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Population
Ecology
What drives population cycles?
ESRM 450
Wildlife Ecology and
Conservation
Population Fluctuations
•  Density dependence tends to push populations toward
carrying capacity, K
•  Consequently, populations do not grow indefinitely (over
long term)
•  Yet they often don’t rest at K either
–  i.e., density dependence doesn’t always lead to a static
equilibrium
•  Instead, at least some variability around K is the rule
Population Fluctuations
•  Fluctuations around K can be dramatic, sometimes
exceeding 1000-fold…
•  …leading many ecologists to
Population Size
-  explore their occurrence in different species
-  and ask why they occur
K
Time
Irregular vs. Cyclic Population
Fluctuations
•  Population fluctuations can be erratic (irruptive)
–  Often due to variation in density-independent environmental
factors that have a large, immediate impact on population size
(e.g., fires, catastrophes)
•  Or they can be periodic (cyclic)
•  Populations exhibiting periodic cycles hit peaks and
valleys in abundance at regular intervals
Evidence for Cycles
•  Records of gyrfalcons (Falco rusticolus)
exported from Iceland in the mideighteenth century indicate dramatic and
regular population fluctuations
Popularity of falconry waned
Why Do Cycles Occur?
•  One answer: Cycles are the result of time lags in
responses of populations to their own density
–  i.e., delayed density dependence
Intrinsic Mechanism For
Population Cycles
•  That is, cycles derive from intrinsic properties of populations
–  No environmental (extrinsic) change needed
•  Populations acquire “momentum” when high birth rates at low
densities cause the populations to overshoot K
•  Overshoot of K causes very low survival and birth rates,
population falls well below K
•  Recovery occurs when birth rates again reflect current, lowdensity, conditions
•  Key point: Population cycles derive from time delays in the
responses of birth and death rates to current environmental
conditions
–  nature of cycle depends on nature of time delay
–  Instant updating, no cycle
The Importance of a Time Delay:
An Empirical Example
–  Laboratory populations of Daphnia
–  at higher temperature (25oC), Daphnia
magna exhibits oscillations:
•  period of oscillation is 60 days (delay 12-15
days)
•  At high density, reproduction stops; doesn’t
begin again until senescent individuals die
off (takes at least 10 days; a slow decline)
•  Next increase phase needs recruitment of
young, fecund individuals
–  at lower temperature (18oC), the
population fails to cycle, because of
little or no time delay of responses
•  Minimal senescent period (rapid die off)
Pratt (1944) Biol Bull
Sources of Intrinsic Time Delays
•  Daphnia example neat because is implicates
demographic process as source of time delay
–  High survival of senescent, non-reproductive adults under
crowded conditions in warm environment
•  Other sources of time delays might include
–  Slow development (current generation a product of past
environment)
–  Food (nutrient) storage: holdover from period of plenty
buffers population from current conditions, for a while…
Why Do Cycles Occur?
•  Another mechanism: Cycles are the result of time
lagged influence of density-dependent extrinsic
factors
–  e.g., delayed density-dependent predation, parasitism,
disease
Time Lag
Predation Rate
Prey Population Size
An Extrinsic Mechanism For
Population Cycles
Time
Because of time lag, prey population overshoots K before predation (parasitism)
drives it back down; can’t grow toward K again until predation (parasitism) rate drops off
A Cycle Caused by Extrinsic
Factors
•  Across boreal forests of North America…
•  Trapping records of the Hudson’s Bay Company show
cycles in populations of snowshoe hares (Lepus
americanus) and Canada lynx (Lynx canadensis)
The Snowshoe Hare Cycle
# of Pelts
•  Fluctuations are dramatic, characterized by a roughly
10-year period
•  Lynx cycle lags a year or two behind that of hare cycle
Mechanics of the “10-Year” Hare
Cycle
•  The 10-year snowshoe hare cycle consists of three
phases
–  Increase Phase
–  Decline Phase
–  Low Phase
Krebs et al. (1995) Science
Mechanics of the “10-Year” Hare
Cycle
•  Increase phase
– 
– 
– 
– 
Time of plenty (lots of winter browse)
High productivity, rapid (exponential) population growth
Before long (2-3 years), K is exceeded
and, all the while, specialized predator (lynx) populations
growing as well
Krebs et al. (1995) Science
Mechanics of the “10-Year” Hare
Cycle
•  Decline phase
– 
– 
– 
– 
Huge momentum shoots hare populations well beyond K
Food becomes limiting (see picture), predation heavy
So, productivity and survival plummet, triggering decline
Decline is swift (1-2 years), driven by time-lagged predation
Krebs et al. (1995) Science
Mechanics of the “10-Year” Hare
Cycle
•  Low phase
– 
– 
– 
– 
Following decline, hares are scarce, allowing food to recover
Predators die off, disperse so survival is high
Yet, low phase can persist for 4-5 years
Why? Answer seems to be stress (time lag); i.e., the “ghost”
of predation risk
Boonstra et al. (1998) Ecology
Southern Snowshoe Hare
Populations
•  Along the southern periphery of the species’ range,
cycle doesn’t seem to exist. Why?
–  Still debated, but…
–  Apparently, increase phase never occurs
–  Instead, hares perpetually suppressed by suite of generalist
carnivores (never die off, disperse)
–  No predation time lag
–  also, fragmentation allegedly intensifies predation