Factors influening basking site selection and basking behaviour of freshwater turtles in a disturbed landscape

Abstract

Ectothermic animals such as Freshwater turtles rely on behavioural thermoregulation- basking, to maintain proper body temperature. It is the time of the day when turtles are exposed to predators and disturbance. Anthropogenic disturbance might hamper the basking behaviour resulting in a cascading consequence affecting body temperature, metabolism, locomotion and ultimately survival. The Chambal River is home to a great diversity of freshwater reptiles while anthropogenic disturbance is very high along the river primarily due to sand mining, illegal fishing, agriculture, livestock grazing and presence of humans. Boat-based surveys with habitat characterization at macro and micro scale were performed to understand the impact of anthropogenic disturbance to basking site selection for turtles. In macro scale, Nilssonia and Batagur showed strong avoidance against highly disturbed stretches. The effect was buffered in the presence of mid-river basking sites. At micro scale, Nilssonia basking site selection was driven by type of Site (shoreline vs Mid river sites) and distance from waterline, while disturbance was not a predictor in the top model. Behavioural observations were done from morning till evening. Basking duration for identified Nilssonia individuals were seen to decrease by 1/3rd under high disturbance, while basking bouts were also shorter. Nilssonia individuals spend more time basking on mid-river sites away from the disturbed shoreline, while the basking bouts were significantly longer on sandbars as compared to disturbed sandbanks. Thus, mid river sites buffer the negative effect of disturbance both in site selection and behavioural aspects. Since mid-river basking sites have limited area, crowding on such sites result in increased negative social interactions. The impact of disturbance was highest on juvenile Nilssonia as they were repeatedly removed from mid river basking sites by larger Nilssonia. Thus, juveniles basked for significantly lesser duration during high disturbance phases as they were unable to utilize mid river basking sites. After the removal of disturbance, it was observed that basking bouts were equally long on sandbank and sandbars and time spent on mid river basking sites were also lowered. This suggests that usage of mid-river sites cannot be simply attributed to thermal properties of the site, but is also, at least partially driven by disturbance at banks. Based on individual ID, the study also revealed how disturbance may impact different individuals in different manners. Thus, mid river basking sites are critical habitat features that allow turtles to bask away from the disturbed shoreline under high disturbance. Even though they buffer the effect of disturbance, it cannot completely remove the negative impact of disturbance. Thus, regulation or removal of disturbance should be first priority for conservation of these species, followed by protection of these critical habitats (primarily from sand mining). The study also suggests to focus on multi-species, multi-scale response to understand how disturbance might be affecting various aspects of its habitat usage and behaviour, along with focusing on individual level responses to develop effective conservation strategies.