Lecture 14 Life Histories Modes of reproduction – sexual vs. asexual k vs r selected species

1. Lecture 14 Life Histories Modes of reproduction – sexual vs. asexual k vs r selected species Survivorship tables

2. Life Histories • An organism’s life history is its lifetime pattern of growth, development, and reproduction • Maximal reproductive success or fitness is constrained by limited resources and an organism must balance trade-offs • Modes of reproduction • Age at reproduction • Allocation to reproduction • Time of reproduction • Number and size of eggs, young, or seeds produced • Parental care

3. Sexual reproduction is the fusion of haploid egg and sperm to form a diploid zygote • A major source of genetic variation due to the recombination of chromosomes during egg and sperm production • Asexual reproduction produces offspring without the involvement of egg and sperm • Individuals are genetically identical to the parent

5. Asexual vs. Sexual Reproduction • Asexual reproduction • Benefits • Offspring are well adapted to current conditions • Potential for high population growth • Costs • Low genetic variability in the population • May be unable to adapt to a change in environmental conditions • Sexual • Benefits • High genetic variability in the population • Increased probability that some individuals will survive environmental changes • Costs • Parents only contribute one half of its genes • Specialized reproductive organs required • Expense of reproduction not equally shared between parents

7. How many young are produced? • Limited access to energy/resources results in trade-off between number and size of offspring • ie.- species producing a larger # if offspring means offspring are smaller, and vice-versa • Parent provides extended care for young  fewer young produced but greater survival rate • The amount of energy invested in reproduction varies for different individuals • Investment in reproduction includes production, care, and nourishment of offspring • An individual’s fitness is determined by the number of offspring that survive to reproduce

8. Common Murre

9. Three Survivorship Patterns • Type I = K selected • Mortality rises in post-reproductive years • Type II • Mortality constant throughout life • Type III = r selected • Many offspring with high juvenile mortality

10. K selected species • Low number of young produced • Offspring size tends to be large • Low mortality of young • Extended parental care • High rate of survival past reproductive age • Long time to maturity • Relativly long life span • Live near carrying capacity

11. r selected Species • High number of young produced • Low parental input to each individual young • Short maturation time • Breed at young age • Produce many offspring quickly • High mortality of young • Nonexistant parental care • Opportunists – populations quickly develop but may crash • Examples: • Waterfleas, insects, bacteria

12. Life History Classification • MacArthur and Wilson • r selection (per capita rate of increase) • Characteristic high population growth rate. • K selection (carrying capacity) • Characteristic efficient resource use. • Pianka : r and K are ends of a continuum, while most organisms are in-between. • r selection: Unpredictable environments. • K selection: Predictable environments.

13. Life Histories – Age Structure and Survivorship in Populations • Cohort populations • Birthrate and survival of young • Competitive ability vs population size – survivorship patterns • Principle of allocation and reproduction • Dispersal and seed size • Ecological succession

14. Cohort – a group of individuals of the same age within a population (individuals born at same time) - see p. 240-241 • Study of cohort provides information about: • Mortality and survival vs. age • Used to construct a cohort life table • Static life table • Pattern of survival  survivorship curve

15. Static life table – ‘snapshot’ of population at a given time • Data corrected to 1000 – actual number sampled 608 • Dall sheep – Murie study 1944 • Collect skulls • Evaluate age of animal at time of death • Allows evaluation of survivorship: percentage of an original population that survives to a given age

16. Plant Succession and Life History Patterns JPGrime (pages 286-288) • Ruderals (highly disturbed habitats) • Grow rapidly and produce seeds quickly. • Stress-Tolerant (high stress - no disturbance) • Grow slowly - conserve resources. • Competitive (low disturbance low stress) • Grow well, but eventually compete with others for resources. Stress: environmental extremes or competition that limits (or provides excess) light, temperature, nutrients

17. Survivorship and Age structure • Age structure: Proportion of individuals in various age classes • Survivorship is the percentage of an original population that survives to a given age • Involves study development of life table • Cohort • Example: Cactus finch • Static • Example: Dall sheep

18. Age Structure Diagrams: Visualization of future population growth

19. What regulates population size?

20. Ch 18 p 344

21. Diffuse predator–prey interactions • The lynx, coyote, and horned owl are responsible for the periodic cycles in the snowshoe hare population • Diffuse mutualism • A single plant species may depend on a variety of animal species for successful reproduction

22. Is regulation top-down or bottom-up? • ie. primary productivity vs. limits imposed by predator populations

23. Hare popul crashes as: 1. Reduced forage  weakened hares, high lynx prdation 2. Forage produced after heavy browsing accumulates plant defense chemicals less palatable Lynx predates weakened hares – eventually crashes

24. Old Field Succession: Dwight Billings • Early species to invade: ‘weedy’ or r-selected species • Do not compete well for resources, high reproductive rate • Shift to k-selected species • Changes in nature of habitat favor species which reproduce successfully at or near carrying capacity

25. Plant Life Histories