Download r-selection

Ecology (BIO C322)
Population Ecology (cont’d)
Metapopulation dynamics
• Metapopulation is a level between the organism and the
population levels of organization
• Defined as subpopulations occupying discrete patches or
islands of suitable habitat that are separated by unsuitable
habitat but connected by dispersal corridors.
• Groups of individuals in each discrete patch may go extinct
at some point in time but patch may be re-colonized by
individuals from nearby patch.
• If colonization and extinction balance over a large area of
landscape, The total population size may remain about the
same.
• The survival of the species may depend more on dispersal
than on births and deaths within the patch.
Energy for Maintenance & Growth
• R  Maintenance energy to be spent in
order to survive.(respiration)
• PN  Growth [in biomass or] in numbers
(Population growth).
• PN  Net energy required for reproduction
• PN  Energy devoted to reproductive
structures, mating activities, offspring
production & parental care.
Costs vs Benefits
• Costs: Loss of time/energy; cost of
maintenance.
• Benefits: Energy obtained from food.
• Increasing energy available for reproduction
accomplished by optimal foraging:defined as
max possible energy return under a given set
of foraging and habitat conditions.
– Selecting larger/more nutritious/easy-to-catch
prey, or;
- By reducing search/pursuit time & effort.
r-selection
• Uncrowded/uncertain environments (subject
to periodic stresses like storms/droughts)
•  Selection favours species with high
reproductive potential.
•  r-selected species.
– High ratio of reproductive to maintenance effort.
– J-shaped population growth pattern.
K-selection
• Crowded environments/low probability of
severe disturbances.
•  Greater energy investment in
maintenance & survival.
•  K-selected species.
– Energy in favour of enhanced competitive
ability;
– Logistic growth pattern.
Practice Concept
• Allocation of energy among the various activities
of an organism reflects balances b/w
advantages & costs.
• First consideration is survival & maintenance of
individual (respiratory component),
• with additional energy allocated to growth &
reproduction (production component).
• Large organisms, like large cities, must allocate
a larger portion of their metabolized energy input
to maintenance,
• than small organisms, which do not have so
much structure to maintain.
Allocations of Net Energy
• Energy expended:
- To cope with competition for common
resources;
- To avoid being eaten by predator;
- To produce offspring.
Figure 6.28 (TB) in tabular form
(C=competition; P=predation)
S.
No.
Situation
Percentage energy for:
Offspring
Competition
Predator
Avoidance
1.
Low C, low P
75
15
10
2.
High C
10
75
15
3.
High P
10
15
75
4.
Equal
selection
pressures
30
40
30
Species 1  r-selected
Species 2, 3, 4  K-selected
Table 6.3 (TB) – Attributes of r- & K-selection
S. No.
Attribute
r-selection
K-selection
1.
Climate
Unpredictable
Predictable
2.
Population size
Variable in time
Constant in time
3.
Competition
Lax
Keen
4.
Selection favours
Rapid development
Early reproduction
Small body size
Many offspring
Slow development
Delayed reproduction
Large body size
Few offspring
5.
Length of life
Short (<1 year)
Long (>1 year)
6.
Stage in
succession
Early
Late (climax)
7.
Leads to
Productivity
Efficiency
Table 6.4 (TB):
Allocation of assimilated energy b/w production
(growth + reproduction) & respiration (maintenance)
Trophic level
Primary consumer
Cotton rat (herbivore)
Secondary consumers
Marsh wren (insectivore)
Red fox (carnivore)
Raccoon (omnivore)
Poikilothermic Arthropods
Pea aphid (herbivore)
Wolf spider (predator)
PN
R
13
87
1
4
4
99
96
96
58
25
42
75
Selection of Prey Size
• Low food (prey) abundance  Prey of all
sizes eaten  “Generalist” feeders.
• High food abundance  Small-sized prey
ignored by predators, large-sized ones
eaten  “Specialist” feeders.
Read this…
• MacArthur (1978) noted that K-selection
prevails in the relatively non-seasonal
Tropics,
• whereas r-selection in seasonal
environments of the North Temperate
Zone,
• where population growth is marked by
exponential growth,
• followed by catastrophic declines during
the winter months.
An example
• Ragweed (Ambrosia) grows in old fields &
recently disturbed places  Produced 50
times more seeds;
• Allocation of greater %
of assimilated energy
to reproduction.
• [Comparison with Dentaria laciniata, a
herbaceous plant in relatively stable forest
floor.]
Goldenrods of genus Solidago
• Six populations selected for study.
• Population 1 in dry, open fields &
disturbed sites  Low leaf biomass, more
of PN allocated to reproductive tissues 
r-selected.
• Population 6 in moist hardwood forests 
More energy allocated to leaf production,
less allocated to reproduction 
K-selected.
Patterns of Dispersion
• Random: When environment is uniform,
no tendency to aggregate.
• Regular or Uniform: Severe competition,
promotes even spacing.
• Clumped or Grouped: Random or uniform.
Allee Principle of Aggregation
• Why individuals aggregate?
– Habitat;
– Climate;
– Reproduction;
– Social behaviour.
• Increased survival of the group in face of
danger; finding resources; modification of
microclimate.
• Limiting effects of undercrowding  Less
chance of finding mate; less probability of
pollination with increasing space.
• Limiting effect of overcrowding 
Competition, pollution.
•  Allee principle of aggregation.
• [Another cause of extinction of small
populations: Reduced genetic diversity.]
Practice Concept
• Biological control of pests is one of the few
research areas in which the Allee effect has been
recognized.
• According to Allee effect, at low population
densities, individuals may not be able to secure a
mate, reducing average birth rates, resulting in
population decline.
• By releasing sterile male flies (pests) into the
natural population, it is possible to reduce the
probability of a female encountering a fertile male.
• This is like artificially creating an Allee effect.
Group Survival Value
• Beehive  Enough heat generated to
enable survival in cold temperatures.
• Bobwhite quail  Group (covey) rest in a
circle, head facing outwards  Defence
strategy.
• Home range: The area in which an animal
or a group is active for its daily needs.
- It varies with body size.
• Refuge or Territory: Defended part of
home range; avoidance of predation;
reproductive isolation.
• Well-defined behaviour patterns in the
territory.
• Song & call (birds): Loud singing (males) after
territory establishment to attract females.
– Individuals unable to establish territories fail to
breed.
• Flashing fangs (mammals).