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Notes and Examples for LIFE HISTORY
- the pattern of reproduction, growth, maturation and dispersal that generates an organism’s FITNESS
The problem is one of ALLOCATION: How does an organism allocate limited resources? (It can’t do
everything.)
FITNESS =
REPRODUCTIVE OUTPUT
X
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
PROBABILITY OF SURVIVAL
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
•
•
Slow metabolic rate; low energy demand
per unit biomass (efficient)
Need less food
SMALL ORGANISMS
•
High metabolic rate; high energy demand
per unit biomass (less efficient)
•
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”)
•
•
Low rmax values
•
Short life, few episodes of reproduction (in
the extreme, one episode – “semelparity”)
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 frequency/
intensity of
disturbance is:
When harshness/stress is:
LOW
HIGH
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.
DEFINITIONS:
Stress (=Harshness) – any factor which slows or limits the ability to accumulate biomass
Disturbance – any factor which removes units of biomass and/or whole individuals
*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.
Genet – physically separate unit of plant biomass with a genotype different from that of its neighbouring
genets; may be composed of a single ramet, or many
Ramet (=“Tiller”) – an attached or separate component of a genet; all ramets of a given genet are identical
in genotype (but likely variable in phenotype, especially size)