The size and number of offspring

According to the classification of Section 4.9, the division of a given reproductive allocation into a smaller number of larger offspring is expected in relatively offspring size-sensitive habitats. Support moving beyond the classification of habitats

Figure 4.28 (a) Relatively low CR habitats (near-vertical fitness contours) are more likely to give rise to semelparity (maximum reproductive allocation: nothing kept in reserve). RRV, residual reproductive value. (b) For Lobelia spp. on Mount Kenya, habitats become lower CR as interflowering interval increases (down the axis) and mean yearly adult survival decreases. Given that the semelparous L. telekii sets approximately four times the weight of seed set by the iteroparous L. keniensis, habitats can be predicted to favor either semelparity (bottom left) or iteroparity (upper right), with a region of uncertainty between the two. Three study populations of L. keniensis have, as predicted, either habitat characteristics favoring iteroparity, or, in the case of an outlying site, characteristics in the region of uncertainty. (After Young, 1990; Stearns, 1992.)

Low CR

Low CR

Semelparity And Iteroparity

Maximum possible allocation (RRV = 0)

Present reproduction (reproductive allocation)

Demarcation region between semelparity and Iteroparity given uncertainty in the fecundity ratio

Semelparity And Iteroparity

0.950

0.980

0.990

0.995

0.998

0.950

0.980

0.990

0.995

0.998

0.999

Mean yearly adult survival

for this is provided by the observations and experiments on guppies described previously (see Table 4.6): offspring size was larger where predation was most concentrated on the smaller juveniles; and also by examples in Figure 4.23 where offspring size was larger in habitats where competition was likely to be most intense - goldenrods in prairies (as opposed to more temporary old field habitats) and Drosophila on pollen (as opposed to rich but unpredictable sources of yeast).

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