Population Dynamics
Molles & Cahill 2008, Chapter 11
Today’s Topics:
• Factors influencing population growth
• Dispersal
• Survival patterns
– Life tables and age distributions
– Survivorship curves
• Practical Applications of Population Ecology
Population growth & decline:
• Populations increase due to
– Birth (natality)
– Immigration
• Populations decrease due to
– Death
– Emigration
Dispersal – movement from one region to another.
(Affects the “immigration” and “emigration” terms in population models)
Many organisms have a separate “dispersal phase”
Molles 2008
Organisms vary widely in their
dispersal abilities
Fig. 11.18, Molles & Cahill, 2008
Rapid spread of Africanized bees (“killer bees”) following their initial introduction in
Brazil in 1957.
Africanized bees
Surrounding a European
Honey bee (pink dot)
http://en.wikipedia.org/wiki/Image:Africanized_Bee.gif
Fig. 11.17
Molles & Cahill, 2008
Note the slower spread of Eurasian collared doves
Molles 2008
Pollen in sediments
- can be used to analyze changes in plant
populations over long time spans,
- can be used to infer rates of plant dispersal
Molles 2008, Chapter 1
Tree migration after the last ice age
ended (about 12,000 years ago).
Davis et al, cited in Molles (2008)
Organisms disperse in response to a
dynamic population of voles (food supply).
Such “cyclical” population dynamics are common in many
predator-prey interactions
Fig. 11.21,
Molles & Cahll, 2008
Example of a classic population study:
Pokki (1981) studied field voles on islands in Finland
• Pokki trapped animals and counted them
• Repeated this method on islands of different sizes over several years
number of inhabited islands
Island size
#
1972
1973
1974
1975
1976
1977
Average
% of
islands
inhabited
tiny (<1 ha)
40
15
12
28
16
4
16
38
medium
18
11
14
14
12
14
16
75
large
13
11
11
10
7
10
12
78
General conclusions:
1) In no year did all the islands have voles
2) Many populations extinctions and recolonizations occurred
3) More extinctions occurred on tiny islands than larger islands
(effect of island size)
Possible reasons: harsh winters and summer drought
Note
effect
of
island
size
With lots of local population (deme)
extinctions, what keeps the larger
population from going extinct?
•
Possible explanations: migration and availability of empty habitat. To
estimate migration, Pokki used mark & recapture techniques (marked animals
and recaptured them). 8.5% of the animals migrated each year.
•
This situation may be common for many populations, e.g. pest populations
and the spotted owl (Pacific Northwest).
•
Conclusions:
To understand population dynamics of these species, we must understand their
dispersal
– Dispersal among metapopulations (see chapter 10) may be particularly important.
– Maintenance of “corridors” between metapopulations may be critical for species
survival
–
Metapopulation – a group of subpopulations living in separate
locations with active exchange of individuals among
subpopulations.
Dispersal in rivers – upstream dispersal
is needed to counteract drift
Migrating snails (Neritina latissima)
Figs 11.22 & 11.23, Molles & Cahill, 2008
Life Tables & Survivorship
Dall sheep
Life table
Survivorship
curve
Fig. 11.1
Fig. 11.2,
Molles & Cahill 2008
3 types of survivorship curves:
Type 1 – death rates go up
for older individuals (e.g.
Humans in US)
Type 2 – death rates similar
for all ages (e.g. certain bird
populations)
Type 3 – death rate highest
for youngest individuals (e.g.
many insects)
Fig 11.7, Molles & Cahill, 2008
Notice the Axes!
Examples of survivorship curves
Figs. 11.4, 11.5, 11.6
Molles & Cahill, 2008
Age Distributions
• Another way to represent demographic
(life table) information
Contrasting age distributions for two
populations of Populus deltoides
(cottonwood).
Figs. 11.8 & 11.9, Molles & Cahill, 2008
Analysis of age distributions across time
reveal disturbance impacts on populations
Age distributions for
Cactus finches
(Geospiza conirostris)
on the Galapagos
Islands.
Note the effect of
periodic drought (tied to
ENSO cycles of wet &
dry years).
Fig. 11.10,
Molles & Cahill 2008
Weather influences population dynamics
Figs. 12.19 & 12.20, Molles & Cahill 2008
Another way to
represent age
distributions
(Molles & Cahill, 2008,
Chapter 12)
The “Demographic Transition”
Model
Explains the transition from a period of
high birth and death rates to low birth
rates and death rates, and is often tied
to “stages of economic development”
(Warren Thompson, 1929)
Example of the “demographic
transition” for Sweden
http://en.wikipedia.org/wiki/Demographic_transition
Age distributions are different for countries in
different stages of the “demographic transition”
http://en.wikipedia.org/wiki/Demographic_transition
Human population growth how and when will it end?
http://en.wikipedia.org/wiki/Demographic_transition