Natural history of Odonata assemblages in tropical streams in Puerto Rico

Abstract

Freshwater macroinvertebrates play an important role in maintaining stream food webs. Odonata (dragonflies and damselflies) are important top predators in these communities and serve as indicators of stream health. Our understanding of odonate assemblages is limited in the Caribbean and the natural history of most odonate species in the region remains unknown. The focus of this research is to study the natural history of odonate species in headwater montane streams following major hurricane impacts in Puerto Rico. We monitored assemblages from August 2018 to July 2019 in two headwater streams within El Yunque National Forest, Puerto Rico. The study streams drain a protected forest, with aseasonal precipitation patterns, relatively constant water temperature, and flashy hydrographs that quickly respond to rain events. We sampled 226 adults and 550 larvae, dominated by three Caribbean endemics: Scapanea frontalis, Macrothemis celeno, and Telebasis vulnerata. Only S. frontalis and M. celeno were abundant enough to assess the temporal patterns and their natural history. Larval density fluctuated throughout the year with short peaks in abundance during different times of the year, according to the species. Small individuals (≤10 mm body length) were more abundant than the large ones. However, all size classes were present during the year. The dominant species, S. frontalis and M. celeno, had continuous development patterns, without identifiable size classes and multiple overlapping generations. The exception was the last stadium that formed a separate group in the body length vs head width plots. Species had clear habitat preferences; S. frontalis was abundant in riffles and preferred areas with high amounts of cobble. Macrothemis celeno prefers pool habitats with fine substrates. While we found trends for negative relations between abundance and discharge, canopy cover, water temperature, and rainfall, none was statistically significant. Observed patterns suggest a lack of strong temporal seasonality in the natural history of Odonata, which coincides with the aseasonal environment of streams draining our study area. Overall, our study is the first to assess temporal variability of Odonata assemblages in montane streams of Puerto Rico and provides information on Caribbean endemic species.