Abstract
Establishing a direct link between climate change and fluctuations in animal populations through long-term monitoring is difficult given the paucity of baseline data. We hypothesized that social wasps are sensitive to climatic variations, and thus studied the impact of ENSO events on social wasp populations in French Guiana. We noted that during the 2000 La Niña year there was a 77.1% decrease in their nest abundance along ca. 5 km of forest edges, and that 70.5% of the species were no longer present. Two simultaneous 13-year surveys (1997–2009) confirmed the decrease in social wasps during La Niña years (2000 and 2006), while an increase occurred during the 2009 El Niño year. A 30-year weather survey showed that these phenomena corresponded to particularly high levels of rainfall, and that temperature, humidity and global solar radiation were correlated with rainfall. Using the Self-Organizing Map algorithm, we show that heavy rainfall during an entire rainy season has a negative impact on social wasps. Strong contrasts in rainfall between the dry season and the short rainy season exacerbate this effect. Social wasp populations never recovered to their pre-2000 levels. This is probably because these conditions occurred over four years; heavy rainfall during the major rainy seasons during four other years also had a detrimental effect. On the contrary, low levels of rainfall during the major rainy season in 2009 spurred an increase in social wasp populations. We conclude that recent climatic changes have likely resulted in fewer social wasp colonies because they have lowered the wasps' resistance to parasitoids and pathogens. These results imply that Neotropical social wasps can be regarded as bio-indicators because they highlight the impact of climatic changes not yet perceptible in plants and other animals.
Highlights
The impact of global warming has become obvious in high latitude regions where melting ice and softening tundra are causing profound changes
Unspoiled tropical rainforests were formerly thought to be essentially unaffected by global warming due to lower increases in temperatures compared to forests at higher latitudes [6]
Elie plantation before and after the 1997-2000 El Nino Southern Oscillation (ENSO) event, we noted a significant decrease in the number of social wasp nests recorded between 1992 and 2006, 2007 and 2010 on the grapefruit trees, palm trees and forest edges lining the plantation, while the difference was not significant between 2006, 2007 and 2010 (Table 1)
Summary
The impact of global warming has become obvious in high latitude regions where melting ice and softening tundra are causing profound changes. Species only survive if their niches are wide enough to overcome the changes related to these new climatic conditions. Niche conservatism with respect to climatic factors exists, so that among closely-related species some are resistant to moving into novel climatic environments [3,4]. In tropical forests, the major threats to biodiversity have mostly been attributed to deforestation and associated events such as fragmentation, habitat loss, fires and local climate change [5]. Unspoiled tropical rainforests were formerly thought to be essentially unaffected by global warming due to lower increases in temperatures compared to forests at higher latitudes [6]
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