R_o = (l_x)(m_x)  replacement rate = # offspring in a lifetime

        where
               l_x = probability of survival from birth to age x  (cumulative)
                        [cf. survivorship = probability of survival to age x+1 from age x ]
               m_x = fecundity (# offspring) at age x

      R_oe^rat low population density

Assumptions:
     Maximum life span is 4 years.
     Iteroparity & semelparity are alternative phenotypes:
          fecundity for each phenotype is genetically determined.
     Survivorship patterns are the same for both phenotypes:
            these are determined by the environment
     Table records total ## offspring (male & female) born to each female.

      50% survivorship        75% survivorship        25% survivorship

      l_x *  m_x =  l_xm_x       l_x *  m_x =  l_xm_x        l_x *  m_x =  l_xm_x
0     1.00  0     0           1.00  0     0.00        1.00  0     0.00
1     0.50  2     1.00        0.75  2     1.50        0.25  2     0.50
2     0.25  3     0.75        0.56  3     1.68        0.063 3     0.19
3     0.125 2     0.25        0.42  2     0.84        0.016 2     0.03
4     0     0     0           0     0     0           0     0     0
R_o              =   2.00                =   4.02               =   0.72

Semelparous strategy: reproduction is entirely in the first year.

      50% survivorship        75% survivorship        25% survivorship

      l_x *  m_x =  l_xm_x       l_x *  m_x =  l_xm_x        l_x *  m_x =  l_xm_x
0     1.00  0     0           1.00  0     0           1.00  0     0
1     0.50  4     2.00        0.75  4     3.00        0.25  4     1.00
2     0.25  0     0           0.56  0     0           0.063 0     0
3     0.125 0     0           0.42  0     0           0.016 0     0
4     0     0     0           0     0     0           0     0     0
R_o              =   2.00               =   3.00               =   1.00