Effectiveness of herbivore exclusion fences for plant conservation depends on management strategy and landscape context

Abstract

Plant conservation fences are widely used to restrict or minimize grazing pressure on endangered plant species or communities, and to ultimately ensure their viability. Despite the wide range of studies on how fences restrict ungulate herbivory and promote plant protection and conservation, information about what factors condition their effectiveness at a regional level is still lacking. For this reason, we analyzed 90 fences located in a biodiversity hotspot (Southern Spain). We used a model selection approach to evaluate the relative importance of management strategy, landscape context and fence shape for determining ungulate detection within exclosures and discussing its ecological and conservation implications. The level of structural damage (fence permeability) was the main factor to explain ungulate detection within fences. As expected, intact or closed fences were able to exclude large herbivores. Interestingly, semipermeable fences (i.e., with an intermediate level of structural damage) also showed a significant reduction in ungulate crossings (approximately 60%). The fences located in higher altitude landscapes and with high proportions of grassland cover showed more frequent ungulate crossings compared to the lower altitude areas surrounded by pine and Quercus spp. formations. Ungulate detection within fences was less frequent when fences were smaller and had a more complex shape. According to our results, fence networks may benefit from including both nonpermeable and semipermeable fences, while prioritizing complex perimeters that adjust to the boundaries of vegetation patches with threatened plant populations and communities. Ungulate fence crossings varied along an altitudinal gradient and vegetation type, indicating that landscape context can also determine the effectiveness of herbivore exclusion fences. We conclude that integrating fence design, management, and landscape context can improve regional-scale implementation of exclosure networks to better safeguard threatened flora.