Download Intra-specific Interactions II

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
Species with a high growth rate are capable
of quickly tracking changes in their
environment
Long-lived, slowly-reproducing species
have populations that do not track
environmental conditions closely
Example: differences in r
From Ricklefs and Miller 2000 p. 350
R versus K selection
•Evolution of life history strategies
The General Idea:
•Organisms dealing with highly variable
environments should have high growth
potential
•Organisms in constant environments show
adaptations for competitiveness and
efficiency of resource use
Pianka’s Summary of r and K Selected
Attributes
(Ricklefs and Miller 2000, p. 674)
Where do we stand now?
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
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. 1995 Nature 375:227-230.
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
•Weather?
•Combination?
Rodent abundances at Kilpisjarvi, Finland
Hansen et al. 1999 Am Nat 154:129-139
The Great Kluane Experimental System
•Large scale experiments with nutrient addition,
supplemental food, and predation control
•Suggested that hare cycles result from threetrophic-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