Status and conservation of the grass snake in Jersey

Global biodiversity losses are being driven by anthropogenic pressures; the most pervasive of which is habitat loss resulting in fragmentation and population isolation. These issues are prevalent throughout Europe due to high intensity agriculture and increasing human population densities. Limitations imposed by resources and the secretive lifestyles of many species hinder the ability of conservationists to undertake status assessments and identify conservation actions. This thesis investigates the threats to an isolated population of grass snakes Natrix helvetica on the island of Jersey, providing recommendations for conservation management and recovery, whilst testing the suitability of tools for monitoring cryptic species. Grass snakes were historically widespread throughout Jersey; however, anthropogenic influences have restricted their distribution to the west and southwest. Furthermore, recent monitoring efforts have detected few individuals and their status is unknown.

Intensive surveys to locate individuals combined with occupancy and N-mixture (abundance) models identified continued occupancy of semi-natural sites in the island's west and southwest, but also highlighted poor detectability of the species unless utilising a large survey effort. Therefore, a large amount of effort is required to determine absence of snakes, and declines in the population cannot be detected with reasonable power. Occupancy models were more reliable than N-mixture models, particularly due to the risks of closure violation when estimating abundance. Nonetheless, N-mixture models estimated an abundance of 48 snakes (95% CI: 23?1279) across the study sites. Radio-tracking also provided evidence for low detection rates. Additionally snakes demonstrated small ranges (mean: 2.48 ha ± 3.54 SD), site fidelity, preferences for ranges close to paths and compost heaps, but avoided crossing roads. Snakes were positively associated with structurally complex habitats including rough grassland, dense scrub and gorse at multiple spatial scales, but negatively with open and wooded habitats.

Species distribution modelling indicated similar habitat preferences to radio-tracking and poor suitability of agricultural habitats. Areas close to amphibian prey populations were also suitable whereas those with high road densities were not. A fifth of Jersey contained priority conservation areas, however almost 90% of these areas do not receive statutory protection. Those in the west and southwest should be prioritised for protection due to their proximity to extant subpopulations. Mitochondrial genes identified the population to belong to a western lineage of grass snakes Natrix helvetica helvetica, with a probable natural colonisation prior to separation from northwest France. Within Jersey, microsatellite markers identified three subpopulations, with significant differentiation between snakes in the south and west. This coincides with a dense urban area, through which connectivity needs improvement.

The Jersey grass snake population can be classified as regionally Vulnerable (D2) under IUCN guidelines. The study illustrates how nature reserves are important for maintaining isolated subpopulations and the potential avenues by which statutory protection, sympathetic management practices and efforts to improve inter-reserve connectivity can contribute to conservation objectives.