1. The Consumer Connection
What does Ecological Stoichiometry have to
say about consumption in food webs?
Matt Whalen
23 October 2012

2. Nutrients are good (and often limiting), so animals should seek them out.
While it was known that consumers can regulate homostasis with
respect to body composition of C, N, P, etc.,
a cost of such regulation (via excretion) was not assumed
Boersma & Elser 2006
- Unimodal response of consumer
growth rates to P levels among
diverse taxa (5 arthropods, 1 snail,
1 fish)
- low-P animals may be more
sensitive to excess P intake
P*Herb P*Carnivore
Growth
P in diet

3. So What?
- could explain why some generalist herbivores consume large
amounts of low-quality (low P) instead of more nutritious parts
- limits to growth, additional stoichiometric constraint...what effect
would this have on ecosystem models?
- Why herbivores expected to be more sensitive? Adapted to
acquiring and storing limiting nutrients, but perhaps not to deal with
excess
- We do know that excretion have energetic costs (think kidneys
working against osmotic pressure), so why was this ignored?
- form of recycled nutrients (organic vs. inorganic) and impact on
autotrophic vs. heterotrophic processes?
- implications for coexistence with additional nonlinearity of growth
response to food quality?

4. Although homeostasis, fitness AND body composition can be affected by
food quality (locust example)
Decreases in food quality could impair
performace of herbivores, what would this
mean for higher trophic levels?
Low quality plant  low quality herbivores
 bad for carnivores?
If eutrophication, how will bottom-up vs. top-
down respond?
Raubenheimer & Simpson 2004 Ecology

5. Generalists may be able to respond to changes in food quality by changing
intake of food OR selectively grazing (either different species, or finding
tastier member of the same species)
Here, Daphnia increase feeding
(beat) rate for low quality foods,
but decease their food intake with
ever increasing food quality
Plath & Boersma 2001 Ecology

6. Balancing Diets – animals may selectively feed on food of different quality to
reach their fitness optima, or at least compromise when optimum cannot be
reached
Generalist and specialist locusts
reached similar target composition
by modifying intake in food mixtures
But, the patterns among food
monocultures (dashed lines) differed
Raubenheimer & Simpson 2004 Ecology

7. The evolutionary side:
How are animals adapted to deal with food quality in their environments?
- Gut length and architecture
- Extraoral digestion
- Generalism vs. specialism
- Feeding at multiple trophic levels
- Seasonal issues in dominance/succession of food groups. Population dynamics
of consumers, resting stages, etc.
- others?

8. The evolutionary side:
Denno and Fagan argue that many predators (herbivore consumers) are
N-limited and therefore would benefit from supplementing diet with N-rich
predators (intraguild predation or cannabalism)
They furthre argue that this possible benefit helps to explain the evolution
of omnivory in the sense of carnivores eating herbivores and other
carnivores
Denno & Fagan 2003 Ecology
Putting the cart before the horse? What about mixing of plants and
animals?
Herbivores are N-rich compared with plants, so consuming them seems
like it would be selectively favored when plant resources are not
unlimited, assuming you have the morphological/physiological capability
Alternatively, if N or P is eaten in excess, might be easier to eat more
lower quality food to balance C deficit

9. 2 Cool Stories
1. Lupines and Lepidopteran herbivores
abundance
Herbivore
Lupine density
P:C P:C
With succession, increased plant densities leads to greater competition and lower
P content in plants. Translates to decreased herbivore fitness! (predation my also
be important here)
Apple et al. 2009 PLoS ONE

10. 2 Cool Stories
2. Spiders and Grasshoppers
A top-down perspective
-Spiders and their pred. cues
change nutrient constraints in
grasshoppers
- “Scared” hoppers have
higher metabolic rate
- increased demand for
digestible C to fuel energy
demand (but lots of crap C)
↑ C:N in hopper bodies
↑ C:N in fecal material
↑ C:N of plant community!
Hawlena & Schmitz 2010 PNAS

11. Nutrient contents of food appear to be very important to consumers, and
can drive ecological interactions, patters of species abundance, evolution of
feeding strategies, etc.
So, what is the correct scale?
- numbers of individuals
- macromolecules (protein, carbs, lipids, etc.)
- elements (N, P, C)
In what situations would each be favored?
Does the scale of the question (high-level ecosystem phenomena vs.
predator-prey interactions)?
Is this a strict decision, or can we combine scales in models when desired?