Population dynamics and mean age at reproduction
In our metapopulation of house sparrows, annual variation in population
size and the strength of density-dependent intra-specific competition
appears to generate temporal variation in the mean age at reproduction.
If individuals produce the same number of recruits, and in the absence
of density regulation, it is expected that individuals that reproduce
earlier in life and thus have shorter generation times will be selected
for, because early reproduction will result in a higher individual
growth rate. Our results suggest that population dynamics affects the
mean age at reproduction because density-dependent competition
constrains the reproductive output of younger individuals. This is
therefore consistent with the idea that density-dependent competition
constrains individual growth rates. Especially for males, older
individuals seem to contribute more to population growth when
intra-specific competition is stronger.
We found strong support for density regulation in the production of
recruits. In years when population sizes were higher than average,
individuals produced fewer recruits. The patterns of density regulation
in recruitment could have been mediated by effects on the reproduction
of parents or via differential juvenile survival. Thus, in years where
population sizes were higher than average, individuals fledged fewer
offspring and/or juvenile survival was lower. In our study, we did not
find that adult survival was density dependent, but given that our
measure of first-year recruitment combines investment in reproduction by
the parents with the survival probability of their offspring, then
juvenile survival may well represent the point in the life-histories of
these house sparrows where survival is most closely density regulated.
When we analyzed how the mean age at reproduction was affected by
population dynamics across years and populations, we found a trend
suggesting that in years where population sizes where larger than
average then the mean age at reproduction was older (Table 3). We also
analyzed how the expected growth of the population, measured as the mean
fitness of the population in a given year, affected the mean age of the
reproducing parents. We found that when the mean fitness of the
population was low then the individuals that managed to reproduce were
older (Table 4, model 1A). In contrast, when populations were growing
either all individuals, even the young ones, managed to reproduce or
individuals that invest more in current reproduction at the expense of
future reproduction or survival were able to contribute
disproportionately to population growth, as predicted by
density-dependent selection theory (MacArthur & Wilson 1967; Engenet al. 2013; Engen & Sæther 2016). Further analyses showed that
this was not solely the result of the age structure of the population
(Table 4, model 1B). These results are thus consistent with classic
density-dependent selection theory predicting that in scenarios with
stronger competition, the individuals being favored will be the ones
that invest more in future reproduction (e.g. by investing in traits
that increase survival and enable them to reproduce later). But is also
consistent with the idea that ‘high-quality’ individuals are the ones
that manage to grow old and can reproduce when density-dependent
competition is strongest.