On the diversity and abundance of riparian odonate fauna ( Insecta ) of the midstream Chalakkudy River , Kerala , India

The riparian Odonate insect diversity of the midstream Chalakudy River at six locations assessed from February 2018 to January 2019 has revealed the occurrence of 25 species of odonates. Among them,10 species are dragonflies belonging to seven genera of the family Libellulidae and the remaining 15 species are damselflies belonging to six families and 11 genera. Five endemic damselfly species have been recorded. Pseudagrion indicum is endemic to the Western Ghats, while the remaining four species, Vestalis apicalis, Libellago indica, Dysphaea ethela, and Heliocypha bisignata, are endemic to India. Diversity indices of the odonates in all the six locations were analyzed and it showed less abundance at sites where tourist activities are more and with thin native riparian vegetation. Further, the study has unequivocally revealed that thick native riparian vegetation is essential for their perching and existence. By and large, the uncontrolled tourism activities and habitat alteration interfere with the density and diversity of these endemic species.


INTRODUCTION
Kerala has a comprehensively documented odonate fauna. The relevant works among them include that of Rao & Lahiri (1982), Mathavan & Miller (1989), Radhakrishnan (1997), Emiliyamma & Radhakrishnan (2002), Emiliyamma (2005), Palot et al. (2005), Adarsh et al. (2014), Varghese et al. (2014), Nair (2017), and Susanth & Anooj (2020). Recent works further added up the rich odonate diversity of Kerala to 174 species (Emiliyamma et al. 2020;Joshi et al. 2020). The seasonal and habitat distribution of Odonata diversity of riparian habitats such as Mula and Mutha river basins in Maharashtra was studied by Kulkarni & Subramanian (2013). Species turn over and abundance of the odonates of riparian zones depends on season and land use types. Endemics and habitat specialists are restricted to undisturbed riverine ecosystems as they possess a narrow range of habitat tolerance. Conservation of riparian zone results in the conservation of endemics of odonates (Subramanian 2007;Subramanian et al. 2008). The present study investigated the odonate diversity and abundance of midstream Chalakkudy river giving special reference to endemics.

METHODS
The survey was conducted once a month from February 2018 to January 2019 by conventional random sampling. Six locations of midstream Chalakkudy River were randomly selected for the observation of odonates. The river is 13.5 km (approximately) long from the first location to last one (Bachan 2003). The details of the study localities are given in Table 1. All the six locations are with rocky river bed and evergreen and semi evergreen forest vegetation. Madhuca neriifolia, Syzigium occidentale, Humboldtia vahliana, Elaeocarpus, and Homonoia riparia are the dominant species of flowering plants in these locations (Bachan 2010). The selected locations have been confronted with anthropogenic disturbances such as habitat alteration due to tourism activities including resorts & commercial establishments, oil palm plantations, and activities of local people. The odonates were documented and identified with the help of photographs, keys, and descriptions given in the literature (Fraser 1933(Fraser , 1934(Fraser , 1936Kiran & Raju 2013). The species richness and abundance were recorded and Simpson & Shannon diversity indices and eveness values were calculated using PAST software. The observed species of odonates were categorized as VC-Very common (180-240 sightings), CO-Common (120-180 sightings), OC-Occasional (60-120 sightings), and RA-Rare (1-60 sightings)) depending upon their occurrence during the survey (Palot et al. 2005;Tiple et al. 2012).

RESULTS
During the study period, 2,186 individuals of 25 species were observed. Out of these, 10 species were dragonflies of the suborder Anisoptera, belonging to seven genera and the family Libellulidae. The remaining 15 species were damselflies under the suborder Zygoptera and they come under 11 genera in six families (Tables 1, 2). Libellulidae is the only anisopteran family, which has been observed among the odonates in the present survey. Orthetrum sabina a well-known cannibalistic dragonfly, has been found to be very common. On the other hand, Onychothemis testacea was encountered very rarely during the present survey. Members of the family Coenagrionidae (6 species) were dominating in the order Zygoptera succeeded by Calopterigidae (3 species) and Platycnemididae (3 species). Vestalis apicalis and Prodasineura verticalis were common but Aciagrion occidentale was observed only sporadically in this region. Out of the 25 species recorded, five species are endemics and they belong to the suborder Zygoptera. But Pseudagrion indicum is endemic to Western Ghats, while Vestalis apicalis is endemic to southern and central India, Libellago indica is endemic to peninsular India, whereas Dysphaea ethela and Heliocypha bisignata are endemic to India (Kalkman et al. 2020). The most dominant endemic species observed in the present survey was Dysphaea ethela and Heliocypha bisignata, which exhibited a minimum level of occurrence. The percentage distribution of each endemic species is as follows: Pseudagrion indicum 9%, Vestalis apicalis 26%, Libellago indica 28%, Dysphaea ethela 34%, and Heliocypha bisignata 3%. The first location Ezhattumugham (L1) harboured as many as 536

Effect of flood
During the month of August of the study period, heavy down pour at Kerala led to a deluge and it badly affected the study areas. Riparian vegetation was totally destroyed. Natural soil texture was lost, soil accumulation could be found in river and river banks. As a consequence, a sudden drop in damselfly diversity

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was noticed just after the flood. Only two species of damselflies were recorded in the first two months after the flood, i.e., September and October 2018. But dragonfly diversity was not much affected. In the succeeding months the species richness and abundance were observed to have rebounded.
Simpson & Shannon diversity indices and evenness values of the six locations were calculated (Table 4). Maximum species richness and abundance were found at Location 1. Simpson and Shannon diversity indices (0.9197 and 2.628, respectively) were found to be equally high for location 2, while the least values were shown by Location 6 (0.8694 and 2.191, respectively). Maximum value of evenness (0.8257) was recorded at Location 3 and a minimum at Location 1.

DISCUSSION
The current study points out the role of native riparian vegetation and the impact of human interference such as habitat alteration by tourism, construction works and plantations on the density and diversity of odonate fauna. Studies revealed that riparian vegetation promotes the occurrence of invertebrates including insects and facilitates suitable habitat for insects by providing food, resting and hiding places for emergent adults and substratum for egg laying. Also the shade cover regulates water temperature and overall quality of the stream (Knight & Bottorff 1981;Ober & Hayes 2008). Moreover, the prey insects are attracted by flowering plants, which in turn form ideal food for odonates. Therefore, these conditions become more pertinent for the carnivorous odonates. The hanging plants and emergent macrophytes furnish perching sites and structures for egg laying and emergence of adults. Literature delineates the role of macrophytes and shoreline structures in oviposition, formation of larval microhabitat, emergence support and adult perching site (Samways & Steytler 1996;Schindler et al. 2003).
In the present study 15 species of damselflies and 10 dragonflies were recorded. As the damselflies are weak fliers, they may depend on their own microhabitat for food and reproduction. But the agile fliers, dragonflies are free to move to more extensive habitats according to their preferences. This is a factor of variation in species richness between the two suborders. The most commonly encountered dragonfly was Orthetrum  This has led to the least diversity indices on species richness and abundance in L6. Another observation noticed in the present study was on the high abundance of endemic species in L1 and minimum distribution at L6. Dysphaea ethela and Heliocypha bisignata were reported to be respectively the common and rarely occurring endemic species. As per the literature, undisturbed riparian forests are typically rich with the presence of endemics (Subramanian et al. 2008). Destruction of riparian flora and fauna could be attributed to damming, tourists activities, construction works and expanding the area for agricultural plantations leading to the declined number of species. For instance, it is evident that the fish fauna of Chalakkudy river is highly threatened by damming, deforestation and pesticide pollution (Raghavan et al. 2008). Habitat alteration interferes with the abundance of endemic odonates and supports the occurrence of generalist species like libellulids (Kalkman et al. 2008;Subramanian et al. 2008), and that is evident in the present study. Research work delineates the resilience capacity of organisms to flood (Death 2008; Golab & Sniegula 2012; Raghavan 2019). Inspite of the destructive flood during the current study, odonates showed a tendency to bounce back to pre-flood conditions within a very short time. Further studies are required to authenticate the same.