LIMNOLOGY
Reading Guide for:
Greig, H., and A. R. McIntosh. 2006. Indirect effects of predatory trout on organic matter processing in
detritus based stream food webs. Oikos 112:31-40.
1. Define a “trophic cascade.”
Predators indirectly affect basal resources through a direct effect on consumers.
2. Contrast autochthonous and allochthonous resource based food webs.
Autochthonous -basal resources are derived from in situ primary production
Allochthonous -basal resources are derived from external sources.
3. Predators can affect basal resources indirectly if they do one of two things.
Suppress the density of consumers or suppress the foraging behavior.
4. Compare and contrast density-mediated and trait-mediated effects of predators on consumers.
A density-mediated effect is when predators reduce consumer density. A trait-mediated effect is
when the presence of a predator causes consumer foraging behavior to change.
5. What is a “compensatory population increase” of consumers in the presence of predators?
A compensatory increase occurs when a consumer that is less vulnerable to predation increases as
a result of reduced density of a competitor that is more vulnerable to predation. Compensatory
dynamics may alleviate the effects of predators on basal resources.
6. List key results from the lethal experiment, the sub-lethal experiment, and the stream survey.
Lethal
Fewer detritivores remained in trout tanks
CPOM loss was significantly lower in trout tanks (presence of trout slowed decomp)
CPOM was highest in high density fishless treatments
Sub-lethal
Trout presence had no effect on detritivore density
Trout presence had no effect on CPOM loss
Supports hypothesis that trout effect is density-mediated rather than trait-mediated
Stream survey
Total shredder biomass was not different between trout and fishless streams
Fishless streams dominated by large detritivore and trout streams by small detritivore
CPOM loss rates highest in fishless streams
Trout have a strong effect on the density of a large detritivore. There is an increase in other
detritivores but they are not capable of compensating for the loss of the large detritivore. The
large detritivore is capable of affecting CPOM loss rates, trout are capable of affecting large
detritivore density, consequently trout can affect CPOM loss rates indirectly through a direct
effect on detritivores.
7. Draw an illustration of the observed links between trout, the shredder community, and detritus
(CPOM and FPOM).
8. Why did small shredders increase in abundance in the trout streams relative to the fishless streams
(the authors give one hypothesis, propose a second one)?
The large detritivore outcompetes smaller detritivores but is highly susceptible to fish predation.
In the absence of fish, the large detritivore dominates the detritivore community. With fish
present, the large detritivore density decreases and this releases the small detritivores from
competition. Because the small detritivores are less susceptible to predation, they flourish in
trout streams.
9. The ability of predators to induce trophic cascades is contingent on 2 things. What are they?
The ability of predators to suppress prey populations well below carrying capacity and on the
degree of functional similarity of species within a particular consumer guild (capacity for
compensatory dynamics).
10. Given your answer above, Why are trophic cascades rare in streams?
High availability of prey refugia makes it difficult for predators to affect consumer density.
Factors other than predation (e.g., high flow events) may keep consumer densities well below
carrying capacity.
Lots of functional redundancy in stream food webs. If one vulnerable consumer is affected by
predators, then that reduction is compensated for by an increase in a less vulnerable consumer.
Fish eat invertebrate predators as well as primary consumers (trophic omnivory). Consequently,
increased predation from fish may result in an overall decrease (or at least no net increase) in
predation on primary consumers.