Topic 13

Properties of Populations and Variation in Life History Strategies

Population ecology is important because all forms of evolution take place at the level of the population

Ecology is the study of organisms and their interaction with the environment

Population environment- study of populations and their interactions with environments, including density, distribution, age structure, and population size

Population- group of individuals of the same species that live in the same place at the same time

Properties: Density, Dispersion, and Demographics

Density

The number of individuals per unit area

How to get to the population density?

By adding births and immigration

Removing deaths and emigration

Immigration and emigration allow for important biological exchange among populations through time (drive gene flow, change allele frequencies)

Dispersion

The pattern of spacing between individuals in a population

1. Clumped: individuals are aggregated into patches

   • Most common due to:

     • resource distribution

     • dispersal limitation, “the apple doesn’t fall far from the tree”, they cannot move far from where they were born

     • social behavior, some organisms live in groups, hunt together, ensures they get enough food, close proximity in order to mate

2. Uniform: individuals are evenly spaced

   • Often due to competition for resources

     • example: penguins at a social distance from each other

3. Random: individuals are spaced randomly/unpredictably

   • Least common

   • If resources are evenly distributed & individuals do not attract or repel each other

Demography

Study of vital statistics and how they change over time

Example vital statistics

• Death rate

• Birth rate

• Age class structure

• Sex ratio

Demographics inference density and dispersion over time

One important measure of demography:

Survivorship curves

How many offspring do individuals produce?

How many offspring survive?

w = (s)(f)(k)/average(s)(f)(k)

How to generate a survivorship curve

• Follow the survival patterns of a cohort birth and death

Three general types of survivorship curves

type I, type II, and type III

Type I

• most individuals lead long lives

• low mortality of juveniles, high mortality in oldest individuals

• overall survival probability (y axis) age in years (x axis)

• note the variability: population may not fall perfectly along the typical curve

Type II

Equal change of death/survival throughout lifetime

• uniform death rate cross age groups

Type III

Very low change of survival for youngest individuals

• mortality is high for juveniles, low mortality for older age classes

We see Type I curve in

• humans, many mammals, some large birds

• often in species that show high levels of parental care & has low depredation of adults

• low mortality in young ages

We see Type II in

• most birds, small mammals, reptiles

• often in species that show parental care & moderate rates of depredation in adults

We see Type III in

• invertebrates, fishes, amphibians, most plants

• typical in species that show no parental care and/or produce MANY offspring

Depredation is the state of where something is hunted

Low depredation means adults are not attacked by predators

Why do some organisms produce many offspring while some produce less?

Organisms vary in life history traits

physiological, behavioral features that affect reproduction, survival, population structure, growth, habitat, competition

1. Age of sexual maturity

2. Size of adult

3. Fecundity: # of offspring produces (per reproductive cycle)

4. # of Reproductive cycles over lifetime

5. Parental Care

6. Longevity

Why do organisms vary in life history traits?

OPTIMALITY THEORY

Optimality theory: model which predicts that the behavior that maximized the different between the costs and benefits of the behavior will result in the highest fitness

Costs:

Energy- energy used for one activity could be used for another (principle of allocation)

Risk- likelihood of injury or death

Opportunity- while performing on activity, cannot perform another

Benefits:

How optimality theory applies

Look at the relationship between life history traits and fitness

If individuals have certain traits they will have great or lower fitness

The ones with most adaptive traits leave the most offspring

Optimal foraging theory: optimality theory applies to foraging behavior

Benefits

• generally in calories

• maybe specific nutrients

Costs/trade-offs

• energy to locate, catch food

• risk of predation

• time

Deviation from optimal foraging

Indicated additional variables should be considered

• risk

• handling costs

• micronutrients

• etc.

Optimality can be applied to reproduction

Optimal litter size

• in low litter sizes, individuals put more energy into offspring and produces higher quality offspring

• life history traits can be variable

• optimality theory applies

Why do we see some sets of life history traits but not others?

The principle of allocation: Energy invested (used) in one trait/characteristic can’t be used for another. It results in trade-offs

The total available energy is split into:

1. Age of sexual maturity

2. Size of adult

3. Fecundity: # of offspring produces (per reproductive cycle)

4. # of Reproductive cycles over lifetime

5. Parental Care

6. Longevity

Longevity is how long an organism lives

When identify trade-offs, remember that organisms cannot have it all, one thing will be maximized at the expense of something else

Variation in life history traits: reproductive events

Semelparity- produce many offspring once in life

Iteroparity- produce few offspring many times in life

Natural selection has resulted in a large variety of life-history strategies

Which strategy results in maximum fitness caries for different species and different environments

• what works well for one organism is going to differ to what works well for another

Life histories are seen to vary along a spectrum

r selection: selection for traits that maximize reproductive success and are favorable in low density environments

• high fecundity

• low survivorship

• short life span

• not investing a lot in survivorship, low parental care, low disease resistance

K selection: for traits that are favorable in high density conditions

• low fecundity

• high survivorship

• long life span

Can vary between or within a species

Different environments favor r or K selected traits

Summary: The environment affects multiple properties of populations. Variation