Life History

life cycle

represents the different stages of an organism's life from conception through death.

life history

the sequence, timing, and nature of events in an organism's life, from birth to reproduction to death.

life history strategy

pattern of life-history traits that has evolved by natural selection over time.

an approach to resource allocation that addresses the questions:

• How long and how big to grow?

• When to begin reproduction and how long to remain reproductively active?

• How many offspring to have and how much to invest in each?

• How much to invest in growth and survival versus reproduction?

no constrains

the fastest reproducing and most fecund individuals (or species) "win"

constraints

both of the organism itself and of the environment it lives in

give rise to trade-offs in how organisms allocate resources.

life-history trade-off

occurs when increasing the fitness of one life-history trait causes a decrease in fitness of another.

has generated an enormous variety of complex life cycles and life-history strategies.

lack hypothesis

states that birds have clutch sizes that maximize the number of chicks successfully fledged.

tested and found that many bird species lay smaller clutches than the hypothesis predicts.

kestrel

experiment supports the hypothesis that smaller clutches led to higher rates of adult survival

reduced parental costs and increasing total lifetime fitness.

Life-history evolution

affected by a variety of different factors, including current versus future reproductive output, risk of predation, and variation in resource availability.

successful life history

produces stable or growing populations.

Demographic parameters

used to predict how a population will grow.

ex. per capita birth rate and per capita death rate.

per capita

Birth and death rates

the number of per capita births and deaths, respectively, in a specified amount of time (usually a year).

Fecundity

is the average number of offspring per reproductive female per unit time.

Per capita population growth rate

(r)

indicates if a population is growing, shrinking, or stable:

Positive (r > 0): The population is growing.

Negative (r < 0): The population is shrinking.

Near 0 (r ≈ 0): The population is stable.

life table data

used to make predictions about future population change

used by demographers to study human populations

Different life histories

can be successful, even under the same environmental conditions.

ex. populations of species with short life spans, high fecundity, and low investment in offspring (such as barnacles) can be just as successful as those with long life spans, low fecundity, and high investment in offspring (such as dolphins).

High birth and death rates

lead to age structures with many more younger than older individuals.

Low birth and death rates

lead to more uniform age structures.

Age structure

is dynamic, because birth and death rates change through time in response to environmental and social factors.

age pyramid

The shape provides clues about that population's demographic parameters and its recent history.

life table organization

organized into discrete age classes.

The choice of age class divisions depends on the organism in question and how much data is practical to collect.

parameters include age class (x), number surviving at the beginning of each age class (n_x), number of births (b_x), survivorship (l_x), and fecundity (m_x).

survivorship

calculated by dividing the number of individuals alive at the beginning of an age class by the number of individuals in the original cohort:

lx = nx/n0

survivorship curve

plots survivorship against age, and characterizes the pattern of mortality within a population.

There are three main types of curves, more easily distinguished when survivorship is plotted on a log axis and age is plotted as percent of maximum life span

type 1

high rates of survival when young and middle aged, but high mortality at old age

type 2

a roughly constant proportion of individuals survive from one age class to the next

type 3

high mortality at young ages and high survival when older

age specific fecundity

calculated by dividing the number of births for an age class by the number of survivors at the beginning of the age class:

mx = bx/nx

net reproductive rate (R0)

average number of female offspring that a typical female is expected to have over the course of her lifetime

R0 = ∑lxmx

generation time (G)

average time that elapses from the birth of a female until the birth of her daughters (i.e., the average age of a mother producing female offspring)

G = ∑xlxmx/r0

per capita population growth rate (r)

describes how quickly a population is growing or shrinking at any given time

estimated from life table data

r = ln(R0)/G

R₀ and r

used to determine how a population is changing over time.

• When a population is growing, R₀ > 1.0 and r > 0.

• When a population is shrinking, R₀ < 1.0 and r < 0.

• If a population is neither growing nor shrinking, then R₀ is 1.0 and r is 0.0.

demographic transition

human populations death rates and birth rates drop.

Because death rates tend to decrease earlier and faster than birth rates do, population growth rates tend to be high during this period of transition.

result of populations developing & industrializing

trade-offs

Individuals have limited resources and energy

results in choices between age at first reproduction, frequency of reproduction, number of offspring produced, and parental care

principle of allocation

constrains trade-offs

Energy used for one function (e.g., growth), cannot be used for another (e.g., reproduction)

competing

demands for energy lead to trade-offs like the one between age at first reproduction and growth

equivalent playing fields

reproducing earlier yields higher population growth rates.

positive relationships between body size and fecundity,

delaying reproduction can sometimes increase fitness.

juvenile and adult survival

determines whether it is better to reproduce early or late

Semelparous

species that reproduce only once, often delaying reproduction to achieve greater fecundity

Iteroparous

species that undergo repeated bouts of reproduction

a strategy that works best when adult survival is high.

reproductive strategies

evolve to maximize overall reproductive output under surrounding environmental and ecological conditions.

r-selected

species well-adapted to disturbed environments that prioritized high growth rates

K-selected

species efficiently compete for resources in more stable environments.

Grime's plant life-history classification

categorized species according to how they responded to stress as well as disturbance

His scheme recognized three primary groups of species:

• ruderal

• stress-tolerant

• competitive

Winemiller and Rose's system

classified fish based on three fundamental population parameters: juvenile survivorship, fecundity, and age at maturity.

scheme identified fish with life-history strategies that were opportunistic (i.e. disturbance adapted), equilibrium (i.e, competitive), and periodic (i.e. stored resources for periods of scarcity).

Life-history strategies

can be flexible, in that individuals of the same species sometimes exhibit different strategies in different environments.