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r and k selection

  1. r and k selection theory AEM 602: Ecology of Plankton and Benthos
  2. • 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
  3. 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 =
  4. •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
  6. • 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
  7. r selected species •Unstable environment •High fecundity •Small body size •Early maturity •Short generation time •Dispersed offspring
  8. •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
  9. • 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
  10. K selected species •Stable environment •Large body size •Long life expectency •High parental care •Fewer off spring
  11. • 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
  12. 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
  13. 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?
  15. 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
  16. • r and K are ends of a continuum, while most organisms are in between – r selection: Unpredictable environments. – K selection: Predictable environments
  17. r vs k
  18. Kirk Winemuller’s (2005) triangular model of life histories for fishes
  19. •  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
  20. • 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
  21. • 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
  22. • 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
  23. r- selected algae k-selected algae Chlamydomonas spp Aphanizomenon sp. Chlorella pyrenoidosa Anabaena sp. Synechoccus sp. Microsystis aeruginosa Scenedesmus obliquus Ceratium hirundinelia Cryptomonas erosa
  24. • 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
  25. • 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.