Life Histories

How organisms grow and produce offspring

Life History

• The schedule of an individual’s life: age at maturity, number of offspring, life span

– Ultimately, all the processes that contribute to the reproductive output of an individual

– Life history traits evolve in populations

Life histories and the principle of allocation

• All organisms take in energy at a limited rate

• Trade-offs in the allocation of resources provide a basis for understanding life histories

Life histories and the principle of allocation

• Trade-off between number and size of offspring:– organisms that produce many offspring must

produce smaller offspring– organisms that produce large offspring can only

produce a limited number

Life histories and survivorship

• Also a trade-off between reproductive effort and survival:• More energy allocated to reproduction means

less energy available for growth and maintenance• Delayed reproduction increases the risk of dying

before reproduction occurs• Adult mortality rates in populations influence

the age at which individuals first reproduce

Life histories and survivorship

• Populations with high adult survival rates should have individuals that delay reproductive maturity and allocate a smaller proportion of their total energy to reproduction

• Populations with high adult mortality should have individuals that reproduce earlier and allocate a higher proportion of their energy to reproductive effort

Life histories and survivorship

Variation in life histories

• Life history strategies can be organized along a continuum from fast to slow:

• r-selected species – life history dominated by intrinsic rate of increase: reach maturity quickly, high fecundity - produce many small offspring

• K-selected species – life history affected more by resources & competitive advantage: mature later or at a larger size, produce fewer, larger offspring

Variation in life histories

• r-selected species – expected to occur in variable or unpredictable environments; Type III survivorship

• K-selected species – expected to occur in more stable environments; Type I or II survivorship

Plant life histories

• Two main environmental variables that dictate plant life history traits (J.P. Grime, 1979):• Disturbance• Stress

Increasingdisturbance

Increasingstress

Plant life histories – Grimes “CSR” model

IncreasingDisturbance(more disturbedhabitats)

Increasing stress (more severe habitats)

Ruderals (Disturbance-TolerantOrganisms)

Stress-tolerantorganisms

Competitiveorganisms

Plant life histories – CSR model

• High Disturbance, Low Stress: Ruderal plants• Grow quickly, reproduce rapidly, produce large numbers of seeds

• Low Disturbance, High Stress: Stress Tolerant plants• Grow slowly, conserve resources for periodic reproduction when

conditions are favorable; low competition

• Low Disturbance, Low Stress: Competitive plants• Can grow quickly, but eventually will compete for resources; life

history will maximize competitive advantage (e.g., later maturity; large size; fewer, larger offspring)

Correspondence between r – K and CSR life history classifications

• ruderal plants tend to be r-selected species

• competitors and stress tolerators tend to be K-selected species

Midterm Review

Benjamin Bloom, 1956

Midterm Review

• Bloom’s Taxonomy 1. Knowledge: arrange, define, duplicate, label, list, memorize, name, order,

recognize, relate, recall, repeat, reproduce state. 2. Comprehension: classify, describe, discuss, explain, express, identify,

indicate, locate, recognize, report, restate, review, select, translate, 3. Application: apply, choose, demonstrate, dramatize, employ, illustrate,

interpret, operate, practice, schedule, sketch, solve, use, write. 4. Analysis: analyze, appraise, calculate, categorize, compare, contrast, criticize,

differentiate, discriminate, distinguish, examine, experiment, question, test. 5. Synthesis: arrange, assemble, collect, compose, construct, create, design,

develop, formulate, manage, organize, plan, prepare, propose, set up, write. 6. Evaluation: appraise, argue, assess, attach, choose compare, defend

estimate, judge, predict, rate, core, select, support, value, evaluate.

Midterm Review

• Biomes– Physical processes on earth that shape wind /

weather patterns (water availability)– Relative energy availability in different areas of

earth– Energy / water availability in different terrestrial

biomes – Major aquatic biomes, and their main physical

properties (light, nutrients, temp)

Midterm Review

• Biomes– Physical processes on earth that shape wind /

weather patterns (water availability)– Relative energy availability in different areas of

earth– Energy / water availability in different terrestrial

biomes – Major aquatic biomes, and their main physical

properties (light, nutrients, temp)

Midterm Review

• Individual Temperature, Water and Energy Relations– Individual adaptations for limiting water loss,

regulating heat gain/loss– Principle of Allocation, energy limitations and

energetic trade-offs

Midterm Review

• Population Genetics– Allele frequencies and genotype frequencies– Hardy-Weinberg equilibrium: p2 + 2pq + q2 = 1– Processes that cause deviation from equilibrium

(i.e., evolution)

Midterm Review

• Population Distribution and Abundance– Different types of distribution patterns of

individuals in populations, and what causes these patterns to occur in nature.

Midterm Review

• Population Dynamics– Different types of distribution patterns of

individuals in populations, and what causes these patterns to occur in nature.

– Life tables, fecundity• Major variables associated with these calculations

– Survivorship, Survivorship curves

Midterm Review

• Population Growth– Different patterns of population growth

• Geometric, exponential, logistic• Major variables associated with these equations

Midterm Review

• Life Histories– Know major patterns in life history trade-offs

• Size, number of offspring, age at maturity, adult survival, etc.

– r- and K-selected life histories– Grimes’ plant life history categories

Midterm Review – Sample Practice Question

You are a gardener and you need to choose a species of tree to plant in your city park. You want a tree that will grow fairly quickly, reach a large size, and live for a long time. If you had to choose a tree from one of Grimes’ three plant life history categories, which do you think would be the most appropriate category to select from, and why?

Midterm Review – Sample Practice Question

You are a land manager who wishes to conserve several separate populations of trout lilies within a national wilderness area. There are many populations threatened by logging, but you are only allowed to focus your conservation efforts on four populations. If you are interested in maintaining the highest level of gene flow possible between the populations you save, should you choose populations that live in areas with or without deer (deer occasionally eat adult lilies)? Support your answer with what you’ve learned about life histories.