1. Chapter 4
Biotics
1) What general pattern did Brooks and Dodson observe in Connecticut lakes?
2) What is the SEH? Which parts are correct?
3) What is the difference between top-down and bottom-up control of lake ecosystems?

2. Pattern Observed by Brooks and Dodson (1965)
Surveyed many lakes in New England and found that Daphnia, Epichura and Mesocyclops were absent from lakes containing Alosa pseudoharengus
ZoopIndicators2.htm
Lakes with alewife were dominated by Bosmina, Ceriodaphnia and Tropocyclops
ZoopIndicators2.htm

3. Alosa pseudoharengus
ans/ans-index.htm
Fish that feeds and grows to maturity in the ocean, then migrates into freshwater to spawn. But some populations have become “landlocked” and live their whole life in lakes.
Unlike other planktivores like bluegill, Alosa will cross the thermal barrier to feed

4. Crystal Lake, Connecticut
Brooks sampled the plankton in 1942
Alosa aestivalis invaded the lake in the 1950s
Dodson was an undergrad at Yale when the lake was sampled in 1964

5. Brooks and Dodson 1965 (over 1350 citations)
To explain their results they proposed size-selective predation and the Size-Efficiency Hypothesis (SEH)
But in 1965 no experimental evidence to support size-selective predation or SEH

6. Size-Selective Predation
First discussed by Hrbáček (1962)
Rozpravy Ceskoslovenské Akademie Ved, Rada Matematickych a Prirodnich Ved
In ponds with < 700 fish/ha (minnows and perch), dominance by large zooplankton:
Daphnia pulicaria ( mm)
Daphnia longispina ( mm)
In ponds with > 30,000 fish/ha dominance by small zooplankton:
Bosmina longirostris ( mm)
Daphnia cucullata ( mm)
But, correlation is not causation

7. Galbraith (1967)
Looked at feeding by rainbow trout and yellow perch in Stager Lake
In Lake
In Guts
Frequency (%)
(Daphnia)

8. Remember from lecture on fish that fish are size-selective because of...
Encounter frequency: Encounter of large prey is higher than small prey
pigmentation also increases visibility
Optimal foraging —take the prey that provides the greatest energy return for cost of capture/handing.
exception to ‘bigger is better’ rule with small fish

9. Size-Efficiency Hypothesis
Brooks and Dodson 1965
1) Planktonic herbivores all compete for the fine particulate matter of the open water
2) Larger zooplankton do so more efficiently and can also take larger particles
3) Therefore, when predation is of low intensity the small planktonic herbivores will be competitively eliminated by large forms (dominance of large Cladocera and calanoid copepods).

10. Size-Efficiency Hypothesis (continued)
Brooks and Dodson 1965
4) But when predation is intense, size-dependant predation will eliminate the large forms, allowing smaller zooplankton to dominate.
5) When predation is of moderate intensity, it will fall more heavily upon the larger species and keep their populations low, so that the smaller species are not eliminated.

11. What have we learned since 1965? Pattern vs. Process
1) Planktonic herbivores all compete for the fine particulate matter of the open water
Only when resources are limiting

12. What have we learned since 1965?
Pattern vs. Process
2) Larger zooplankton do so more efficiently…
Not necessarily
Results of competition are condition dependent

13. What have we learned since 1965?
Pattern vs. Process
2) …and can also take larger particles
But large zooplankton may miss the smaller particles

14. 3) Therefore, when predation is of low intensity the small planktonic herbivores will be competitively eliminated by large forms (dominance of large Cladocera and calanoid copepods).
Competitive dominance is not only a function of body size
When predation by fish is of low intensity on the large grazers, it is also of low intensity on big predatory invertebrates like Leptodora, Mesocyclops and Epichura
Predatory invertebrates like Leptodora, Mesocyclops and Epichura eat small planktonic herbivores

15. 4) But when predation is intense, size-dependant predation will eliminate the large forms, allowing smaller zooplankton to dominate.
Eliminating the larger forms includes eliminating the predators of the smaller forms

16. 5) When predation is of moderate intensity, it will fall more heavily upon the larger species and keep their populations low, so that the smaller species are not eliminated.
It is not that simple....
To fully explain size structure of the zooplankton assemblage, need to consider basin shape, zooplankton behavior, competition and predation (including type of predator)

17. There is no question that Brooks and Dodson observed a very general pattern
Brooks and Dodson’s 1965 result set the stage for the next 40 years of research in aquatic communities.
In the 1980s, Carpenter and Kitchell took it one step further
Trophic Cascade Hypothesis

18. Trophic Cascade Hypothesis
Carpenter et al. 1985
Increasing piscivore biomass…
Brings about a decrease in planktivore biomass
An increase in herbivore biomass
And a decrease in phytoplankton biomass