Spatial Biology of Northern Watersnakes (Nerodia sipedon) Living along an Urban Stream
Fifty Nerodia sipedon living along two kilometers of an urban stream in northeastern Pennsylvania were radio-tracked over three activity seasons, yielding more than 2520 relocations. Half the stream length is urbanized, flowing through a city park at the head of the study area and an industrial area at the downstream end; the half between is relatively natural, flowing through a conservation corridor. Individual snakes exhibited high site fidelity. For 82% of relocations, snakes were within a one-meter radius of places they had previously occupied, most often using exactly the same hole, rock, branch, or cover object. Snakes were found to have moved to a different site at 27% of relocations. When snakes moved, they returned to previously occupied places 56% of the time. Females exhibited significantly greater site fidelity than males, and snakes occupying the urban half of the study site exhibited significantly greater site fidelity than snakes found in the natural half. Individuals varied greatly in the amount of space they utilized. Snakes had a mean minimum convex polygon (MCP) activity area of 1.13 ha. Snakes appeared to use the stream to travel between locations separated by more than 100 meters. Original MCPs overestimated space use by including large terrestrial areas that were never occupied by and were unsuitable to snakes. Fixed kernel methods underestimated space use by producing multiple small, disjunct contours. Both methods often largely or completely excluded the stream. To address these problems, we provide additional estimates of space use that sum the MCPs encompassing all of a snake's locations within 100 meters of each other and add the area of stream connecting the most upstream and downstream locations.Abstract
Aerial photograph of the study site with urban and natural areas labeled. The most frequently used hibernaculum is indicated with a white asterisk. Approximate boundaries of the urban and natural areas are indicated with black lines across the stream. White bar = 100 m.
Location points (white dots) and the minimum convex polygon boundaries (black lines) for snake 48 in A, snake 47 in B, and snake 8 in C. The MCPs that connect distant terrestrial locations almost completely exclude the stream, and in A and B the majority of the MCP is more than twice as far away from water as the furthest observed snake location. White bar = 100 m in A and B and 50 m in C.
Location points and home range boundaries determined by various methods for snake 25. The minimum convex polygon is shown in A. In B the 50% and 95% fixed kernels using LSCV are very small and are indicated by solid white lines, the 95% kernel using the reference smoothing parameter is indicated by a small dash line, and the 95% fixed kernel using LSCV but only including each location once is indicated with a large dash line (largest). The home range boundary determined by our corridor method is shown in C. White bar = 100 m.
Contributor Notes
Associate Editor: M. J. Lannoo.