El Niño events, host plant growth, and migratory butterfly abundance in a changing climate

Background/Question/Methods

Range and phenological shifts of butterflies are models for the effects of environmental change on insect populations generally.  A major gap lies in our knowledge of climatic effects on tropical insect populations. Tropical forests are experiencing a pantropical increase in ambient temperature, and in many regions, the severity and duration of seasonal drought has also increased. Previously we showed that greater leaf productivity by larval host lianas during El Niño in Panama is associated with greater abundance of migrating adult Aphrissa statira butterflies (Pieridae), with the greatest larval damage to the hostplants occurring during the first four weeks of the wet season. Common to many deserts, savannahs, and tropical wet forests, this pattern of increased primary productivity resulting in insect outbreaks and migration during El Niño years is called the El Niño Migration Syndrome. El Niño causes prolonged drought in many tropical rainforests, and climate change is projected to have a similar effect. To investigate the generality of the El Niño Migration Syndrome in a Neotropical wet forest, we quantified the abundance of migrating Marpesia chiron butterflies (Nymphalidae) over 17 years and the production of new leaves of their hostplants over 9 years on the isthmus of Panama.  We asked whether increased abundance of migrating insects and productivity of their food plants were associated with El Niño and La Niña events?

Results/Conclusions

We found that the quantity of M. chiron migrating across the Panama Canal was directly proportional to the sea surface temperature (SST) anomaly of the Pacific Ocean, which characterizes El Niño and La Niña events.  We also found that the production of new leaves by its larval host trees, namely Brosimum alicastrum, Artocarpus altilis, and Ficus citrifolia, was directly proportional to the SST anomaly, with greater leaf flushing occurring during the period of the annual butterfly migration during El Niño events.  Combining these and our previously published results for the migratory butterfly Aphrissa statira and its host lianas, we conclude that dry season rainfall and photosynthetically active radiation can serve as primary drivers of larval food production and insect population outbreaks in Neotropical wet forests, with drier years resulting in enhanced plant productivity and herbivore abundance. Insect populations should closely track changes in both frequency and amplitude of the El Niño Southern Oscillation with climate change.