Trophic Dynamics and Herbivory
1. In the relationship: productivity = ingestion minus respiration minus egestion (P=I-RE), and where production = Assimilation minus Respiration (P=A-R),
How does a) food 'quality' affect the equations?
b) the energy used to capture food affect the equations?
2. An elk herd that has no predators is not increasing or decreasing in energy content on
an annual basis (i.e., average herd size and average animal size is constant). If births and
deaths are occurring, is there any “production”? How can you diagram this? (can you put
this in terms of boxes and arrows for energy?)
3. Why, in almost all terrestrial ecosystems known or imagined, does more energy go to
detritivores than herbivores?
4. Can energy cycle? Can a hunk of energy travel several times through a food web?
5. Trophic level analyses (biomass or energy pyramids) may be interesting descriptions
and interesting ways to compare ecosystems. However, what's the "fairy tale" in this
picture. Why do most ecologists ignore trophic level analyses?
The great debate. Two famous ecologists debated the issue, "The effect of
herbivory on net primary productivity"
One ecologist presents a 30-minute talk on plant defensive chemistry and argues that
herbivory has had a negative effect on overall NPP.
The other ecologist presents a 30 minute talk on plant relative growth rates and
herbivory. The ecologist argues that herbivory increases plant production.
Outline these two talks, listing the points that would support their respective arguments.
2) Why is the response of agricultural crops to herbivory almost always negative?
3) Explain a situation where herbivory could be described as a mutualistic
(plus-plus) relationship between a plant and an animal.
4) Give examples where herbivores are capable of changing ecosystem structure.
5) What percentage of NPP is consumed (on average) by forest herbivores?
6) What percentage of NPP is consumed on heavily grazed (but sustainable) grasslands?
6b) What percentage of NPP is consumed in oceans? Why the difference between the
forests and the oceans?
7) Name a chemical of very large molecular weight that functions to dilute resource
quality and therefore discourage herbivory?
8) Name some chemicals that are small molecular weight that function as herbivore
poisons or feeding deterrents.
9) Do anti-herbivore chemical defenses contain materials other than carbon, hydrogen,
and oxygen? If so, what are these called and where are they found?
10) Some plants are only fed upon by 'specialist herbivores'. What's special about
these animals?
11) Explain how the relative growth rate (RGR) of a plant can a) increase, b) remain
the same or c) decrease when plants are grazed by (say) ungulates.
12. If lignin protects plants from herbivores, why can we argue that lignin production
reduces plant NPP? How does lignin content of plants change when soils are more
or less fertile? Why should they do that?
13. How can consumption by herbivores lead to a stable grazing lawn? (i.e., not have
the biomass changing through time?)
14. What factors may cause the relative growth rate of foliage to increase after other
foliage is removed by herbivores?
15. There is less evidence (but some) that plants exhibit “overcompensation” to root
herbivores, but your instructor believes this will be shown. Explain how punching
holes or removing plant root tissue might result in an INCREASE in plant
productivity.
16. How could herbivory ‘drive’ succession? How could herbivory inhibit or
suppress succession? Can you give some examples?