AEC 360  Ecology

Lecture 9:  Life Histories

I.  What traits promote reproductive success, or fitness?

    A.  Life History:  Attributes of the schedule of an individual's life

    B.  Maturity, age at first reproduction

    C.  Parity, number of episodes of reproduction

    D.  Fecundity, # of offspring

    E.  Senscence, the end of life

II.  Why not evolve all life history characteristics to maximize fitness?
    A.  Life histories should evolve to improve reproductive success

    B.  ALL characteristics can't simultaneously improve, because there are always tradeoffs

    C.  Resources are limited, and improving one characteristic usually means diverting energy from another

    D.  Problem of allocation

       - Life history traits often represent an optimal allocation (Fig. 8.4).

    E.  Phenotypic plasticity refers to response of a species' characteristics to its environment, Evolution occurs through changes in genotype or allele frequencies

        1.  Reaction norms refer to the relationship of the phenotype and environment (Fig. 8.12)

III.  In nature, life history traits are related to environment   
    A.  Life history trait variation is often determined by habitat or environmental condition

     B.  Life history traits are often correlated so we frequently see particular groups of traits (Table 8.1)

Fast: short liferapid development, early maturity, high reproductive output, low parental investment

Slow:  long life, slow development, late maturity, low reproductive output, high parental investment


     C.  Plants strategies can be classified based on the environment where they thrive (Fig. 8.2)
        Stress tolerators, disturbance tolerators, and competitors


IV.  Tradeoffs that involve costs and benefits to reproductive success determine life history traits
    A.  Age at First Reproduction (Fig. 8.6)
        1.  Benefits: earlier reproduction causes higher population growth rates

        2.  Costs: Lower survivorship

    B.  Reproductive Effort  (Fig. 8.5)
        1.  Benefits: More effort leads to more offspring and higher population growth rates

        2.  Costs: Lower survivorship of parent and offspring

    C.  Annual vs. Perennial
        1.  Annuals produce many seeds, but must get established

        2.  Perennials are established, but must survive winter and can't allocate as much to seeds

    D.  How many times to reproduce?
        1.  Semeparous: Reproduce once when conditions appropriate

          -favored when cost of reproduction is high or when reproduction can be timed to occur during favorable period


        2.  Iteroparous: Reproduce many times

    E.  Why do organisms senesce?
        1.  Definition: Increase in mortality and decline in fecundity
        2.  Accumulation of molecular defects
        3.  Evolution of repair mechanisms in old age may exact costs on survival earlier
        4.  Selection on changes in survival and fecundity at old age is weak