Transcript Life histories

Life History Patterns
Life histories of species are the set of parameters (including
the ones in a life table) that are important in describing the
factors critical in survivorship and reproduction of the
species.
In addition to survivorship and fecundity in the life table,
things like whether there is parental care, how many and how
large offspring in a litter are, the timing of litter production,
and a number of other factors are important in describing
a life history.
When we consider why a population is successful or is
endangered, we evaluate the life history to find answers.
The same basic approach, that is looking at the balance
between birth rate and death rate that was important in
looking for an equilibrium population size,
is useful in understanding how environmental contamination and harvesting affect natural populations and their size.
Review: Environmental contamination, for example DDT,
reduced fecundity in predatory birds. The DDT
affected peregrine falcon shell glands. Females laid eggs
with thinner shells that broke when the female tried to
brood them. The result was seriously reduced fecundity.
Viewed as a balance between birth and death, that reduced
the carrying capacity for peregines...
What about harvesting? Harvesting may not affect the
fecundity of surviving individuals, but clearly increases
the death rate. A good example here is the near extinction
of the blue whale. The survivorship of the whales declined
considerably even during the 20th century. That decline
could also be shown as an increased death rate.
Mouth of a blue whale –
100’ long, ~160 tons, and
hunted to near extinction.
Sperm whale, another
endangered species, and
also a plankton feeder in
Antarctic waters. It is
hunted by Japanese whalers
for “research”.
The curves on the last slide were survivorships. Here is a
generalized version of that as a set of birth and death
curves, showing the lowered equilibrium population size
with harvesting...
Another review point: the Allee effect
We already know that r is, for many species,
density-dependent. According to the basic logistic model
the relationship is linear. Remember:
1/N dN/dt = r (K-N)/K = r (1 - N/K)
This is a linear equation. If we plot r versus N instead, r
is positive for population sizes less than K, and negative
above K.
This suggests that when species are rare, their population
growth and r should be maximal. Then why do rare species
go extinct? If protected, rare species should recover if this
simple model is correct.
However, rare species have gone extinct even when
protected. Why?
The answer is that the simple linear relationship between
r and N isn’t always accurate, particularly at low density.
At low density, r may actually decline to negative values.
The reason is that mates become more difficult to find when
density is low. As a result, fertilization and birth rates drop.
This suggests that an already small (threatened, endangered?)
population may continue to decline when at low density,
rather than recovering…
Eventually, we surmise, to extinction
The Allee effect is suspected to have acted in a number of
extinctions or near extinctions. One example:
the black-footed ferret
The black-footed ferret is a member of the weasel family
that lived in the plains and prairies of central North America.
It was listed as threatened in 1967 and endangered in 1973.
Their main food is prairie dogs.
Black-footed ferret from the SSP
reintroduction plan population.
Prairie dogs were essentially eliminated as pests in
agriculture. In addition, the habitats of the ferret were
fragmented; only small isolated areas were left for ferrets.
By 1985 there were only 2 small populations (total ~10)
of the ferret left. One, from South Dakota was placed in a
captive breeding program. It failed, and all died.
The other seemed to be successful, but with a plague of
canine distemper prairie dogs and its spread to ferrets, only a
small number remained alive. Some of these were collected
for captive breeding (luckily) before extinction of the wild
population.
The future of the species now rests with the offspring of 12
animals in that captive breeding program.
There are strong indications of success in this recovery
program!
There are two points to make from this:
1) Ferret numbers declined even before disease due to
the isolation of small, remnant populations. The Allee
effect is believed to have affected the success of these
populations.
2) Also affecting the success of these small populations
would have been genetic inbreeding and resultant loss of
fitness in offspring.
That loss of fitness likely leads to further declines in
population size. An example: the cheetah
The African cheetah is known to be very highly inbred.
Apparently, only a small handful of individuals survived a
crunch (properly known as a genetic bottleneck) ~10,000
years ago. All members of the species are descended from
those few survivors.
How do we know they are highly inbred?
• Skin grafts among “unrelated” individuals are generally
successful.
• Feline leukemia spread through an entire colony of
captive cheetahs, even though cats in general are highly
resistant to the disease.
• Cheetahs are monomorphic at all 55 enzyme loci tested.
How does this relate to the question of population size
declining?
Captive populations of cheetahs in zoos, carefully bred to
pair only “unrelated” individuals, suffer among the highest
rates of infant mortality in mammal breeding programs.
Why?
With inbreeding, low frequency recessive lethal genes are
exposed, and offspring survival is significantly reduced.
However, wild African cheetah populations seem to breed
successfully, and high infant mortality is due to predation on
cubs by lions and hyenas.
Now let’s get back to the main story of the day…
Life History Characteristics…
Some basic life history characters are:
• life span
• frequency of reproduction
• energy allocated to growth and reproduction
There are some curious and amazing extremes...
Most plants either have short lifespans and reproduce once,
or have long lifespans and reproduce repeatedly.
Short lifespan and single reproduction: weeds like thistles
Long lifespan and repeated reproduction: a maple tree
But…
The century plant (the agave from which tequila is made)
grows for ~100 years (thus its name), then reproduces once.
There are bamboos that delay as long as 115 years before a
single bout of reproduction. Even more remarkable, they
are synchronized in that reproduction no matter where they
grow (Russia, China, Japan, Alabama). How and why?
Another wierdo is the Samoan palolo worm.
• It lives most of the year as a sexually immature animal,
called an atoke.
• During the breeding season a part develops into a sexually
“ripe” worm, called an epitoke.
• During swarming, which occurs at a precise phase of the
moon in October-November, these posterior parts, swollen
with gametes, break free, swim to the surface,
and, just before sunrise, discharge the gametes.
• When gametes are discharges, the sea is said to look like
milk.
atoke
epitoke
Animals (or plants) that reproduce only once are called
semelparous, whereas animals or plants that reproduce
repeatedly are called iteroparous.
The extreme examples thus far have been semelparous. You
would expect mammals, with generally long lifespans, to be
iteroparous.
A last extreme example is a semelparous mammal.
It is a marsupial mouse from Australia, Antechinus.
Species in this genus from tropical areas and from desert
areas are iteroparous. However, the species from seasonal
deciduous forest has semelparous males.
Why?
The slow development of marsupials means that there is
only one opportunity for a litter during a summer.
When males mature they fight so aggressively to get and
keep mates that they suffer from Selye stress syndrome.
The “disease” results in a hypertrophy of the adrenals, and
insufficient response from the adrenals as seasonal, climatic
stress ensues.
The males die after only one bout of reproduction.
They are semelparous.
Even among more typical life histories, there is considerable
variation…
Species
Albatross
Gull
Gecko
Uta
life span (yr)
lifetime egg prod.
50
8
3
3
In spite of these large differences, each species is
“successful”.
40
20
3
>100
Are there patterns in life history that relate to features of
environment, habitat, or behaviour?
Yes! Robert MacArthur and E.O. Wilson, in a seminal book
(1967) advanced the idea that life histories evolve to cope
with environmental pressures. Today that seems fairly
obvious
They compared the expected patterns in extreme types of
environment…
Harsh environments versus equable (mild) ones
Here are the various life history characteristic “opposites”
they associated with these environments...
Life History
Characteristic
Harsh
“Risky”
Stable, equable
Mild
Population growth
Opportunistic
Equilibrium
Age of maturity, 
early, rapid
maturation
delayed
Adult body size
small
large
Frequency of
reproduction
semelparous
iteroparous
Litter size
large litters of
small offspring
fewer large
offspring
Life History
Characteristic
Mortality pattern
Harsh
“Risky”
Stable, equable
Mild
crashes
density-independent
density-dependent
Parental care
usually none
variable
life history pattern
“big bang”
maximize “r”
equilibrium
life history “strategy”
r-strategy or
r-selection
K-strategy or
K-selection
Type of species
annual plants
higher vertebrates
insects, plankton & some inverts
many invertebrates
Remember that r and K strategies are extremes. There is a
continuum of strategies between these extremes...
So far I haven’t mentioned how these strategies relate to
the allocation of energy to reproduction and growth.
The Principle of Allocation can be stated as follows:
Organisms are faced with limited budgets of resources and
energy that must be allocated to growth, maintenance, and
reproduction. Allocation to any one of these functions
reduces the amount that can be allocated to the others.
Thus there are trade-offs among allocations to growth,
maintenance, and reproduction.
The pattern of trade-offs selected by evolution is the one
that maximizes lifetime reproductive success.
We would expect different allocation patterns in a species
with a high qx and a rapidly declining lx than one with high
survivorship and low mortality. Why?
Allocation to
Mortality
Future
growth
reproduction
Offspring
High
not likely
low
high
Low
likely
high
low
The high mortality strategy and allocations fit an “r-strategy”
When mortality is low, allocations fit a “K-strategy”
The results of this analysis can be further refined…
What is the survivorship schedule?
If mortality occurs mostly in adult ages, then an rstrategy can be expected - reproduce heavily as early
as possible, you may not survive to try again.
But…
If mortality falls mainly on the juveniles, there are
two possibilities. One, seen in long-lived trees, is a
highly iteroparous, K-strategy. Allocation of energy
to reproduction is reduced or limited.
The other is a strategy called bet hedging. There are
a number of bet hedging strategies. One of the most
common is spatial.
Bet-hedging in a heterogeneous space…
Adults spread the risk. They release offspring in different
areas. Some places will be ‘good’ for this species, and some
will be ‘bad’.
But by spreading offspring into different areas, at least some
offspring will survive.
This is the ecological version of “Don’t put all your eggs in
one basket”.
This is one form of bet-hedging based on variation over
space. There are other forms…
Spatially, this is what we mean by bet hedging…
If some patches are good and some poor, putting “all your
eggs in one basket” is a risky strategy
There are many approaches to spreading the risk spatially:
1. Juveniles may rapidly disperse from their birthplace to
other patches.
2. Female insects frequently lay their eggs on many
different plants (though the plants may all be from one
species).
3. In plants, there are many strategies for seed dispersal.
a. with attached “parachutes” (dandelions, goldenrods)
b. by having awns that drill, corkscrew-like, into animal
fur
c. by being super light to be carried on the wind (orchids)
d. by floating on water (coconuts)
e. by occurring within attractive fruits and being adapted
for gut passage (apple seeds)
Dandelion – a seed dispersed by parachute
Porcupine grass – a (painfully) animal dispersed seed
Orchid pods – the seeds are described as “dust”
and weigh only micrograms each.
Review
The r-K continuum (and the r-K selection hypothesis)
suggests that life history features are fine-tuned by natural
selection.
Natural selection optimizes the match between life histories
and the environment.
What does optimal mean here? An optimal life history
strategy gives the highest lifetime reproductive success.
-- Bet hedging is one “optimal strategy” where juvenile
mortality is high