Fecundity without nectar is insufficient for the persistence of a blue butterfly.

Organisms with complex life cycles undergo ecological transitions between life stages, often resulting in stage-specific resource use. The relative contribution of each stage-specific resource to vital rates influences population dynamics and subsequently whether habitats can support viable populations. In lepidopterans, survival to reproduction requires sufficient resources for immature life stages, but the extent to which resources for adults are critical to population persistence is variable. We studied Boisduval's blue butterflies (Icaricia icarioides), in a greenhouse experiment, to quantify the effect of the adult diet, nectar, on vital rates. Butterflies fed ad libitum produced 3.4 times more eggs, on average, over their lifetime and lived 6 more days relative to those which only had access to water. We used these experimental data to parameterize a population model to test if vital rates with and without nectar result in viable population growth rates. Despite individual females laying 68 eggs without nectar, we found that Boisduval's blue butterfly populations will not persist without the improved fecundity associated with nectar resources (λ < 1). In this species, although amino acids in the adult diet contributed to various improvements in fecundity, these improvements did not translate to improvements in population growth rates. Incorporating our experimental vital rates into a population model indicates that the relative abundance and quality of nectar can alter at what threshold other resource(s) are limiting the population.