Population structure and dynamics of Nerothemis tulla (Odonata: Libellulidae) with water availability in rice field ecosystem

Abstract

Dragonflies are important insects in rice field ecosystem (RFE) as pest predators. Rice fields in return serve as a good breeding and feeding ground for dragonflies though RFE undergo several changes during a crop cycle. However habitat conditions vary mainly on the climatic regions and farming practices. The selected RFE in wet zone lowland for sampling comprised deferent habitats, two shallow streams, non cultivated fields, cultivated fields, small water pools, ponds and water wells. The most common dragonfly in marshes and paddy fields, Nerothemis tullia (Pied Parasol) was studied to understand its population structure and dynamics with the water availability.

Sampling was conducted from May 2010 to November 2010 covering one cultivation cycle. Two sampling methods i.e., random sampling along 5m X 50m transects and systematic sampling in pre-identified colonies were used. Colonies were identified in advanced, numbered consecutively and every third colony was observed. Nerothemis tulla adult male, adult female, juvenile male and terneral female were distinguish by general mophological characters and recorded separately. Activeness of Nerothemis tulla individuals were recorded as number of catches (of prey) or tries for per 10 minutes. Other associated dragonflies and damselflies were also recorded. Water level and flow rate in streams were measured as indicators of water availability in RFE.

Systematic sampling recorded higher density (2.50 individuals/m²) while random sampling produced lesser density (0.19 individuals/m²). However random sampling method recorded higher diversity of Odonata in RFE.  Number of individuals were positively correlated (r =7.68) with water level. Flow rate had no significance relationship with individual density.  Activity of colony peaks between 9.00 hr to 11.00 hr and minimum around 13.00 hr. Second peak occurred around 16.00 hr - 17.00 hr. Number of both male and female juveniles sharply reduced towards the drier months whilst number of both adult male and female increased. During the dry period in September, density reduced to minimum and juveniles became scares. In November with the beginning of rain, number and activity of Nerothemis tulla increased and more colonies observed at water holes and in field proper of paddy fields. Sampling along transects can be recommended for study of odonata diversity whilst systematic sampling is more suitable for study of population dynamic and structure. The study is being continued for second crop cycle.

Though common species are neglected in conservation practices, consequences of sudden decline in common species that occur in higher number, such as Nerothemis tulla is unpredictable. This study implies the necessity of their conservation as they have complex ecosystem interactions and intense ecosystem service.