-based (alkaloids)
c. the evolutionary dance between plants and their herbivores
Boiled down: Optimization: how to maximize the benefit
and/or minimize the loss under given set of resource
limitations. (we only see the survivors...)
C. Evolutionary view of herbivory
1. evolution of defensive chemicals have energy and nutrient
costs that could have gone into reproduction or growth.
2. even when herbivores not present, costs are genetically
programmed.
agricultural plants bred for productivity. Defenses
applied by humans.
The field of coevolution developed around the analysis of
plant-herbivore interactions and the discovery of the huge number
of plant chemicals that appear to have evolved as plant defenses.
First group:
QUANTITATIVE INHIBITORS: reduce energy gain by
lowering assimilation efficiencies. Plants simply dilute system
with lignin, tannins and (perhaps) cellulose so that resource
quality is "low" or ratio of carbon to essential nutrients is very
high. Simply costs more to extract these...and starvation can
result if a) energy gain insufficient or 2) nitrogen (or even
vitamin) uptake insufficient to replace loses due to
maintenance....
Many consumers have no trouble finding or consuming vegetation.
Their guts are therefore almost always full. So, if the quality of
what they are eating declines, their growth rates will also
decline. While the argument has been made that reduced quality
means that herbivores must consume more...and hence up their amount
of consumption..., the slowed growth will undoubtedly pay benefits
to the plant in terms of reduced reproduction potential of the
herbivores and potentially higher rates of predation. (A herbivore
busy with a meal is likely more vulnerable to predation.)
QUALITATIVE INHIBITORS.
Specific, nonstructural chemicals that poison specific herbivores.
two groups:
a) Alkaloids: nitrogen based chemicals that are not
chemically stable, but have very specific effects...our favorite
drugs...nicotine, caffeine, ... coevolutionary result...consumers
detoxify, modify or even use chemicals to their benefit...lesson to
chemical companies here...plants involved in a defensive arms race
where they can't win, but, once committed, there's no turning back?
b) carbon based terpines, isoprenes, phenolics...obviously,
volatile.
II. Herbivory as an Ecosystem Process
A. Effects on Plant NPP
1. emphasis of ecological rather than evolutionary
perspective.
a. "grazed" vs "ungrazed"
b. grazing intensity
c. grazing frequency
2. The Grazing "Optimization" hypothesis (McNaughton 1979)
a. interpretation of graph
1. verbal model (learn this one!)
2. mathematical model
NPP = biomass*relative growth rate
NPP = B*Ps
g/m2/time = g/m2 * g/g/time
3. Outcomes of model:
undercompensation, compensation,
overcompensation
B. Effects on Plant species composition
l. stable grazing lawns (Serengeti, Shortgrass Steppe
2. unstable grasslands: Tallgrass, Great Basin
McNaughton in a citation classic (American Nat.
l979)...presented the counterintuitive finding known as the
"Grazing Optimization Hypothesis", that grazing can maximize NPP in
a system.
How can this possibly occur?
counter-intuitive at first.
It's not magic...but it is a little
Plant productivity = leaf area (biomass) times photosynthetic rate.
or, NPP = B*Ps
Herbivory reduces leaf area (with each bite), so 'B' decreases,
what about Ps?
Three outcomes are possible:
1. Ps can remain unchanged so that B*Ps becomes smaller with every
bite.
2. Ps can increase enough so that its increase matches the decrease
in biomass. Hence, productivity would be unchanged in the face of
herbivory.
3. Ps can increase faster than B decreases! This would allow for
greater NPP to be produced with less biomass.
This generates three possible outcomes...three curves that are
presented in class, but can be described as:
Undercompensation, (what happens in my garden). Note that if a
plant is not limited by any resources, then Ps will already be
maximized, so that Ps remains unchanged with herbivory, but the
amount of biomass producing that productivity declines, and NPP
therefore declines!
Compensation: appears to occur in many forest situations where low
to moderate herbivory does not appear to either increase or
decrease NPP. The plants clearly do a bit better with their
remaining biomass, but the net result is that NPP is unchanged.
Overcompensation: This is what we see in most grasslands. The
plants often produce more NPP than they would without herbivores!
Part of the reason for this is that Ps is adversely affected by
other limiting variables (like light or water), so that removal of
leaf area can potentially increase those resources, thereby
increasing the NPP rates.
Overcompensation often requires that a limiting nutrient like
nitrogen also become more available. This can be done two ways
1. rapid recycling of readily mineralizable materials like urea.
2. reduction in microbial immobilization due to a reduction in the
C:N ratio of litter, and/or an increase in the quality of those C:N
substrates. (litter that plants have not have a chance to use
resorption on, litter that may not have been heavily lignified,
etc.)
Also, there's evidence that plants do allocate resources to storage
that they "are forced to use" when grazed. That's like moving your
money from a checking account (no interest) to an account that
provides a return on your money (or, in this case, on your
biomass). By making new leaves and new roots rather than storing
carbohydrates and nutrients, NPP is stimulated.
How do you measure NPP under extensive herbivory?
1) Enclosures vs exclosures...enclosures will give you the
response of a community to the ungrazed condition...but is this a
valid estimate of NPP for grazed sites? NOT if: regrowth
vegetation exhibits any changes in relative growth rates. Hence,
permanent exclosures (example some on Boulder green space, will
tell you the species comp and NPP of ungrazed system, but will not
tell you how much productivity will occur under grazing.
2) Artificial grazing: control the amount removed. Artificial
because missing many direct and indirect effects of herbivory: a)
selective grazing, b) saliva???? (insect galls) c) trampling d)
feces and urine deposition. Get good numbers, but may not match
reality.
3) Moveable exclosures: create small exclosures. Sample inside,
outside exclosure. Move the exclosure to new grazed spot. Wait
until grazing event has occurred. Sample inside,
outside...move...The data provide weekly estimates of consumption,
which, when added over the growing season and combined with the
last estimate of plant biomass, provide an estimate of NPP under
grazed conditions.
By repeating this in different pastures with different grazing
intensities, the McNaughton model can be tested.
III. Effects of herbivores across space and time.
A. Time scale for plant and ecosystem response.
1. Instantaneous
leaf (Ps) response, plant (Ps) response
2. Growing season
nitrogen mineralization
3. Decades-Centuries
soil organic matter accumulation
effects on water, nutrients, soil texture, etc.
B. Space scale
1. generalists (eat everything)
consequences to plant species composition: minimal
2. specialists (selective eaters)
consequences to plant species composition: maximal
C. Consequences of frequency and intensity
(assume generalists)
1. High intensity changes limiting resources
2. High frequency influences plant storage.
(herbivory forces plant to use reserves)
IV.
Herbivory as the good, bad, and ugly?
A. Indirect effects
1. soil disturbance, compaction
a. loss of cynaobacterial crusts in certain rangelands
b. effects on water runoff
2. creators of microsite/habitats
a. watch your step
b. nutrient hotspots
B. Herbivore effects on biological diversity (Science, April
1998)
1. Historically, herbivores created disturbance patches
within a sea of low resource availability
2. Grazing as mechanism for redistribution of nutrients
3. Currently, mowing with removal (no return of nutrients)
may reduce trend towards nitrogen saturation.
Grazing of Western Lands is controversial, and there appears to be
substantial confusion about what's happening.
How can grazing be a sustainable activity leading to a stable
grassland, yet in other cases grazing leads to instability and the
conversion of grasslands to shublands and deserts?
THE RESPONSE OF AN ECOSYSTEM IS THE INTERACTION BETWEEN THE FORCING
FUNCTIONS (THE INPUTS) AND THE STATE OF THE SYSTEM AT THE TIME OF
THESE INPUTS.
Ok...what does that mean?
First, let's consider grazing as an "input" rather than as a
within-ecosystem component. We can do that, and our system then
becomes the vegetation-soil system.
1. If the only propagules in the system are those of the dominant
vegetation (the grasses) the system will persist even if
overgrazed...at least, it will for awhile.
In terms of either persistence or fitness...as long as the plant
species is harmed relatively less than every other species then it
"benefits" from grazing, right?
2. If, however, seeds of either woody species or exotic vegetation
are present and these are not grazed. a different result can occur.
Grazing results in the removal of the canopy and increased
inorganic nitrogen in the soil as a result of increased
mineralization and decreased plant uptake by the dominant species,
then the species composition is likely to shift.
The winner of that competition is determined by who eventually can
hog the "most limiting resources"...usually the water, but,
perhaps, nitrogen, other elements, or light.
If, in case 2, grazers are left on the system "too long", the
conversion occurs. Once grazers are removed, the system may not
return to a grassland. Why? The simple rule...the species that
can control the most limiting resources wins. Once woody or exotic
species are given an advantage, those same species MAY be able to
hold that advantage if they can out-compete for the critical
resources. ALSO...the rules of "the most limiting" resources could
be changed as well. If for example, woody species can hog the
light, then they can out-compete those grassland species that
require full sunlight. Before the woody species got there, light
may never have been limiting! So, part of the story is that the
biota can change the rules...and we can't always a priori tell which
rules are going to be changed!
B. Irruptions of Herbivores
You may remember why a species exhibits exponential
growth. The species tend to have high potential growth rates,
(high 'r-selected traits), and resources go through a period when
these are not limiting. Grasshopper outbreaks in North America and
lemming outbreaks in the tundra are good examples. (regretfully, so
is the "human outbreak", but we don't study it as such!).
What's important to us here, is that the ecosystem
characteristics...NPP, nutrient cycling, etc., do show strong
effects as a result of these outbreaks.
Herbivore outbreaks in Colorado historically included “soil grub
outbreaks”, where root-feeding beetles would disturb the prairie, and
Grasshopper outbreaks. Neither of these natural disturbances have been
seen in decades.
In the mountains, however, a variety of wood-boring beetles
periodically reach high densities and have killed many trees. This is
a natural process, but likely has been influenced by fire suppression
and climate change variables. Clearly, this is an example of how
Consumers affect plant successional processes. (see also the figures
on our web site.)