11 - Life Histories

What is a life history?

Set of traits that determine an organism’s growth, reproduction, and survival, shaping how it allocates resources across its lifespan

What are key life history traits?

Number of offspring, size of offspring, age/size at maturity, and reproductive effort

What is reproductive effort?

The amount of energy, time, and resources an organism invests in producing and caring for offspring

What does life history theory assume?

Energy is limited, so organisms cannot maximize all functions (growth, survival, reproduction) at once

What is the principle of allocation?

Energy used for one function reduces the amount available for others, creating trade-offs

What general patterns are predicted?

Organisms with few offspring tend to produce larger offspring; organisms that mature early tend to have smaller body sizes

Trade-offs:

What is the offspring size vs. number trade-off?

Organisms must choose between producing many small offspring or fewer large offspring due to limited resources

Why does this trade-off exist?

Energy constraints prevent organisms from maximizing both offspring number and size simultaneously

Examples of offspring size vs. number?

Fish (like darters) produce many small eggs; sharks produce fewer, larger offspring; sunfish produce many small offspring

What pattern is seen in fish reproduction?

Species that produce more eggs tend to produce smaller eggs

Life History Decisions:

What are two key life history decisions?

When to mature (age at first reproduction) and how much energy to invest in reproduction

Why is age at maturity important?

Earlier maturation allows faster reproduction but often results in smaller body size and lower survival

Why is reproductive allocation important?

Allocating more energy to reproduction can reduce energy available for growth and survival

Trade-offs:

What is the survival vs. reproduction trade-off?

Higher reproductive effort often leads to slower growth, smaller size at maturity, and lower survival

What are the risks and benefits of delayed maturation?

Benefit: larger size and potentially higher reproductive success; Risk: dying before reproducing

Evolution:

How does mortality influence life history strategies?

High adult mortality favors early maturation and higher reproductive effort, while low mortality favors delayed maturation

Prediction:

What does life history theory predict about mortality and maturation?

As adult mortality increases, organisms should reproduce earlier and invest more in reproduction

Evidence:

What pattern is observed in snakes and lizards?

Species with longer lifespans (higher survival) tend to mature later

What pattern is observed in fish?

Species with higher mortality rates tend to mature earlier

What is the gonadosomatic index (GSI)?

A measure of reproductive investment, calculated as the proportion of body mass devoted to reproductive organs

How does mortality relate to GSI?

Species with higher mortality rates tend to have higher GSI, meaning greater reproductive investment

Overall relationship between mortality and life history traits?

Higher mortality → earlier maturation and greater reproductive effort; Lower mortality → delayed maturation and reduced reproductive effort

Study:

What did the pumpkinseed fish study examine?

How differences in survival rates across populations affect age at maturity and reproductive allocation

What was the main prediction tested in the pumpkinseed study?

Higher adult survival relative to juvenile survival should favor delayed maturation and lower reproductive effort

Life Strategies:

What are r-selected species?

Species adapted to unstable or frequently disturbed environments where rapid reproduction is advantageous

What traits characterize r-selected species?

Early maturation, short lifespan, many small offspring, and high reproductive rates

What are K-selected species?

Species adapted to stable environments where populations are near carrying capacity (K)

What traits characterize K-selected species?

Delayed maturation, longer lifespan, fewer but larger offspring, and greater competitive ability

How does carrying capacity relate to K-selected species?

As populations approach K, resources become limited, increasing competition and favoring efficiency and survival

Are r- and K-selection strict categories?

No, species exist along a continuum with a mix of traits depending on environmental conditions

Plants:

What factors determine plant life history strategies?

Levels of environmental stress (e.g., poor nutrients, drought) and disturbance (e.g., fire, grazing)

What are competitive plant species?

Species in low-stress, low-disturbance environments that invest in growth and resource competition

What are stress-tolerant plant species?

Species in high-stress, low-disturbance environments that grow slowly and conserve resources

What are ruderal plant species?

Species in high-disturbance, low-stress environments that reproduce quickly and produce many seeds

What are characteristics of annual plants?

Rapid growth, early reproduction, short lifespan, and production of many seeds

What is predicted for plants in low stress and low disturbance environments?

They evolve strong competitive ability to efficiently use available resources