Transcript Life History Strategy

Outline
1. Density dependent population dynamics: logistic
equation
2. Cyclic and chaotic populations
3. Life history strategies
4. K vs r selection (MacArthur)
5. C-S-R models (Grime)
6. Examples of adaptive life history traits and
strategies
Life table and population ecology
resources
1. USGS interactive population model for
spectacled eiders
http://www.absc.usgs.gov/research/speimod/
2. Sample life table calculations from University of
Cincinnati Clermont College:
http://biology.clc.uc.edu/courses/bio303/life%20t
ables.htm/
3. Interactive models for human population
http://simon.cs.vt.edu/geosim/IntlPop/
Density dependent growth
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As intrinsic rate of increase goes up, behaviour
of model changes:
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Carrying capacity: one equilibrium value for N
Stable limit cycles: N oscillates among several values
As R increases, number of values in cycle doubles (for
2.1<R<2.57)
Eventually (R>2.57) dynamics are CHAOTIC. Not
random, highly density dependent, but unpredictable.
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Time lags can also create cycles:
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Resource availability changes with time and
population size (eg herbivores and food source)
dN/dt=rN((K-N(t-T))/K)
Classic example: lynx and hares…
Alee effect
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Can get density dependent effects that work in
the opposite direction, especially for small
populations
For example: reduced reproduction if population
is too small can accelerate decline to extinction.
This “inverse density dependence” is called an
Alee effect.
Metapopulation
A “population of populations”.
Dynamics driven by dispersal between patches
(colonization) and extinction of sub-populations.
Metapopulation
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Usually more dispersal between close patches,
less extinction in large or good-quality patches.
“Island Biogeography”
Small, poor patches may be “population sinks”
(cannot sustain population on own; are
maintained by dispersal from “source”
populations.
Summary
Demography affects current distributions, historical
range shifts/spread, gene frequencies, and
population structures.
Population dynamics important for commercial
species: yield, growth, survival, etc.
Use population models to create management plans
for both endangered and invasive species
Herbivory (eg stock production) can affect population
parameters of range species: Riginos and
Hoffman (2003). Journal of Applied Ecology
40:615-625.
Life History Strategies
Principle of allocation: individuals have limited
resources to spend on growth, maintenance and
reproduction
Life History Strategy: Pattern of resource allocation
(developed via natural selection)
Successful strategy maximizes reproduction,
survival and/or growth in given environment
Assumption: there is a tradeoff between fecundity
and survival. “Cost-benefit analysis”
Examples of tradeoffs
Current and future reproduction. Poa pratensis
number of inflorescences in year one negatively
related to number in year two
Colonization and competition. Large numbers of
small seeds VS few large seeds.
Examples of tradeoffs
Current and future reproduction. Poa pratensis
number of inflorescences in year one negatively
related to number in year two
Examples of tradeoffs
Current and future reproduction. Poa pratensis
number of inflorescences in year one negatively
related to number in year two
Colonization and competition. Large numbers of
small seeds VS few large seeds.
Root and shoot growth.
Examples of tradeoffs
Current and future reproduction. Poa pratensis
number of inflorescences in year one negatively
related to number in year two
Colonization and competition. Large numbers of
small seeds VS few large seeds.
Root and shoot growth.
Herbivore defense and competitive ability. Defensive
chemicals or structures are costly to make and
may reduce growth/reproductive output.
Examples of tradeoffs
Current and future reproduction. Poa pratensis
number of inflorescences in year one negatively
related to number in year two
Colonization and competition. Large numbers of
small seeds VS few large seeds.
Root and shoot growth.
Herbivore defense and competitive ability. Defensive
chemicals or structures are costly to make and
may reduce growth/reproductive output.
Growth and reproduction. Smaller growth increment
or foliage production after large seed crop for
trees
Allocation to resource acquisition
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Limiting resource model: there is a minimum
level of a resource that is necessary for positive
net growth
Can see tradeoff between resource capture
mechanisms (eg root vs shoot)
Resource ratio hypothesis: mechanism for
competition and succession.
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Coexistence if two species limited by different
resources
Shift in composition through succession because shift
from limited soil resources to limited light
Allocation to survival
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Longer lived species delay reproduction
Longer life span means investment in
maintenance and structure
Disturbed, ephemeral, and variable
environments favour short life span
Less disturbed, moderate environments favour
longer life span
Ephemeral- life span less than 6 months
Annual
Life histories
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Ephemeral – life span less than 6 months
Annual – complete life cycle in one year
Biennial – vegetative one year, reproductive in
following year
Perennial – lives for more than two years
Allocation to reproduction
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Monocarpy or semelparity: Single period of
reproductive activity followed by death.
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Polycarpy or iteroparity: Multiple episodes of
reproduction in lifetime.
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These two strategies have advantages and
disadvantages...
Allocation to reproduction
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Monocarpy or semelparity: mobilize large
reserves, synchronize across region and swamp
predators, minimal investment in structure
(annuals).
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Polycarpy or iteroparity: Multiple episodes of
reproduction in lifetime, greater overall potential
fitness, don’t have “all eggs in one basket”
Classifying life history strategies
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K and r selected species
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A continuum between allocation to reproduction and to
survival.
At one end: selection maximizes intrinsic rate of
increase for population. This means seed output,
dispersal, etc. This is r-selection. Occurs where
mortality is DENSITY INDEPENDENT
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At other end: selection maximizes competitive ability
or survival to maintain population near carrying
capacity. This is K-selection. Occurs where mortality is
DENSITY DEPENDENT
Characteristics of r vs K
r selected species: variable climate, low competition,
short life span, seed bank, seed dispersal,
semelparous strategy. Type I survivorship. e.g.
Cheatgrass.
K selected species: predictable climate, high
competition, constant high population size,
strong competition, no seed bank, delayed
reproduction, iteroparous strategy. Type III
survivorship. e.g. Sugar maple.
R, C, and S strategists
Another classification for life histories (Grime 1977)
R=Ruderal; colonizing species. Good dispersers, high
reproduction. Temporary habitats with high resources.
Allocation to reproduction. EG ‘weeds’
C=Competitive species. Good at obtaining resources.
Predictable habitats with high resources. Allocation to
growth. EG rhizomatous grasses.
S=Stress-tolerant species. Can persist in harsh, low resource
environments. Allocation to maintenance. EG lichens.
R, C, and S strategists
Species tend to have combinations of these traits.
C
CS
CSR CR
SR
S
R
Lab: life cycle diagrams, matrix models, life tables,
and their applications for management
Next lecture: metapopulations, life history strategies,
allocation.