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 competitionOrganisms are pushed to use their resources more efficiently

late reproductionlong generation timefew offspringe.g.Primates

SharksTurtle 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 moreNo energy expenditure

Eggs have protection from predators/ harsh conditions

High chance of offspring survival

Drawbacks High levels of mortality

Energy expenditureSome 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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