Intra-specific Interactions II What are the implications of density dependence in real populations? Do natural populations show fluctuations that could 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 density dependent forces- the world is a variable place Variation can be predictable (seasons, diurnal, tide cycles) or stochastic (weather patterns, catastrophes such as severe fires) These will affect populations differently

Population fluctuations The range of fluctuations a population may show is also a function of its growth rate Long-lived, slowly-reproducing species have populations that do not track environmental conditions closely Species with a high growth rate are capable of quickly tracking changes in their environment

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 growth potential Organisms in constant environments show adaptations for competitiveness and efficiency of resource use The General Idea:

Pianka’s Summary of r and K Selected Attributes (Ricklefs and Miller 2000, p. 674)

There is no apparent link between population growth rates and fluctuations, and life history characteristics… Theory a departure point for work in evolution of life history strategies in 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 density dependence: Delayed density dependence

Are the patterns observed for the discrete logistic equation really biologically possible? Actually, yes. Experimental and theoretical work by Constantino et al Nature 375:

Another example: blowflies A.J. Nicholson explored how delayed density dependence might induce population cycles using blowflies First experiment: larvae given limited food, adults fed unlimited food supply Delayed density dependence in fecundity led to dramatic population cycles

Example: blowflies Restricting adult food supply acted to make deleterious effect of density immediately apparent 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 to population density Limiting factors: Effects on survival and reproduction are independent of population density (note that intra-specific interactions are not the only possible regulating factors…)

Limitation versus Regulation in Natural Populations How much is density dependence responsible for the patterns of abundance that we see? Are populations more regulated or limited? This question has been the basis for an often-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 although size fluctuates, it does so in a constrained manner 3). The population demonstrates return tendency, i.e., it increases when small and 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 are harvested, or killed through incidental causes (toxic spills) have died anyway due 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 Am Nat 154: 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 Science 269:

The cycles are disappearing…. WHY? Hornfeldt Oikos 107:

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