Climate change impact on Amazonian ant gardens

Abstract

In Amazonia higher Atlantic sea surface temperatures, greenhouse gasses, deforestation and El Niño events result in the greater frequency of severe droughts, although total rainfall has increased due to wetter rainy seasons, something confirmed in French Guiana from available climatic data (1980–2017). Aiming to study the impact of rainfall on ant gardens (i.e., arboreal ant-epiphyte mutualisms that depend on the atmosphere for water; AGs) initiated by the ponerine ant Neoponera goeldii, we conducted surveys around the Petit Saut and Régina areas (mean annual rainfall: ≈3,000 mm and ≈4,000 mm, respectively). Each year, near the end of the dry season we recorded the number of these AGs in 10 × 5 m sections parallel to the roadsides. The Petit Saut survey (1993–2017) revealed that AG density along roadsides varied only slightly in "wet zones" situated along ditches, whereas in "dry zones" where the soil seasonally dries out it dropped sharply during the drastic 1997 dry season. Then, this density, low due to recurrent droughts, dropped again during the drastic successive 2015–2016 dry seasons. In the Régina survey (2006–2017), we had the opportunity to follow the establishment of AGs in a "dry zone". It was represented by a typical sigmoidal curve and then it stabilized with AG densities higher than at its peak in 1996 in dry zones of Petit Saut, showing the importance of rainfall. Here, too, the drastic 2016 dry season adversely affected the AGs. Finally, the epiphytic composition of the AGs was mainly represented by Aechmea mertensii (a tank bromeliad), Anthurium gracile (Araceae) and Codonanthe crassifolia (Gesneriaceae), but AGs with the tank bromeliad are more resistant to droughts. These AGs are at risk in dry zones if drastic successive dry seasons occur in the future as global warming intensifies while those developing in riparian areas might survive.