Moth assemblage response to hurricane disturbance in a tropical rainforest

Aura M. Alonso-Rodríguez, Rubenstein School of Environment and Natural Resources, University of Vermont, Burlington, VT and Pablo E. Gutiérrez-Fonseca, Department of Biology, University of Costa Rica, San José, Costa Rica

Background/Question/Methods
Cyclonic storms dominate the disturbance regime in many tropical forests, yet their potential impacts are still unknown for many taxa. Hurricanes Irma and María (both category 4) struck the island of Puerto Rico in September 2017, causing an estimated mortality and/or severe damage of 23-31 million trees across the island. This habitat transformation may have severe consequences for insects such as moths (nocturnal Lepidoptera), which generally depend on host-plant availability for their survival. In this study, we assessed hurricane impacts on moth assemblages in two of the dominant forest types of the Luquillo Mountains, Tabonuco (Dacroydes excelsa) and Sierra Palm (Prestoea montana) forests. Both species have adapted to withstand hurricane winds in different ways: Tabonuco trees drop their leaves and intertwine their roots to create an underground network that serves as an anchor, while Sierra Palms have thinner trunks that can bend and flex in the wind. Moths were collected monthly in 6 sites (3 per forest type) from Apr-Aug 2017 (pre-hurricanes) and Oct 2017-Mar 2018 (post-hurricanes). In order to maximize the chances of collecting a representative sample of the assemblage, we restricted sampling to new moon nights and used automated light traps operated during the entire night.
Results/Conclusions
All moths of >1cm wingspan were processed and separated by morphospecies, which resulted in 5985 individuals distributed in 233 morphospecies. Moth richness and abundance was similar in both forest types before the hurricanes, but was significantly higher in Tabonuco forests when compared to Sierra Palm forests after disturbance. We also found a complete shift in moth species composition and a significant increase in species dominance after hurricane disturbance, regardless of forest type. This was mainly driven by increased abundance (and a few outbreak events) of several species from the Pyraloidea superfamily. Our results seem to indicate that, previous to disturbance, Tabonuco and Sierra Palm forests provide similar habitat for moth assemblages in the Luquillo Mountains. However, after the onslaught of such extreme climatic events, Tabonuco trees may play a key role in providing shelter for moths and potentially other invertebrates in an otherwise decimated forest. As hurricane frequency and intensity continues to increase due to climate change, these remaining old-growth forests may be vital for conserving biodiversity. Furthermore, with so much recent attention on the global decline of insect populations, understanding how this may be driven by climatic phenomena becomes increasingly important, especially in the tropics.