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Life histories
• Diagrams summarize average life
history events (usually with 1-year
time steps)
• Result of natural selection
– Represent successful ways of allocating
limited resources to carry out various
functions of living organisms
Tradeoffs
• Size (survival) vs. number of
offspring
• Fecundity vs. adult survival
• Age at first reproduction
• Fecundity and growth
Phenotypic plasticity: Solid line indicates that
reaction norms can evolve!
Figure
10.7
Environmental variation in
biological conditions
• Test for phenotypic plasticity by
transplanting individuals (similar
genotype) to several environments
• Expose to predators, different
resources
Membranipora membranacea: bryozoan common in the PNW
Modular species: grows by
making new feeding units at
the edge of the colony.
Chemicals released by
predatory nudibranchs
induce the formation of
spines.
Spines protect the
bryozoan from predation
by nudibranchs. Why not
have them all the time?
Induced defense reduces growth rate
Induced defense reduces reproduction
Time to metamorphosis depends on food availability
In this case, both time and size at metamorphosis
are affected
Figure
Phenotypic plasticity
• Modular organisms can respond to
environmental cues and alter the
characteristics of new modules
• Genotype x environment interaction:
plasticity itself can adapt
Grime’s plant life histories
• Competitive: Large, fast potential growth
rate, early reproduction, vegetative spread
• Ruderals: high potential growth rate, early
reproduction, production is largely seeds,
seed bank/ easily spread seeds
• Stress tolerators: slow growth rate, late
reproduction, little energy to seeds
•These data show that
plant species coexist best
at low rates of
fertilization, and
moderate rates of
disturbance in Minnesota
prairies.
•The authors suggest that
coexistence is enhanced
due to tradeoffs between
competitive and
colonization abilities.
Wilson and Tilman 2002
Colonization ability
(Seed dispersal)
Competitive ability
(Rate of nutrient use)
•At low disturbance,
lots of perennial plants,
shrubs
•At high disturbance,
lots of annual plants,
easily dispersed
•But as soil is
fertilized, competitive
plants become more
common throughout
disturbance gradient
Wilson and Tilman 2002
Populations:
Distributions/ Life tables
Ruesink lecture 4
Biology 356
Ecology includes the study of patterns in the
distribution and abundance of species
• Distribution is the spatial
arrangement of organisms within a
species
– What is their total range?
– Are there particular habitat
associations?
– Within a habitat, is the species clumped,
random, or more evenly spaced?
Total species on
earth
Historical filter:
Which taxa
evolve?
Physical filter:
What is activity
space?
Biological filter:
Effects of other
species?
Three distinct ways that populations
are distributed in space
Figure 13.5
Aspen: Clumped, random, or evenly spaced?
Figure 13.7
Desert shrubs: Clumped, random, or evenly spaced?
Figure
13.6
Distribution often depends on
the scale of observation.
How is this species distributed
across its geographic range?
How is it distributed within
glades?
How are individuals distributed
within an aggregate?
Figure 13.3
Ecology includes the study of patterns in the
distribution and abundance of species
• Abundance is the number of
individuals in a population
– Biologically, a population is a group of
regularly-interbreeding individuals
– Operationally, it is the size of the
researcher’s study site
On average, every individual produces one
successful offspring (replaces itself)
• This case represents species that
have populations in a dynamic steady
state
– Births equal deaths
Elephant seals: 1890 = 20; 1970 = 30,000
Figure
14.12
English ivy
Scot’s broom
Japanese oysters
European starling
Invasive species
• Non-indigenous = alien = exotic =
introduced = non-native
• Why (historically) aren’t all species
everywhere?
• Many species evolved in allopatry and
have remained isolated by
biogeographic barriers
Ruesink et al. 1995
Bioscience
Cumulative number of species
Current
rates of
invasion are
orders of
magnitude
higher than
in the past
In the Galapagos, number of alien plant species is
closely related to the human population on the islands
Invasion pathways
• Purposeful
– Planned releases
– Imports
• Accidental
85% of exotic woody species in the
U.S. were introduced for
horticulture
English holly
English ivy
Purple
loosestrife
European
buckthorn
Costs and benefits
• Estimated annual cost of invasive
species in US = $137 billion based on
50,000 species ($41 B from crop
weeds and pests)
• Estimated annual benefits = crops and
livestock
• Pimentel et al. 2000 Bioscience
Population ecology
What makes a good invader?
• Life history traits of successful
invaders from first principles
• High reproductive rate
• Modular
• Broad activity space = generalist, not
specialist
Evidence – freshwater fishes with
smaller body size are more likely to
invade (shorter time to maturity)
Proportion established
1
0.8
0.6
0.4
0.2
0
0
50
100
150
Length (cm)
200
250
Some species increase rapidly
• Newly-introduced species
• Species that can “outbreak”
• Species hunted to low population
levels and then protected
– But sometimes they do not recover
• Births exceed deaths
Is harvest or hydropower most responsible for the
decline of Columbia River Chinook salmon?
Kareiva et al. 2000
Runs in different seasons are genetically distinct
Harvest
Spawn andHatcheries
die
Chinook spend 3-5 years
in the ocean before
returning
Habitat
Hydropower
Smolts
migrate to estuary
Some species are in decline
• Most threatened and endangered
species
• Deaths exceed births
Ecologists have developed a simple (!) way of
summarizing birth and death schedules
• Follow the fate of one cohort through
the lifespan OR Track the birth and
death rates of each life stage
100 eggs
Survive and
grow to 25
tadpoles
Survive and
grow to 10
young frogs
Survive and grow to 5
adult frogs, each of
which lays 20 eggs
Life table for frog example
Age (x) Number
alive
0
100
1
25
2
10
3
5
Survivorship
(lx)
Mortality
rate
(mx)
Survival rate
Fecundity
(sx)
(bx)
100 eggs
Survivorship
1.0
Survive and
grow to 25
tadpoles
0.25
Survive and
grow to 10
young frogs
0.1
0.05
Survive and grow to 5
adult frogs, each of
which lays 20 eggs
Life table for frog example
Age (x) Number
Survivorship
alive
(lx)
0
100
1.0
1
25
0.25
2
10
0.1
3
5
0.05
Mortality
rate
(mx)
Survival rate
Fecundity
(sx)
(bx)
100 eggs
Survive and
grow to 25
tadpoles
Survive and
grow to 10
young frogs
Mortality rate
0.75
0.6
0.5
??
Survive and grow to 5
adult frogs, each of
which lays 20 eggs
Life table for frog example
Age (x) Number
Survivorship
Mortality
rate
Survival rate
Fecundity
(sx)
(bx)
alive
(lx)
0
100
1.0
0.75
0.25
1
25
0.25
0.6
0.4
2
10
0.1
0.5
0.5
3
5
0.05
(mx)
100 eggs
Survive and
grow to 25
tadpoles
Survive and
grow to 10
young frogs
Fecundity
0
0
0
20
Survive and grow to 5
adult frogs, each of
which lays 20 eggs
Life table for frog example
Age (x) Number
Survivorship
Mortality
rate
Survival rate
Fecundity
(sx)
(bx)
alive
(lx)
0
100
1.0
0.75
0.25
0
1
25
0.25
0.6
0.4
0
2
10
0.1
0.5
0.5
0
3
5
0.05
(mx)
20
Compare to life history diagram
20
0
1
0.25
2
0.4
3
0.5
Differences: Time steps need not be one year.
All individuals move from current stage to the next.
Life tables
• x = age
• lx = survivorship up to age x (proportion
living)
• mx = mortality between age x and age
x+1
• sx = survival rate from age x to age x+1
• bx = fecundity (births) at age x
Table 14.4
Net reproductive rate
• R0
– “R nought”
– net reproductive rate, that is, number of
offspring produced by average individual
• R0 = S lx bx
Life table for frog example
Age
(x)
Mortality Survival Fecundity
Num Survivors
hip
rate
rate
(bx)
ber
(lx)
(mx) (sx)
alive
0
100 1.0
0.75
0.25
0
1
25
0.25
0.6
0.4
0
2
10
0.1
0.5
0.5
0
3
5
0.05
20
l x bx
Life table for frog example
Age
(x)
Mortality Survival Fecundity
Num Survivors
hip
rate
rate
(bx)
ber
(lx)
(mx) (sx)
alive
0
100 1.0
0.75
0.25
0
1
25
0.25
0.6
0.4
0
0.25 x 0 = 0
2
10
0.1
0.5
0.5
0
0.1 x 0 = 0
3
5
0.05
20
0.05 x 20 = 1
lx mx
1.0 x 0 = 0
Net reproductive rate
• Note that R0 does not explicitly
determine population growth rate,
which depends on both how many and
when offspring are produced
– Offspring produced earlier lead to higher
population growth
• Need to include average age at which
an individual gives birth (generation
time)
Life table calculations
• R0 =Net reproductive rate = S lx bx
• T = generation time = S x lx bx/ S lx bx
• For frog example, T =
(3)(0.05)(20)/((0.05)(20)) = (3)/(1) = 3
• These time steps need not be “years”
Age structure describes the relative contribution
of different age classes to total population size
Figure 14.4
Population growth rate and age
structure influence each other
• Age structure can influence population
growth
100 eggs
Survive and
grow to 25
tadpoles
Survive and
grow to 10
young frogs
Adult frogs eaten by
horde of herons
Total population
135 individuals
No reproduction
So during this
time period the
population has
to shrink in size.
Only mortality
occurs.
Eggs destroyed
by UV radiation
Total population
135 individuals
135 x 25/40 = 84
135 x 10/40 = 34
135 x 5/40 = 17
17 x 20 = 340 eggs
(compared to 100
from “normal” age
structure)
Population growth rate and age
structure influence each other
• Population growth rate also influences
the age structure of the population
Population structure also varies for populations with
different rates of population growth
Births stable
since 1950
Births stable
since 1990
Figure 14.8
Next week
• In class assignment on population
dynamics
• We will relate life tables to the
exponential and logistic population
growth you learned in Bio 180
• Bring a calculator!