Download Life History 1

Life History 1
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We envy birds their ability to soar above the mundane concerns of our daily lives
As carefree as a bird, the saying goes…
But the lives of birds are anything but free of care
Birds face a constant struggle against overwhelming odds to keep themselves alive and raise
their young
Rreproductive potential of birds is tremendous, and populations with no limits will grow
exponentially
But birds rarely achieve this exponential ideal, because of the limits imposed by their
environment
Populations of most species of birds are remarkably stable over time
Density-dependent factors, like resources and predation, and density-independent factors like
sunshine and rainfall combine to regulate avian populations
To understand how these forces act on populations of birds we have to understand how bird
populations are structured
A population’s age structure is determined by the number of individuals of each sex in each
age class
One of the simplest ways to keep track of how a population is growing and changing is to
construct a life table of that population
Life tables record births and deaths (vital statistics)
Life tables can be constructed from many types of information, such as:
> population census
> skull measurements on dead animals
> birth and death dates on tombstones
The original use of life tables was to build “actuarial tables” for the insurance industry
Insurance agents needed a reliable way to estimate the probability that a potential client would
die within a certain interval of time
Life tables can also be used to calculate R, the Replacement Rate, or net reproductive rate
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R = lx bx , summed over all age intervals:
> R = ∑ lx bx (discrete form) ...or..
> R = ∫ lx bx dx (continuous form)
Humans as a species should fall under the logistic model of population growth
Logistic model assumes
> Continuous breeders - like bacteria, humans…
> Growth is limited
Life tables reveal many interesting things about how populations are structured
Show us that birth and death rates for most species are different for different age classes, and
differ between males and females
Show us natural selection can act differently at different ages, or between the sexes
Pattern of age-specific births and deaths also varies from species to species
One way to visualize these different patterns is to construct a survivorship curve, a plot of lx
against time (remember, lx=ax/a0)
Survivorship curves for various sheep and deer show Type I survivorship
Notice that these curves can be different for males and females (a and d), and can vary from
one habitat to the next (c - shrub vs. chaparral)
This diagonal survivorship curve, shown here for the lizard Sceloporus, represents a different
pattern of survival
For these animals there is a constant probability of death at all age intervals
This type of curve, called a Type II curve, is also typical of birds
Birds have a very high mortality rate in their first year of life, and a fairly constant rate after
that
The final type of survivorship curve, Type III, is typical of many fish and most invertebrates
Juvenile mortality is extremely high, but the survivors of this initial blitz enjoy a low
mortality later in life
The crawfish, for example, has several hundred young at one time, but only a few survive the
first few weeks
A single fish can lay up to one million eggs, but most of these perish as juveniles
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Life tables show us that different species have different life histories
Life history traits are the pattern of age-specific birth and mortality of a population or species
These traits are ultimately under genetic control, so they not only vary to some extent within a
population, they can be inherited
Because they can vary and are inherited, they can also evolve
• Life history traits include:
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Size (large or small body)
Rate of growth and development (fast or slow)
Age at first reproduction (early or late)
Reproduce all at once OR in a series of events
Have many small offspring OR few large offspring
Reproductive Effort (the total energy allocated to repr.)
Longevity (short or long life)
Dispersal ability (near or far)
All of these traits affect survival, so natural selection should favor certain combinations over
others in certain environments
Making one choice precludes others (ex. many young = small young)
And certain traits are always associated with other traits (ex. longevity, iteroparity)
If an organism puts too much energy into reproduction, it might not have enough energy to
maintain itself, and might die before reproducing again
If it puts less energy into reproduction, fewer offspring might survive, but the effort could be
repeated
The sum of all the choices an organism makes creates its life history strategy
A life history strategy is a set of co-adapted traits designed by natural selection to solve a
particular ecological problem
Every choice the organism makes on how to allocate its limited lifetime energy involves a
trade-off, affecting all other choices
Life history strategies represent a compromise between conflicting demands
Strategies represent a choice about how to allocate limited time and resources, how to balance
reproduction and survival
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Many strong forces limit the growth potential of bird populations: climate, limited habitat,
predation, diseases, accidental death, and starvation
These forces and others work to keep bird populations in check
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Loss of habitat is becoming increasingly critical for the survival of many birds…
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Climate imposes broad limits on ranges and population size
> directly through extremes of temperature and rainfall
> indirectly through the effects of climate on food supply (seeds, fruits, and insects)
The passing of El Niño always leaves several million seabirds dead in its wake
Changes in salinity and temperature of the seawater affect the anchovy populations they rely
upon for food
We often see such events at second hand in irruptive invasions
During years of poor seed production, large flocks of northern coniferous birds like the
Evening Grosbeak and the Purple Finch invade the south, showing up at bird feeders and in
roadside flocks
Snowy Owls may also invade in large numbers when lemming populations are at a low ebb
In the invasion of 1945-1946 over 14,000 Snowy Owls were seen in southern Canada and
New England
Lack of suitable habitat is a limiting factor for many species
In some cases these limitations are natural, but increasingly they are due man
Lack of habitat often acts on bird populations through limiting food supply
Lack of suitable nesting habitat can also be a major problem
The Red-cockaded Woodpecker has become increasingly rare as the old Longleaf pine forests
it used to nest in have fallen to the lumberman’s axe
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Trees must be at least 80-100 years old to be suitable for nesting
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These older trees must also be infected with red heart fungus
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The fungus softens the heart wood of the tree just enough for the woodpecker to carve out a
nest
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They then drill additional resin holes, which elude a sticky trap for predatory rat snakes that
climb the pines searching for the nests
Populations of these woodpeckers are limited by the number of old infected pines
Not all habitat changes caused by humans are harmful
In New York State, the decline of small family farms has caused widespread secondary
succession
As these old field reached the shrubby stage, they once more became suitable nesting habitat
for Great Blue Herons
Species is becoming far more common as it reoccupies its former northern range
(McCrimmon 1981)
Predators can be a major limiting factor
A large Brown Noddy breeding colony of 35,000 birds (Anous solidus) in the Dry Tortugas
was plundered by rats for 19 years until only 400 birds survived (Robertson 1964)
Predators can include mammals like rats, cats, skunks, foxes and weasels, reptiles like snakes
and gators, other birds like hawks, owls, and crows
Marra (2011) ~80% of mortality in juvenile Catbirds in Washington suburbs was due to
predators, of whom 47% (ironically) were cats!
Smithsonian/Fish & Wildlife report 2013 – cats are deadlier than we suspected
Cats kill 2.4 billion birds per year (and 12.3 billion mammals)
2-4 times higher than previous estimates!
In the Saskatchewan River delta in Canada, about 10% of the ducklings born each year are
snatched to a watery grave by the Northern Pike
Even predatory birds are themselves limited by predation
Lockie’s (1955) study of the Short-eared Owl (Asio flammeus) found that only 2 of 24 nests
managed to fledge young due to predation by crows and foxes
Diseases and parasites are also a major source of mortality in birds
Large numbers succumb to the same types of fungal, bacterial and viral pests that we do, plus
scores of nematodes, cestodes, ticks, mites and other invertebrate pests
In one sample of 175 scoters of various species, Bourgeois and Threlfall (1982) found that
91% of the birds were infected by one or more of 45 different species of parasites
Peters (1936) drew up a list of 198 different ectoparasites known to infect some or all of 255
species of birds found east of the Mississippi
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These parasites included lice, fleas, various flies and their larvae, mosquitoes, black flies,
leeches, ticks, and several species of skin and feather mites
Baby birds in the nest are especially prone to parasite infection
The rapid development of nestlings might be partly an adaptation to get them out of the nest
before parasite loads build up to critical levels
In 1973, a colony of Sooty Terns nesting in the Seychelle Islands deserted their entire brood
due to a heavy infestation of ticks
Leaving all the remaining young behind to starve…
We ourselves have become a primary source of avian mortality
> directly by hunting birds for food or sport
> indirectly by converting their habitats to farms and factories
Over 57 million birds are fated every year to become road kill
Another 80 million birds perish every year from colliding with plate-glass windows
Food limitation has historically been viewed as the most critical factor limiting bird
populations
Peter Grant’s study of Darwin’s Medium Ground Finch is a classic example of how closely
reproductive success is tied to food availability
Biomass of finches was strongly correlated with seed biomass, and the loss of these seeds
caused widespread starvation
When food is limited, adult birds may be unable to breed
When food is abundant, adults have enough food left over to provide an optimal diet for
nestlings and fledglings
The importance of food supply to reproductive success has been demonstrated through
> manipulating brood size
> removal of mates or helpers
> artificial food supplementation
These studies show that parental birds must pay a substantial energetic cost to meet juvenile
food demands
Birds are mainly limited by food supply, especially during non-breeding season when food
resources are relatively low
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Studies by Tom Sherry and Dick Holmes have shown a strong relationship between
reproductive success among wild passerines and food resources (Holmes et al. 1986, 1991)
Reproductive success in Black-throated Blue Warblers is strongly correlated with caterpillar
biomass
Colonial wading birds are known to be typically food-limited birds
Rapid increase in populations of several species of wading birds in Louisiana is due to
supplemental food from crawfish farms, new food source peaking during the wading bird
nesting season
Having considered the many ways in which birds can die, the enormous reproductive potential
of birds makes very good sense-it is a highly adaptive trait
Birds must make up their population losses each year by recruitment
Recruitment of new birds into the population can come from survival of the young-of-the
year, or through immigration from other local populations
Population size is driven by annual recruitment, which in turn is mainly driven by annual
fluctuations in food supply
Survival rates vary greatly from species to species, being higher for the larger seabirds,
raptors, and wading birds, and lower for songbirds
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An Albatross only has 3 chances in a hundred of dying in any given year
Song Sparrows experience over 70% mortality each year of their relatively brief lives…
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Birds have a Type II survivorship curve
The rate of mortality is roughly the same from year to year for Herring Gulls
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Notice the sharp dip in the first year, when the loss of eggs and chicks is quite high
If gulls make it through the first year, they have a reasonable shot at a long life
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Juvenile mortality is uniformly high for most species of birds
The first graph shows percent survival for Ruffed Grouse, the second for Bronzed Grackles,
during their first summer and autumn
This poses special problems for breeding birds, who must adjust the pattern of their
reproduction to balance the pattern of their mortality
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Consider these extremes of avian life history traits
Small birds, like ducks and robins, have relatively short lives
They must mature quickly and put enormous energy into raising several young in each of the
few years they will live
Large birds, like eagles and albatross, have relatively long lives
They’ll be around for a while, so they can mature slowly
Once they reach reproductive age, they parcel out their lifetime’s reproduction in a long series
of reproductive events, with relatively few young in each brood