Download Demography and Populations Survivorship

Demography and Populations
• Demography is the
study of fecundity and
survival
• Four critical variables
– Age of first breeding
– Number of young fledged
each year
– Juvenile survival
– Adult survival
Survivorship
Mammal
Bird
%
%
Surviving
Surviving
Age
Age
1
Avian life history patterns
• Mortality rates high in first
year of life, then drop off
• Reproductive success
improves with age and
experience
• Long-lived species tend to
have low reproductive rates
and take longer to reach
maturity compared to shortlived species
• Ecological factors influence
life history patterns in
predictable ways
From Ricklefs. 2000. Condor 102:9-22
Extremes of avian life history patterns
Species
Survival
before
breeding
Age of first
reproduction
Fecundity
Adult mortality
rate
Albatrosses and
eagles
Moderate
(30%/year)
Late
(8 – 10 years)
Low
(0.2 young/year)
Low
(5%/year)
Ducks quail and
small passerines
Low
(15%/year)
Early
(1 year)
Moderate
(3 young/year)
High
(50%/year)
2
Components of a life table
Σ
1.01
• Sx = annual
survivorship
• Lx = probability of
surviving to age x
• Bx = age-specific
fecundity (number of
female young
produced each year
by adults in this
cohort)
• (Lx) (Bx) = expected
annual fecundity
• R0 =  (Lx) (Bx) = net
reproductive rate
Σ
0.52
Equal lifetime fecundity
Expected annual fecundity = (Lx) (Bx)
Lx = Probability of surviving to age x
Bx = Age specific fecundity
Lifetime fecundity = Area under curve
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Survival rates
Seabirds > Landbirds
Large > Small
Tropical > Temperate
Adult vs juvenile survival
Survival in Florida Scrub Jays
Fledging weight
and survival in
Great Tits
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Tropical vs temperate survival rates
• Karr (1990) found no differences
between Maryland and Panama
• Oatley and Underhill (1993) found
survival in African species higher
than similar European species
• Johston et al. (1997) found
Trinidadian birds survive better
than N. American birds
Avian longevity
Maximum ages (wild birds):
Small passerines: 10-15 years
Waterbirds & raptors: 20-30 years
Longevity records:
65 year old Laysan Albatross
36 year old Eurasian Oystercatcher
34 year old Great Frigatebird
Captive birds live longer than wild birds
Some captive parrots >80 years
Life expectancy = (2-m)/2m
Where m = annual mortality
AMOY (m) = ?
m = 5%
(2-m)/2m = 19.5
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Fecundity
• Number of young raised
to independence
– Annual fecundity
• Number of nesting attempts
• Nesting success
• Clutch size
– Lifetime fecundity
• Age at first breeding
• Breeding frequency
• Lifespan
Fecundity – age at first breeding
Recall that:
dN/dt = rN
Where:
N = population size
t = time
r = instantaneous population growth rate
Alternatively:
(Log e R0)/T = r
Where:
R0 = net reproductive rate = Σ LxBx (R0 > 1 increase, R0 < 1 decrease)
T = generation time = Σ xLxBx / R0 = 2.6 years for Screech Owl
What to do?
Note: the age at first breeding has a disproportionate effect on the potential growth rate of a
population (r). For example, doubling Ro (via higher fledging success) increases r by 31%,
But….. Reducing T by 50% increases r by 100%. Therefore individuals that can breed earlier
should (all other things being equal) because that will give them the higher payoff in lifetime
fecundity…. (all other things are not equal).
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Fecundity – age and experience
• Older California Gulls
spend more time
attending their nests and
young and are more
successful than young
birds
• Delayed breeding is
favored in long-lived
species and where cost of
breeding is high
• Cost of first breeding may
be reduced by males of
some species that retain
cryptic female plumage
Fecundity – number of nesting attempts
• Number of attempts
reflects:
– Length of nesting
season
– Food supplies
– Predation pressure
• Tropical species:
– Two to six
broods/season
– Longer renesting
intervals
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Fecundity – nesting success
Temperate > Tropical
Hole-nesters > Open-nesters
• Primary causes of nest
failure
Large, well tended > Small, precocial
–
–
–
–
–
Predation
Starvation
Desertion
Hatching failure
Adverse weather
Fecundity – clutch size
• Passerines, 2-12
(usually 2-6)
• Precocial species,
up to 20
• Hummingbirds and
doves, always 2
• Most pelagic
seabirds, 1
8
Evolution of clutch size:
4 major hypotheses
1.
Lack's Food Limitation Hypothesis: parental ability to care
for nestlings dictates clutch size.
2.
Tradeoff Hypothesis: future survival dictates
maximum possible clutch sizes.
3.
Predation Hypothesis: nest predation selects for
smaller clutches because they are less conspicuous
and minimize short-term losses.
4.
Seasonality Hypothesis: clutch size reflects the
seasonal availability of resources relative to
population size.
Lack’s food limitation hypothesis
• Birds raise maximum
number of young they
can feed
• Clutch size manipulation
experiments confirm this,
but there are many
exceptions
• Alternative hypotheses
– Tradeoff hypothesis
– Predation hypothesis
– Seasonality hypothesis
In the population of Great Tits in
Wytham Wood, broods of 10 – 12
chicks were the most productive. The
average clutch size is 8.5
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Why don't birds lay more eggs?
Relationship between reproductive
effort and parental fitness is a major
goal of life history theory
–
–
–
–
Lack (1947) submits that
clutch size has been shaped
by natural selection to
reflect (on average) the
number of young adults can
feed
25 year cycle of interest in
topic
Early experiments focused
on chick stage
97 studies to date
Modification of Lack's theory to
include trade-offs
Counting young fledged doesn’t accurately measure parental fitness
Trade-offs in parental and offspring survival, and lifetime reproductive
performance may be critical
Current evidence conflicting
Daan et al. (1996) parental survival reduced
when Kestrel brood enlarged
Orell et al. (1996) brood enlargement did not affect adult survival in
Willow Tit
Life history attributes depend on "state-dependent" variables McNamara and
Houston (1996)
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Estimating the true cost of reproduction
Estimates of egg composition indicate that
the energy required for egg production
range from 13 - 41% BMR in passerines to
200% BMR in waterfowl
- Need more direct measures.
Ward (1996) finds 5% BMR in doubly
labeled water studies of Barn Swallows
Incubation costs are also not trivial, 19 50% above BMR. Experiments show low
nestling growth rates or fewer young
fledged by birds that incubate extra eggs
Full cost studies of Collared Flycatcher,
Goldeneye, Gannet, and Common Tern
are revealing.
Estimating the true cost of reproduction
future work
•
How are costs of egg production, incubation, and
chick rearing stages partitioned?
•
How do stages of nesting cycle interact?
•
How important are state-dependent variables,
especially the quality of individuals at fledging?
•
How important are factors other than the parent’s
ability to produce and provision a brood, such as
predation and the length of nesting season?
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The predation hypothesis
Three ways predation could
select for small clutch size
1. Larger clutches take longer to lay
(longer exposure to predators)
2. Larger broods are noisier and
more conspicuous (more likely to
attract predators)
3. Longer breeding seasons could
favor more nesting attempts with
fewer eggs per attempt
Age-related mortality explains life history strategies
of tropical and temperate songbirds
Thomas E. Martin Science 2015;349:966-970
Conceptual framework for life history strategies.
Over the circle of life, mortality risk varies
across life stages (gray ring) and exerts
selection on growth strategies (orange boxes)
and parental strategies (yellow boxes). Blue
arrows reflect positive selection; red arrows
reflect negative selection; black arrows reflect
the influence of life history traits on each other.
Nest and fledgling predation exert opposing
selection on length of the nestling period.
Fledgling predation risk favors longer nestling
development to enhance locomotor traits (i.e.,
longer wings), but longer periods increase nest
predation risk. Increased parental investment
and steady mass growth allow enhanced wing
growth without extending the length of the
nestling period and increasing predation risk.
Parental investment is a function of total
parental effort (total provisioning rate)
partitioned among young, where total parental
effort is a result of adult and nest mortality. The
higher parental investment that facilitates the
longer wings favored in tropical birds is
achieved by small clutch sizes.
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Fig. 2 Growth and nest predation on three continents.(A) Peak growth
rate is faster in species with higher nest predation risk but is slower in
tropical species with the same level of risk as temperate species, while
controlling for mass (table S1A). Growth rate is the conventional peak
rate of growth, Ki (see Fig. 3A). (B) Nestling period covaries with growth
rate, but tropical species have shorter nestling periods for the same
growth rate as temperate species (table S1B).
The seasonality hypothesis
• Magnitude of seasonal
flux in resources
shapes clutch size
(Ricklefs 1980)
• Clutch size increases
with A/E ratio
• Northern Flicker clutch
size increases by 1 egg
per 100 of latitude
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Rates of population growth
Recall that R0 =  (Lx) (Bx) = net reproductive rate
R0 > 1 = population growth
R0 < 1 = population decline
ln R0/T = r = intrinsic rate of natural increase (where T =
generation time)
r > 0 = population growth
r < 0 = population decline
Exponential growth can be
described by:
dN/dt = rN
R0 = 1.25
r = 0.23
House Finch introduction to Eastern U.S.
Growth potential differs among species
• K–selected species
– 10% – 30%/year
– R0 = 1.1 – 1.3
– r = 0.09 – 0.26
• r-selected species
– 50% – 100%/year
– R0 = 1.5 – 2.0
– r = 0.41 – 0.70
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Eventually, negative feedback
slows growth
K = Carrying capacity
Population Regulation
Density dependence – the tendency for the death
rate in a population to increase, or the birth rate to
decrease, as population density increases
Population regulation – maintenance of average
population size via density dependent processes
Population limitation – ceiling on population
growth (carrying capacity, K)
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Factors that limit populations
Habitat
Weather
Parasites & Disease
Food is the factor that limits
most bird populations
• Irruptions of Snowy Owls
and northern finches is
directly tied to food
supplies (small mammals
and conifer seeds).
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Density-dependent population regulation in Great Tits
A = Deciduous
B = Mixed
C = Pine
Fecundity varies as a function
of population density
Populations are limited during the
winter where juvenile survival is
closely tied to the abundance of
seeds from Beech trees.
Density dependent
juvenile survival limits
population size from
year to year.
Populations are limited during the
winter where juvenile survival is
closely tied to the abundance of
seeds from Beech trees.
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Population management requires
good demographic data
Summary
• Variations in demographic parameters
(breeding success, mortality, immigration,
and emigration) determine population size
• Biotic (habitat, food, parasites, predators) and
abiotic factors (weather) can set limits on
population size
• Bird populations are regulated primarily by
density dependent processes
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