General Ecology: Lecture 7
Population Growth 2
I.
II.
Population fluctuations and cycles [Figs. 11.10-11.13]
A.
Local phenomena
1.
Localized populations fluctuate more than the entire population
B.
Resilience
1.
Defined: Rate at which a population springs back to its equilibrium after
jumping above or dipping below it due to a “disturbance”.
2.
Effects of body size and lifespan
a.
Smaller/shorter lived animals show both wider swings and greater
resiliency. Why? (2 reasons—know them!)
C.
Oscillating populations
1.
Defined: Populations that seem to have regular cycles [Fig. 11.12]
2.
Key hypotheses for why populations oscillate
a.
Time lags (already described)
b.
Predation and oscillation have been linked
c.
Environmental conditions (changes in food, habitat)

Example: cycles of El Nino and kelp growth
d.
Changes in gene frequencies

Might be random, due to genetic drift

Might be linked to changing environmental conditions that
cause shifts in genotypes. Think about this!
e.
Parasite life cycles

Example: mosquitoes carrying malaria thrive in wet conditions
f.
Behavior
Extinction
A.
What makes a species vulnerable to extinction?
1.
Small size. Why?
a.
Low density:

Difficulty in finding mates (Example: queen conchs in FL)

Vulnerability to hybridization
b.
Predation or environmental disaster will likely claim a higher
proportion of the population (although…)
2.
Isolation. Why?
a.
Restricted gene flow (understand why this is a problem)
3.
The combination of being small and being isolated makes populations
vulnerable to inbreeding and genetic drift.

How can these be deleterious to the population’s survival?
B.
Mass extinctions [Fig. 11.14]
1.
Permian extinction (~250 mya) appeared to claim up to 96% of all species
2.
Cretaceous extinction (~60 mya) (dinosaurs, ammonites),
C.
Process of extinction
1.
Role of local extinctions
a.
Species extinction begins with the loss of localized populations

Often occurs due to habitat destruction, followed by
marginalization of any remaining members

Fragmentation: continuous populations  metapopulations
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2.
3.
4.

Inbreeding and genetic drift occur (Why a problem?)
Deterministic causes of extinction
a.
Occurs due to force or change with continued effects from which
there is no escape for the species

Example 1: Asteroid/climate change theory (Cretaceous)

Example 2: Loss of the Great auk, Pinguinus impennis

Distribution

Hunting for food, bait and feathers, eggs, skins
Stochastic causes of extinction
a.
Occur due to random events that are not continuing

Examples of stochastic events

Usually just reduces a population, but small populations can be
wiped out.
Links between deterministic and stochastic causes of extinction
a.
Heath hen example (see text for details!)
Study questions
1.
What is meant by “resilience” of a population? Why are species comprised of small
individuals often more resilient than those comprised of large individuals? (HINT: Two
factors)
2.
List/explain (1-2 sentences each) several factors that might cause a population to oscillate.
Be familiar with the specific examples given in class for oscillations due to environmental
conditions and oscillations due to parasites.
3.
What characteristics of a population or species make it vulnerable to extinction? List and
then explain the cause/effect relationships.
4.
What deleterious effects might inbreeding and genetic drift have on populations to make
them more vulnerable to extinction?
5.
Does extinction of a species occur all at once? Explain.
6.
What is the largest major extinction that occurred on earth? You should be able to name it
and indicate approximately when (how many millions of years ago) it apparently occurred.
7.
What extinction event is responsible for the disappearance of dinosaurs, ammonites (giant,
shelled cephalopods) and many other species on earth? You should be able to name it and
indicate approximately when (how many millions of years ago) it apparently occurred.
8.
Define and compare the causes of deterministic vs. stochastic extinctions.
9.
Use the heath hen example to explain how deterministic and stochastic factors can work in
tandem to cause extinction.
10.
What human-related factors are most responsible for recent extinctions?
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