University:
University of Houston
Course:
ENGL 1301 | First Year Writing I
Academic year:

2022
Views:

197

Pages:

2
Author:

Anabella Delacruz

Page 1 of 2 Lecture 8 How many times should an organism reproduce? Let's consider the extreme case - semelparity vs iteroparity Semelparity - one reproductive event during life- span (=annual) Iteroparity - more than one reproductive event during life-span. (=perennial) Cole (1954) showed that all else being equal, a semelparous organism could achieve the same reproductive rate of population growth by simply increasing the number of offspring it produced by one greater than an iteroparous individual. Example (see handout) Not realistic because survival is never 1.0 for either juvenile survivorship in annual (semelparous), nor is adult survivorship in perennial (iteroparous) population ever equal to 1.0. Let's change juvenile survivorship but make it equal for both semelparous and iteroparous species. Where, (Ba) = # of offspring produced by annual breeder. (C) = juvenile survivorship (Bp) = # offspring of produced by perennial breeder. (P) = adult survivorship so, with same fecundity and juvenile survivorship for both: (Ba) (C) < (Bp) (C) + P (10) (0.5) < (10) (0.5) +1 = 6 vs 5 for annual but this time if we add one more offspring to annuals the result is different: (11) (0.5) < (10) (0.5) +1 5.5 < 6, iteroparity wins So, juvenile survivorship can affect the advantage in favor of iteroparity, even with adding one more offspring to annual fecundity, all else being equal. What we need to consider is the survivorship of adults relative to that of offspring to understand the conditions under which semelparity and iteroparity may be favored: We can rewrite our equation to read: (Ba) C= (Bp) C + P, assuming C is constant in both then: Ba= Bp +P/C, Now, if P is high (adult survivorship of iteroparous species), for semelparous strategy to be equal or have an advantage over iteroparous form it will have to have higher number of offspring. If C (juvenile survivorship) is low relative to P, then iteroparity is favored. More realistic, Ba will be greater than Bp, so what are the conditions that will favor iteroparity? e.g. where perennial fecundity is 1/2 that of an annual. 20=10 + 1/0.1 juvenile survivorship need only be equal or greater than 10 % before the perennial strategy is favored. In general, if Ba < Bp + P/C then, iteroparity is favored Ba>Bp +P/C then, semelparity is favored Page 2 of 2 e.g. with some adult mortality (P= 0.5) 20=10 + 0.5/0.05, juvenile survivorship need only be < 5% before perennial strategy is adopted. Conditions favoring iteroparity adult survivorship is high juvenile survivorship is low Conditions favoring semelparity adult survivorship low Juvenile survivorship high One last consideration not mentioned is the problem associated with variance – i.e. variation in fecundity and mortality associated with environmental factors – flood, fire, outbreak of disease, etc. If variance in juvenile survivorship is high à iteroparity will be favored If variance in adult survivorship is high à semelparity will be favored May lead to "bet-hedging" strategy, iteroparous even when most of the time juvenile survivorship is high. Also, semelparous organisms often have mechanisms to thwart variance in juvenile survivorship, e.g. seed banks where not all seeds germinate simultaneously.