Seasonal specialization drives divergent population dynamics in two closely related butterflies

Abstract

Seasons impose different selection pressures on organisms through contrasting environmental conditions. How such seasonal evolutionary conflict is resolved in organisms whose lives span across seasons remains underexplored. Through field experiments, laboratory work, and citizen science data analyses, we investigate this question using two closely related butterflies (Pieris rapae and P. napi). Superficially, the two butterflies appear highly ecologically similar. Yet, the citizen science data reveal that their fitness is partitioned differently across seasons. Pieris rapae have higher population growth during the summer season but lower overwintering success than do P. napi. We show that these differences correspond to the physiology and behavior of the butterflies. Pieris rapae outperform P. napi at high temperatures in several growth season traits, reflected in microclimate choice by ovipositing wild females. Instead, P. rapae have higher winter mortality than do P. napi. We conclude that the difference in population dynamics between the two butterflies is driven by seasonal specialization, manifested as strategies that maximize gains during growth seasons and minimize harm during adverse seasons, respectively.

Flight behaviour diverges more between seasonal forms than between species inPierisbutterflies

“…Seasonal differences in population dynamics driven by climate have also been reported for P. napi and P. rapae (Okamura et al, 2019;von Schmalensee et al, 2023). While P. rapae is more abundant during summer, P. napi has a higher overwintering survival than P. rapae, resulting in higher abundances during the spring (von Schmalensee et al, 2023).…”

“…We caught Pieris napi and P. rapae during the morning (∼8:00 – 11:00) in the surroundings of České Budějovice, Czechia (48°59’36"N 14°26’10"E). Pieris individuals captured between May and mid-June represented the spring form, whereas the ones captured during July and August represented the summer form (von Schmalensee et al, 2023). Flight trajectories were filmed between 10:00 and 16:00 in a tunnel-like cage (6 m × 2.5 m × 2.5 m, length × width × height).…”

“…We caught P. napi and P. rapae in field during the morning (∼8:00 – 11:00) in the surroundings of České Budějovice, Czechia (48°59’28.3“N 14°26’29.6”E). Pieris individuals captured between May and mid-June represented the spring form, whereas the ones captured during July and August represented the summer form (von Schmalensee et al, 2023). A third generation of adults may fly during September and October during warm years (Benes et al 2002), but these individuals have not been included in this study.…”

“…While P. napi usually flies in habitats such as forest edges and feeds on biennial and perennial plants, P. rapae explores temporary and degraded habitats with ephemeral plants (Ohsaki & Sato, 1999), including agricultural landscapes (Ryan et al, 2019). Seasonal differences in population dynamics driven by climate have also been reported for P. napi and P. rapae (Okamura et al, 2019;von Schmalensee et al, 2023). While P. rapae is more abundant during summer, P. napi has a higher overwintering survival than P. rapae, resulting in higher abundances during the spring (von Schmalensee et al, 2023).…”

Predictable local adaptation in butterfly photoperiodism but not thermal performance along a latitudinal cline

“…Still, the ability to track optimal temperatures is itself temperature-dependent as colder temperatures restrain activity, which offers an additional explanation for the low T min in larvae (Mellanby, 1939). Little is known about female microclimate choice in egg-laying butterflies-studies on closely related species may suggest a bet-hedging strategy (Hopper, 1999;Root & Kareiva, 1984)-but recent studies reveal that P. napi females likely choose microclimates that correspond with the thermal niche of the offspring (Vives-Ingla et al, 2023;von Schmalensee et al, 2023).…”