Intra-specific Interactions II

What are the implications of density dependencein real populations?

Do natural populations show fluctuations thatcould be the result of density dependence?

Density dependence as a regulating factor

Fluctuations and cycles in natural populations

Not all fluctuations are the result of densitydependent forces- the world is avariable place

Variation can be predictable (seasons, diurnal,tide cycles) or stochastic (weatherpatterns, catastrophes such as severefires)

These will affect populations differently

Population fluctuationsThe range of fluctuations a population may

show is also a function of its growth rate

Long-lived, slowly-reproducing specieshave populations that do not trackenvironmental conditions closely

Species with a high growth rate are capableof quickly tracking changes in theirenvironment

Example: differences in r

From Ricklefs and Miller 2000 p. 350

R versus K selection

•Evolution of life history strategies

•Organisms dealing with highly variable environments should have high growthpotential

•Organisms in constant environments showadaptations for competitiveness and efficiency of resource use

The General Idea:

Pianka’s Summary of r and K SelectedAttributes

(Ricklefs and Miller 2000, p. 674)

There is no apparent link between populationgrowth rates and fluctuations, and lifehistory characteristics…

Theory a departure point for workin evolution of life history strategiesin regulated populations

Where do we stand now?

Back to dynamic patterns: Cycles can result from demographic processes

These are the result of time lags and densitydependence:

Delayed density dependence

Are the patterns observed for the discretelogistic equation really biologically possible?

Actually, yes.

Experimental and theoretical work byConstantino et al. 1995 Nature 375:227-230.

Another example: blowflies

A. J. Nicholson explored how delayed densitydependence might induce population cyclesusing blowflies

First experiment: larvae given limited food,adults fed unlimited food supply

Delayed density dependence in fecundity ledto dramatic population cycles

Example: blowflies

Restricting adult food supply acted to makedeleterious effect of density immediatelyapparent

Food-limited adults cannot lay as many eggs;very high densities of larvae prevented.

Population fluctuations disappeared….

Example: blowflies

(From Ricklefs and Miller 2000 p. 358)

Which brings us to population regulation….

Regulatory factors:The strength of the effects is related topopulation density

Limiting factors:Effects on survival and reproduction areindependent of population density

(note that intra-specific interactions are notthe only possible regulating factors…)

Limitation versus Regulation in Natural Populations

How much is density dependence responsiblefor the patterns of abundance that we see?

Are populations more regulated or limited?

This question has been the basis for anoften-rancorous debate for decades

Population Regulation

Populations are regulated if they display three phenomena:

1). The population persists through time

2). Population size is bounded so that althoughsize fluctuates, it does so in a constrainedmanner

3). The population demonstrates returntendency, i.e., it increases when smalland declines when large

(Hixon et al. 2002)

WHY DO WE CARE?

Important implications for applied population management:

Additive vs. compensatory mortality

The question: would individuals that areharvested, or killed through incidentalcauses (toxic spills) have died anywaydue to density dependence?

Understanding cycles in natural systems:The case of the vole and hare cycles

Charles Elton and the Lemming Myth

….Possible causes?

•Intraspecific competition over food

•Predation

Rodent abundances at Kilpisjarvi, Finland

Hansen et al. 1999 Am Nat 154:129-139

•Combination?

•Weather?

The Great Kluane Experimental System

•Large scale experiments with nutrient addition,supplemental food, and predation control

•Suggested that hare cycles result from three-trophic-level interaction

-Krebs et al. 1995. Science 269:1112-1115

The cycles are disappearing….

WHY?

Hornfeldt 2004. Oikos 107:376-392

Summary

•Populations show dramatic changes in behavior due to growth rate and DD

•Populations can be limited, regulated, or both

•Regulated populations show boundedness,return tendency, and persistence

•Population fluctuations subject of much inquiry; r and K selection, cycles

Ungraded Writing Assignment

• Explain the difference between population regulation and limitation, and list the three phenomena necessary for regulation to occur