13. Trade offs in Life’s Histories

Life History

Description of major characteristics of an organism from its birth to its death

Key Traits of Life History (5)

• Body size

• Fecundity (# offspring per reproductive episode)

• Parity (# reproductive episodes over lifetime)

• Maturity (age at first reproduction)

• Aging/Senescence (lifespan/survival)

Parity in Animals

number of reproductive episodes over lifetime

Semelparity/semelparous: one reproductive event

Iteroparity/iteroparous: multiple reproductive events

Parity in Plants

Monocarpic: flowers and sets seeds only once

Polycarpic: flowers and sets seeds multiple times

NOT the same as annual/perennial plant

Allometry

Study of scaling between body size and various biological traits and functions (shape, anatomy, physiology, behaviour)

• Body size influences relationships with temperature, energy, water, nutrient acquisition

• Taking into account surface area/volume

• Ex: estimating fish biomass in fisheries

Principle of Allocation

Amount of energy and time available to each organism is limited therefore when these factors are allocated to one function, it reduces energy and time available for other functions

Energy Budgets

Need to allocate energy between and within parts of energy budget

• Trade offs between reproduction and other activities

Examples of trade offs

• Offspring vs. Number

• Seed size vs. Dispersal distance

• Offspring size vs. Gene flow

• Survival vs. Age at maturity

• Size vs. Maturity and behaviour (game theory)

Offspring Size vs. Number of offspring produced

• Species with larger females lay larger number of eggs (positive relationship) BUT species producing larger eggs lay fewer eggs (negative relationship)

Seed size vs. Dispersal distance

Dispersal: permanent movement of individuals (propagules) from one population to another (NOT MIGRATION)

Seed dispersal: spread of seeds from mother plant across space

• larger seed mass, the wider plants can disperse

• Only one of many factors influencing how far seeds disperse

Migration

Seasonal movement of individuals from one location to another

Offspring Size vs. Gene Flow

Gene Flow: transfer of genetic material from one population to another

• Smaller offspring have greater dispersal distance (low genetic isolation and high gene flow)

• Bigger offspring have shorter dispersal (high genetic isolation and low gene flow)

• Ex: fish laying smaller eggs lay more eggs (higher fecundity)

Survival vs. Age at Maturity

Reproductive Effort: allocation of energy, time and other resources to reproduction, including formation of eggs and offspring care

• Investing early in reproduction means more energy cannot be allocated to growth and survival

• Higher survival rate = reproducing later

• Lower survival rate = reproducing early

Size vs. Maturity and Behaviour

Variation within species can be substantial as they represent alternative but successful evolutionary strategies

• Game Theory

• Disruptive selection

Game Theory**

Members of the same species mature at different times

r Selection

Small and fast characteristics

• high reproductive output

• Put less energy into growth

• Dies faster

• Not as favoured

K Selection

Large and slow characteristics

• lower reproductive output

• Put more energy into growth

• Longer growth

• Better in more stable, less disturbed environments

Types of Life History Classifications

• r and K selected species

• Grime’s Triangle

• E-P-O

Life History Classifications - Grime’s Triangle

CSR triangle → Competitive, Stress tolerant, Ruderal

• Few plants fall into the “corners“

• Most species fall within the triangle

Grime’s Triangle: Ruderal

• Thrives best under high disturbance and low stress (many weeds)

• Rapid growth

• Produces many tiny seeds

• Short life (often annuals)

• Little maintenance and growth investment of large structure

Grime’s Triangle: Stress-Tolerant Plants

• Occupy environments of high stress and low competition

• Invest in physiological stress-tolerance instead of growth

Grime’s Triangle: Competitive Plants

• Occupy environments of low disturbance and high productivity

• FAST growth

Life History Classification: E-P-O Approach

Equilibrium, Periodic, Opportunistic Scheme