LIFE HISTORY - is the particular pattern of reproduction, growth, maturation and dispersal that generates an organisms success in leaving descendants (FITNESS)

 

Problem:  How does an organism allocate limited resources? (It can’t do everything.)

 

 

FITNESS  =

 

REPRODUCTIVE OUTPUT

X

PROBABILITY OF SURVIVAL

 

When/how often to reproduce

When to cease reproduction

Energy required for displays
- behavioral
- morphological

How many offspring to produce:
- per brood
- per lifetime

Invest in parental care, or not

 

How long to continue growth:
- short time (end up small)
- long time (end up large)

Predator defences

Disease resistance

Special skills, e.g. competitive ability

Dispersal:
- when?
- how often?
- how far?

 

 

 

WHAT ARE THE ADVANTAGES OF BEING LARGE OR SMALL?

 

LARGE ORGANISMS

SMALL ORGANISMS

 

 

· Slow metabolic rate; low energy demand per unit biomass (efficient)

 

· High metabolic rate; high energy demand per unit biomass (less efficient)

· Need less food

 

· Need more food

· Relatively well buffered against abiotic fluctuations

 

· Usually more sensitive

· Mass, aggressiveness and weaponry are possible options; advantage in a competitive world

 

· These options not so available

· Long life with multiple episodes of reproduction (“iteroparity”)

 

· Short life, few episodes of reproduction (in the extreme, one episode – “semelparity”)

· Low  rmax values

· High rmax values

 

 

So overall:

 

IT PAYS TO BE SMALL WHEN TIMES ARE GOOD - LOTS OF FOOD, LOW FLUCTUATIONS ETC.....BUT....WHEN TIMES ARE BAD, LARGE ORGANISMS HAVE THE ADVANTAGE.

 

 

Therefore:

 

(A). Under conditions of INTENSE COMPETITION for resources, individuals that divert energy to competitive abilities (growth, defense, etc.) will leave more successful offspring than those merely continuing to reproduce.

· i.e. K-selection characteristics have selective advantage under intense competition.

 

(B). By necessity, such organisms must have LOW rmax values.

· i.e. they have higher ra values (not rmax) under competitive environments than "reproducers"  Note: a K-strategist will leave fewer offspring under intense competition than under low competition, but under intense competition will leave more than an  r-strategist.

 

(C). Under conditions of LOW COMPETITION, individuals that devote resources to reproduction as soon as possible will leave more successful offspring than those who devote energy to growth etc. - best strategy is to keep 'G' (generation time) small.

· i.e. r-selection characteristics have selective advantage under low competition.

 

 

CHARACTERISTICS OF SPECIES WITH

C, S, & R STRATEGIES*

 

The expected life-history traits will be C-, S-, or R-selected types…:

 

When harshness/stress is:

 

 

 

 

LOW

HIGH

 

 

 

 

 

 

When frequency/

intensity of

disturbance is:

LOW

C

S

 

 

 

 

 

HIGH

R

-

C - SELECTION: COMPETITORS - selected for high competitive ability which depends upon plant characteristics which maximize the capture of resources in productive, relatively undisturbed environments.

 

S - SELECTION: STRESS-TOLERATORS - has brought about reductions in both vegetative and reproductive vigour, adaptations which allow endurance of continuously unproductive environments e.g. arctic, alpine or arid environments.

 

R - SELECTION: RUDERALS - associated with short life span and high seed production in highly disturbed but potentially productive environments e.g. arable situations.

 

 

*GRIME, J.P. (1977). Evidence for the existence of three primary strategies and its relevance to ecological and evolutionary theory. 

American Naturalist 111:1169-1194.

roy turkington, biol 303 - life history