BISC302: Exam 3 – SimU Text: Life History
Happens in many ways: Single-celled organisms can split in half.
Many plants and some animals can break off a piece of their body and have it re-grow into a new individual.
The number of offspring or eggs produced per unit time.
The ability to produce a large number of offspring over a short period of time is when it is high.
In human demography, it is the physiological ability to reproduce, as opposed to fertility, which is the actual rate or number of children born per number of people.
Thus, the number of individuals in a successful population, would, on average, stay constant or grow over several generations.
Thus the population would have, on average, fewer individuals each generation, which could ultimately culminate in extinction if a better strategy does not evolve
Examples are birth and death rates, fecundity, and age at first reproduction.
Known or estimated they can be incorporated into mathematical models to develop hypotheses about life history traits.
In age-structured models, they are often represented in life tables by a standard set of symbols.
If there is no immigration or emigration, it is simply the difference between the birth rate and the death rate for the population.
r = b – d
Mathematical models describing population growth refer specifically to r as the intrinsic population growth rate.
– Negative (r < 0): the population is shrinking. - Near 0 (r = 0): the population is stable.
b ≈ Births / ( t Navg )
(t = time; Navg = average number of population throughout t)
This is done by first dividing the population into different age classes. For instance, a species that lives on average for one year might be divided into 1 month age classes, so all individuals less than one month old are grouped together; those from 1 – 2 months are grouped together; and so on.
For species that only reproduce once per year, the natural age class width is one year. For other species, the width of each age class is picked for convenience, depending on what analysis is being done.
For instance, if a species only reproduces once per year, all the individuals in a population that were born in the same year would be considered part of the same ___
Almost all plants and fungi because they are rooted in a fixed location, but this term is also used to refer to immobile animals, such as barnacles and mussels, that cement themselves to a rock.
In practice, this is often measured as the number of offspring that are produced that themselves grow up to produce offspring. But the theoretical definition includes all future generations that come from the individual, or more generally, the genetic contribution of this individual to future generations.
For instance, limited resources constrain how fast an organism can grow.
Not all need be resource related. For example, a certain body architecture might limit the size that an organism can reach, as is the case for non-woody plants which cannot grow as tall as a typical tree without falling over.
In some species, they can actually mate with themselves (both egg and sperm for the offspring come from the same parent).
In other species, individuals can be this but still need to seek out another individual to mate with.
May vary depending on environmental conditions.
A species life history strategy represents the complete suite of this that have been selected for during its evolution.
For example, energy spent on growth cannot be spent on producing eggs.
For example, laying 12 eggs in a good year might be great, but in a sparse year it could threaten survival of the entire clutch.
Used geometric mean relative fitness on Great Tits population.
Found that fledging more birds in good years is NOT worth the risk of fledging fewer birds in poor years, when there is high variation in year-to-year food availability.
The penalty for laying too many eggs in bad years is greater than the benefit of laying a few more eggs in good years
When x = 0 on the life table, it represents the number of births that occurred for this cohort during the first age class—i.e., between the ages of 0 and 10. Because sometimes young do not have offspring.
The rate that only considers females that are of reproductive age.
– Listed by age, class or stage (x).
The parameters usually include the:
– births (bx),
– deaths (dx),
– fecundity (mx or fx), and
– survivorship (lx)
– mortality (qx)
– Cohort or Dynamic
– Static (time-specific)
The proportion of a cohort of individuals that survives to a given age.
It is the preferred one.
Consists of following a cohort of individuals from birth to death (until the last individual dies) . As the cohort ages, age-specific fecundity and survival can be calculated from birth to death at each age.
It is the simpler one.
A researcher examines a population at a single point in time and assumes that the population’s age structure and all age-related mortality and fecundity schedules are not changing over time.
This is the stable age distribution method and has the benefit of easier data collection.
The drawback is potential inaccuracy, because often the assumption of stability is violated.
Most populations will arrive at a this distribution if their demographic parameters are not changing, such as when age-specific birth and death rates stay constant over time.
(Static life table)
This notation is often used in life tables or when reporting statistics for age-structured populations.
For example, in a life table with 10-year age classes, n30 is the number of individuals in the population that are between the ages of 30 and 40 years old.
(n0 = the initial cohort size)
= b0/ l0
For example, if 800 females were alive at age 20, and as they aged from 20 to 30 they produced 40 female babies, the fecundity for age class 20 would be:
f20 = b20 / n20 = 40/800 = 0.05
= d x / l x
= lx – lx+1
Survivorship is most often plotted as either the number of individuals out of a cohort of 1000 surviving to age class x or as the proportion of a cohort surviving to age class x.
Can help ecologists make inferences about populations when direct observations are difficult. For example, these curves can be constructed by analyzing skeletal remains.
– Type I
– Type II
– Type III
Used to estimate the growth rate of the population.
The sum of survivorship multiplied by fecundity for each age class:
For example, if R0 = 2.0, the next generation will be twice as large as the current.
This means the population will decline by that proportion each generation.
For example, an R0 value of 0.7 means that the next generation will be 70% as large as the previous generation.
T = ∑ [x(lx)(mx) / R0]
Can also be estimated from fecundity and survivorship data in a life table.