r and k selection theory
AEM 602: Ecology of Plankton and Benthos

• Seek to explain the evolution of organism traits as adaptive
responses to environmental variation and differential mortality
or resource allocation to life (Roff 1992; Stearns 1992)
• Examine how traits are inter-correlated and constrained by
ecological factors
• Predict demographic response to disturbances at variable
spatial and temporal scales (Murphy,1968; Roff, 1988; Orzack
and Tuljapurkar, 1989)
• Invoked to predict relative influences of density-dependent
versus density-independent ecological influences on life
stages and age classes (Reznick et al. 2002)
• The most notable example is the theory of r and K-selection
(MacArthur and Wilson 1967) based on their work on island
biogeography
Life history theories

Populations grows, shrinks, or remain stable,
depending on rates of birth, death, immigration, and
emigration
Population growth curves show change in population
size over time.
Population growth
crude birth rate +
immigration rate
crude death rate+
emigration rate
–Growth rate, r =

•Exponential growth can not
continue forever
•The exponential growth of
many real populations begins to
level off as the density
approaches the carrying
capacity(K) of the environment
•Carrying capacity of a population is the maximum
density of a population that the environment can
support over a sustained period without damage to
the environment

SURVIVORSHIP CURVE

• In some environments, organisms exist
near their asymptotic density (K) for much
of the year
– These organisms are subject to K-selection
• In other habitats, the same organisms
may rarely approach the asymptotic
density but instead remain on the rising
portion of the curve for most of the year
– These organisms are subjected to r-selection

r selected species
•Unstable
environment
•High fecundity
•Small body size
•Early maturity
•Short generation
time
•Dispersed offspring

•Found in disturbed, variable or
unpredictable environment
•Productivity – high
grow rapidly
• Good colonizers
•Life span – short;
mortality – high
•Mode of life- wide range of feed

• Species that are r-selected ~ less interspecific
competition
– Hence, evolve no mechanisms for
strong competitive ability
• Algae, bacteria, rodents, annual plants,
sardine, squid, krill and most insects
• They reproduce and disperse rapidly when conditions are
favorable or when a disturbance opens up a new habitat
or niche for invasion, as in the early stages of ecological
succession

K selected species
•Stable environment
•Large body size
•Long life
expectency
•High parental care
•Fewer off spring

• Species – highly demanding ,
well adapted
• Productivity – since adapted- low
• Prolificity – low – long period of
sexual maturity, broods limited
• Mode of life- feed on small part of
food chain
• Population density- no of
descendents depend on
environmental potential

Species that are K-selected exist
under both intra- and
interspecific competition
Organisms are pushed to use
their resources more efficiently
late reproduction
long generation time
few offspring
e.g.Primates
Sharks
Turtle
Whale

No parental
care
Care of laid
eggs
Care of
young
What is it? No contact with
offspring after eggs
are laid
Guarding and/or
incubating eggs to
hatching
Care of young after
hatching/birth
Benefits Free to mate more
No energy
expenditure
Eggs have
protection from
predators/ harsh
conditions
High chance of
offspring survival
Drawbacks High levels of
mortality
Energy expenditure
Some mortality after
hatching
Very high levels of
energy expenditure
– may not be able to
mate for many years
after offspring birth
Examples Reef fish, frogs,
turtles
Seahorse, diamond
python, cephalopods
(eg. Octopus, squid)
Humans, primates.
Mammals (milk),
Some catfishes &
Tilapia spp.
Parental care or not?

CONTINIUM

LOGISTIC LAW
• dN/ dt = rN (K − N)
K
• If N is far below K, the growth realization factor will
be close to 1, and the population will show exponential
growth
• But as N begins to approach K, the growth realization
factor approaches zero, and the rate of population
growth drops to zero

• r and K are ends of a continuum, while
most organisms are in between
– r selection: Unpredictable environments.
– K selection: Predictable environments

r vs k

Kirk Winemuller’s (2005) triangular
model of life histories for fishes

• Opportunistic species:
– short generation times and small body sizes
– produce lots of eggs but do not provide parental care
– thrive in environments that are dominated by
ecological disturbance and can often be thought of as
colonizing species
– most comparable to r-selected strategists
– Example is a killifish

• Periodic species:
– long-lived and have large body sizes
– produce lots of eggs but don’t provide parental care
– This strategy works best when variation in juvenile survival rates
is greater than variation in adult survival rates
– Example is a swordfish
– In fact, this appears to be the most common life history strategy
throughout the fish world

• Equilibrium species:
– are comparable to K-selected strategists
– fairly long-lived and vary in body size
– have few offspring, but they provide parental care to those
offspring
– This strategy appears to be favored in environments that are
highly density-dependent or stressful
– Examples include amblyopsids (cave fishes), cichlids,
and syngnathids (seahorses, sea dragons, and pipefishes).
amblyopsids cichlids syngnathids

• R- selected species are characterized by small size, high growth
rate and short cell cycle and are able to increase in population
rapidly under conditions of low population density and low species
competition. These organisms are particularly prominent in
temperate lakes during the clear-water phase. During this time,
grazing pressure by herbivores is patchy and intense, and algal
growth is limited to short periods in those parts of the lake with lower
densities of zooplankton
• K-selected species, conversely, typically have large size, low growth
rate, and long cell cycles and are adapted to conditions of high
population density and high competition
r-selected and k-selected algae

r- selected algae k-selected algae
Chlamydomonas spp Aphanizomenon sp.
Chlorella pyrenoidosa Anabaena sp.
Synechoccus sp. Microsystis aeruginosa
Scenedesmus obliquus Ceratium hirundinelia
Cryptomonas erosa

• The distinguishing feature of the r- and K-selection paradigm was
the focus on density-dependent selection as the important agent of
selection on organisms’ life histories
• This paradigm was challenged as it became clear that other factors,
such as age-specific mortality, could provide a more mechanistic
causative link between an environment and an optimal life history
(Stearns 1976,1977)
• The r- and K-selection paradigm was replaced by new paradigm
that focused on age-specific mortality (Stearns, 1976; Charlesworth,
1980). This new life-history paradigm has matured into one that
uses age-structured models as a framework to incorporate many of
the themes important to the r–K paradigm
Conclusion

• Winemuller, KO. 2005. Life history strategies, population regulation,
and implications for fisheries management. Can. J. Fish. Aquat.
Sci. 62: 872-885.
• Roff, D.A. 1992. The evolution of life histories: theory and analysis.
Chapman and Hall, New York.
• Stearns, S.C. 1992. The evolution of life histories. Oxford University
Press, Oxford.
• Murphy, G.I. 1968. Patterns in life history and the environment. Am.
Nat. 102: 391–403.
• Roff, D.A. 1988. The evolution of migration and some life history
parameters in marine fishes. Environ. Biol. Fishes, 22: 133–146.
• Orzack, S.H., and Tuljapurkar, S. 1989. Population dynamics in
variable environments. VII. The demography and evolution of
iteroparity. Am. Nat. 133: 901–923.

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