Submitted to: Global Change Biology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/24/2009
Publication Date: 5/28/2009
Publication URL: hdl.handle.net/10113/50073
Citation: Srygley, R.B., Dudley, R., Oliveira, E.G., Aizprua, R., Pelaez, N.Z., Riveros, A.J. 2010. El Niño and Dry Season Rainfall Influence Hostplant Phenology and an Annual Butterfly Migration from Neotropical Wet to Dry Forests. Global Change Biology. 16:936-945. Interpretive Summary: We know very little about the causes of insect outbreaks in the tropics. To understand what features influence the timing and magnitude of insect migrations, we counted the number of butterflies crossing our study site on the Panama Canal each year for 16 years and simultaneously collected data on local rainfall, winds, sunlight, and temperature. The most abundant of the butterflies Aphrissa statira migrated earlier when the wet season began earlier and when the moisture in the soil during the latter part of the dry season was higher. The migration peaked approximately four weeks after wet season began. The butterfly larvae feed on new leaves of two hostplant species, and the timing of the migration was associated with new leaf production on one hostplant but not the other. The number of migrating butterflies was greatest during El Niño events, which are characterized by higher than normal sea surface temperatures in the equatorial region of the Pacific Ocean. El Niño has a drying effect on the Panama Canal region, and both hostplant species responded to El Niño with greater production of new leaves. In contrast, new leaf production and the number of butterflies migrating was lowest in La Niña years when Pacific Ocean surface temperatures are lower than normal. During one exceptional El Niño event, the number of butterflies migrating was well below that predicted, and yet the dry season in Panama was also unusually wet. Thus dry season rainfall, under the influence of both El Niño and local climatic conditions, appears to be a primary driver of larval food production and population outbreaks for this migrating butterfly. Understanding how global climate cycles and local weather influence tropical insect migrations improves our ability to predict the ecological and economic effects of climate change.
Technical Abstract: We censused butterflies flying across the Panama Canal at Barro Colorado Island for 16 years and butterfly hostplants for eight years to address the question: What environmental factors influence the timing and magnitude of migrating Aphrissa statira butterflies? The peak migration date was earlier when the wet season began earlier and when soil moisture content in the dry season preceding the migration was higher. The peak migration date was also positively associated with peak leaf flushing of one hostplant (Callichlamys latifolia) but not another (Xylophragma seemannianum). The quantity of migrants was correlated with the El Niño Southern Oscillation, which influenced April soil moisture on Barro Colorado Island and total rainfall in the dry season. Both hostplant species responded to El Niño with greater leaf flushing, and the number of adults deriving from or laying eggs on those new leaves was greatest during El Niño years. The year 1993 was exceptional in that the number of butterflies migrating was lower than predicted by the El Niño event, yet the dry season was unusually wet for an El Niño year as well. Thus dry season rainfall appears to be a primary driver of larval food production and population outbreaks for A. statira. Understanding how global climate cycles and local weather influence tropical insect migrations improves the predictability of ecological effects of climate change.