Life History Patterns

Life histories of species are the set of parameters (including the ones in a life table) that are important in describing thefactors 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 howlarge offspring in a litter are, the timing of litter production,and a number of other factors are important in describinga life history.

When we consider why a population is successful or isendangered, we evaluate the life history to find answers.

The same basic approach, that is looking at the balancebetween birth rate and death rate that was important in looking for an equilibrium population size, is useful in understanding how environmental contamin-ation and harvesting affect natural populations and their size.

Review: Environmental contamination, for example DDT, reduced fecundity in predatory birds. The DDTaffected peregrine falcon shell glands. Females laid eggswith thinner shells that broke when the female tried tobrood them. The result was seriously reduced fecundity.Viewed as a balance between birth and death, that reducedthe carrying capacity for peregines...

What about harvesting? Harvesting may not affect thefecundity of surviving individuals, but clearly increasesthe death rate. A good example here is the near extinctionof the blue whale. The survivorship of the whales declinedconsiderably even during the 20th century. That declinecould 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 ageneralized version of that as a set of birth and deathcurves, showing the lowered equilibrium population sizewith harvesting...

Another review point: the Allee effect

We already know that r is, for many species, density-dependent. According to the basic logistic modelthe 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, ris positive for population sizes less than K, and negativeabove K.

This suggests that when species are rare, their populationgrowth and r should be maximal. Then why do rare speciesgo extinct? If protected, rare species should recover if thissimple model is correct.

However, rare species have gone extinct even when protected. Why?

The answer is that the simple linear relationship betweenr 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 whendensity 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 ofextinctions or near extinctions. One example:

the black-footed ferret

The black-footed ferret is a member of the weasel familythat 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 12animals 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 acrunch (properly known as a genetic bottleneck) ~10,000years ago. All members of the species are descended fromthose 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 thistlesLong 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 asingle bout of reproduction. Even more remarkable, they are synchronized in that reproduction no matter where theygrow (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.

epitoke

atoke

Animals (or plants) that reproduce only once are calledsemelparous, whereas animals or plants that reproducerepeatedly are called iteroparous.

The extreme examples thus far have been semelparous. Youwould expect mammals, with generally long lifespans, to beiteroparous.

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 desertareas are iteroparous. However, the species from seasonaldeciduous forest has semelparous males.

Why?

The slow development of marsupials means that there isonly one opportunity for a litter during a summer.

When males mature they fight so aggressively to get andkeep mates that they suffer from Selye stress syndrome.The “disease” results in a hypertrophy of the adrenals, andinsufficient response from the adrenals as seasonal, climaticstress ensues.

The males die after only one bout of reproduction. They are semelparous.

Even among more typical life histories, there is considerablevariation…

Species life span (yr) lifetime egg prod.

Albatross 50 40Gull 8 20

Gecko 3 3Uta 3 >100

In spite of these large differences, each species is“successful”.

Are there patterns in life history that relate to features ofenvironment, habitat, or behaviour?

Yes! Robert MacArthur and E.O. Wilson, in a seminal book(1967) advanced the idea that life histories evolve to copewith environmental pressures. Today that seems fairly obvious

They compared the expected patterns in extreme types ofenvironment…

Harsh environments versus equable (mild) ones

Here are the various life history characteristic “opposites”they associated with these environments...

Life History Harsh Stable, equableCharacteristic “Risky” Mild

Population growth Opportunistic Equilibrium

Age of maturity, early, rapid delayedmaturation

Adult body size small large

Frequency of semelparous iteroparous reproduction

Litter size large litters of fewer largesmall offspring offspring

Life History Harsh Stable, equableCharacteristic “Risky” Mild

Mortality pattern crashes density-dependentdensity-independent

Parental care usually none variable

life history pattern “big bang” equilibriummaximize “r”

life history “strategy” r-strategy or K-strategy orr-selection 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 acontinuum of strategies between these extremes...

So far I haven’t mentioned how these strategies relate tothe allocation of energy to reproduction and growth.

The Principle of Allocation can be stated as follows:

Organisms are faced with limited budgets of resources andenergy that must be allocated to growth, maintenance, andreproduction. Allocation to any one of these functionsreduces 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 onethat maximizes lifetime reproductive success.

We would expect different allocation patterns in a specieswith a high qx and a rapidly declining lx than one with highsurvivorship and low mortality. Why?

Allocation toMortality 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 r-strategy can be expected - reproduce heavily as earlyas possible, you may not survive to try again.

But…

If mortality falls mainly on the juveniles, there aretwo possibilities. One, seen in long-lived trees, is ahighly iteroparous, K-strategy. Allocation of energyto reproduction is reduced or limited.

The other is a strategy called bet hedging. There area 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

furc. 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