Reconstruct cohort reproduction with the correct survival denominator

1. Frame the biological question

A life table follows survival and reproduction by age or stage. Cohort survivorship is measured from an initial cohort, whereas conditional survival asks what fraction of those already alive at one age reach the next. Mixing these denominators changes the interpretation of mortality and reproduction.

2. Connect mechanisms and evidence

The supplied female-based cohort has survivorship l0 = 1, l1 = 0.6 and l2 = 0.3. Reproduction per surviving female is zero at age 0, a at age 1 and b at age 2. Expected contribution per initially born female is lx times mx, giving net reproductive rate R0 = 0.6a+0.3b.

The mean age of reproduction in this schedule weights age by its expected contribution: (0.6a+2×0.3b)/R0. This is a cohort reproductive-age mean, not automatically the exact generation time of a growing stable population. Conditional survival from age 1 to age 2 is 0.3/0.6 = 0.5, not 0.3.

R0 above one means more than replacement in this stipulated lifetime schedule, but overlapping generations prevent treating it as a one-year population multiplier. Sex ratio, migration, changing conditions and density dependence can alter real growth. Population sampling must also retain spatial frames, detection and independent units rather than treating every repeated sighting as a new individual.

3. Reconstruct a worked comparison

At a = 2 and b = 2, age contributions are 1.2 and 0.6, so R0 = 1.8. Mean parental age is (1.2+1.2)/1.8 = 4/3 intervals. Half of the age-1 survivors reach age 2 despite birth-based survivorship being 0.3.

4. Predict and investigate

Predict first: Does age-2 survivorship 0.3 mean only 30% of age-1 survivors reach age 2?

Compare schedules with reproduction concentrated early versus late while retaining survival. Find two schedules with equal R0 but different mean reproductive age and explain why their annual dynamics need not match.

Retain the first prediction, controlled settings, complete ledgers and one conflicting or null comparison. These are original supplied calculations, not observed populations or completed fieldwork.

5. Transfer to a changed case

Apply the mechanism to a changed case

For a = 1 and b = 4, calculate R0 and mean reproductive age.

Retain your calculation, units and assumptions in the lesson notebook before opening this public practice response.

Compare a worked response

Contributions 0.6 and 1.2 give R0 = 1.8, again. Mean age (0.6+2.4)/1.8 = 5/3 intervals, later than the worked schedule despite the same lifetime net reproduction.

6. Defend a conditional conclusion

Separate mechanism, population pattern and causal evidence. Identify a missing biological process and an independent observation that could discriminate alternatives. Preserve first reasoning and substantive human critique in the General Biology II workspace. Use supplied or ethically archived information only; no animal manipulation, habitat intervention or organism release is requested.

Supporting scientific reference, checked 30 October 2026. Original prose, models and packets; no source exercise or figure reproduced. Actual field sampling and independently assessed transfer remain separate evidence gates.