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?

Chapter 4Biotics

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

Lakes with alewife were dominated by Bosmina, Ceriodaphnia and Tropocyclops

www.potomacriver.org/ ZoopIndicators2.htm

www.potomacriver.org/ ZoopIndicators2.htm

Alosa pseudoharengus

Unlike other planktivores like bluegill, Alosa will cross the thermal barrier to feed

www.anr.state.vt.us/dec/waterq/ 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.

Crystal Lake, Connecticut

Alosa aestivalis invaded the lake in the 1950s

Brooks sampled the plankton in 1942

Dodson was an undergrad at Yale when the lake was sampled in 1964

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

Size-Selective Predation

In ponds with > 30,000 fish/ha dominance by small zooplankton:

Bosmina longirostris (0.3-0.4 mm)Daphnia cucullata (0.7-0.8 mm)

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 (2.0-2.3 mm)Daphnia longispina (1.4-1.8 mm)

But, correlation is not causation

Galbraith (1967)Looked at feeding by rainbow

trout and yellow perch in Stager Lake

In LakeIn Guts

(Daphnia)

Freq

uenc

y (%

)

1. Encounter frequency: Encounter of large prey is higher than small prey

pigmentation also increases visibility

Remember from lecture on fish that fish are size-selective because of...

2. 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

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).

Size-Efficiency Hypothesis Brooks and Dodson 1965

2) Larger zooplankton do so more efficiently and can also take larger particles

1) Planktonic herbivores all compete for the fine particulate matter of the open water

4) But when predation is intense, size-dependant predation will eliminate the large forms, allowing smaller zooplankton to dominate.

Size-Efficiency Hypothesis (continued) Brooks and Dodson 1965

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.

What have we learned since 1965?

1) Planktonic herbivores all compete for the fine particulate matter of the open water

Only when resources are limiting

Pattern vs. Process

What have we learned since 1965?

2) Larger zooplankton do so more efficiently…Not necessarily

Pattern vs. Process

Results of competition are condition dependent

What have we learned since 1965?

2) …and can also take larger particlesBut large zooplankton may miss the smaller particles

Pattern vs. Process

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

Eliminating the larger forms includes eliminating the predators of the smaller forms

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.

To fully explain size structure of the zooplankton assemblage, need to consider basin shape,

zooplankton behavior, competition and predation (including type of predator)

It is not that simple....

Brooks and Dodson’s 1965 result set the stage for the next 40 years of research in aquatic communities.

Trophic Cascade Hypothesis

In the 1980s, Carpenter and Kitchell took it one step further

There is no question that Brooks and Dodson observed a very general pattern

Trophic Cascade HypothesisIncreasing piscivore biomass…

Brings about a decrease in planktivore biomass

An increase in herbivore biomass

And a decrease in phytoplankton biomass

Carpenter et al. 1985