Experimental warming impact on a native butterfly (Pieris canidia), an invasive butterfly (P. rapae), and their nectar plant (Bidens pilosa var. radiata) across altitude?

Pollination is an important ecosystem service. Given that climate warming has reportedly affected the growth and development of pollinators and/ or plants, likely shifting current plant-pollinator interactions (e.g. mismatches) across spatial gradients (e.g. altitude or latitude), this study examines 1) how warming may affect the growth and development of pollinators and nectar plants (flowers), 2) whether warming will affect the interactions between pollinators and nectar plants, 3) whether the effects above, if any, will vary across altitude (spatially) and between native and invasive pollinators, and 4) how plant-pollinator interactions vary temporally across altitude in the field. This study examined a nectar plant and its two pollinators. Bidens pilosa var. radiata, a notorious invasive plant in Taiwan, has become an important nectar resource for many pollinators, including two common butterflies, the native Pieris canidia and invasive P. rapae. To understand warming impact on these butterflies and their nectar plants across altitude, this study included a laboratory experiment and field survey. The laboratory experiment collected the seeds of B. pilosa var. radiata and females of the two Pieris species (F1 for study) from three sites each at medium (~1000 m a.s.l.) and low altitude (~100 m a.s.l.). Species from medium or low altitude were each raised in three growth chambers (control temperature, 3oC, and 6 oC warming); the control temperature for medium and low altitude was set at 16.8 oC and 22.8 oC, respectively, based on field data. The field survey examined the population density of both Pieris and the flower traits of B. pilosa across altitude over months. The results showed that warming impact on pollinators, plants, and their interactions varied with altitude. First, warming affected the native and invasive Pieris butterflies differently across altitude. In specific, warming reduced the adult longevity and forewing length of low-altitude invasive P. rapae, but of medium-altitude native P. canidia. Second, warming affected nectar plants (B. pilosa) differently across altitude. For example, warming facilitated flowering (earlier flowering and more flowers) and reduced flower reflectance in medium-altitude plants, but had no such effects on low-altitude plants. Third, warming also affected pollination activity differently between the two pollinators across altitude. For instance, warming increased the frequency and duration of flower visits by low-altitude P. rapae. In addition, the field survey showed that both Pieris and flower traits varied spatially (altitude) and temporally (months). Low altitude had a higher Pieris density; low-altitude B. pilosa plants (total sugar weight in flowers, flower reflection) and medium-altitude B. pilosa plants (sucrose concentration in flowers) might have a different strength in attracting pollinators. Many of these traits also varied with months. In summary, warming may differently affect the ecosystem service (a function of pollinator longevity and flower visit) provided by the two Pieris pollinators across altitude, and the different response of medium- and low-altitude plants to warming together suggests an upcoming shift in pollinator-plant interactions across altitude under climate warming, potentially shaping local plant-animal communities.