Parental care amplifies effects of altered brood size on offspring production

Recruitment is usually negatively density-dependent in relation to number
of offspring produced. However, parental care can alter the pattern, as
behaviours that maximise individual fitness are not necessarily beneficial
at the population level. We show that in the threespine stickleback, the
quality of male parental care is positively density-dependent in relation
to number of eggs within a nest, which reverses negative density-dependent
egg survival. This is because males invest more in larger broods, while
favouring future reproductive opportunities when broods are small. Thus,
positively density-dependent parental investment amplifies changes in
offspring production when males receive more or less eggs than expected.
Notable, parental care investment is mal-adaptive at the individual level
when the number of eggs received decreases. Given that female fecundity is
changing in many environments because of human disturbances, the results
indicate that parental care can contribute to alter population dynamics.
At a broader level, the study exemplifies how behaviours that have evolved
to maximise individual fitness under pristine condition can become
mal-adaptive under disturbed conditions and influence both individual
fitness and population processes. Considering that human activities are
rapidly transforming environments, such mal-adaptive behavioural responses
could be common and magnify negative effects of human activities on
populations.