Learning from past designs: improving amphibian fences using an adaptive management approach

Abstract

Fences have been widely used to exclude, manage, or monitor both native and invasive amphibian populations. Given that fences are artificial barriers that impact animal movements within the landscape, it is critical they do not allow for unwanted movement or lead to unintended animal welfare risks. We have carried out a literature review to identify features that have been used for amphibian fences, as well as aspects of fence design, installation, and maintenance that have limited their effectiveness. We also describe our own application of adaptive management to amphibian exclusion fences, in which we detected flaws and improved features, and monitored the effectiveness of these changes. Based on an exploration of the literature and our experiences, we found several key attributes to fences that must be considered when created for amphibians, including height, lip barriers, underground barriers, support frameworks, gates, seams, clearance zones, and moisture refuges. We found that studies commonly do not detail all of these aspects of their fences, and that few openly describe flaws in the design, installation, and subsequent maintenance of their fences. This is potentially concerning because it may limit chances to make improvements to fence designs that are specific for amphibians. We subsequently provide considerations and recommendations for each key fence attribute, along with maintenance and monitoring advice. These take into account intended fence purpose, desired fence permeability, and project constraints for a variety of amphibian types, life histories, and developmental stages. They are intended to be used by managers to assist in designing an effective fence for their target species. Some of our recommendations to reduce animal welfare risks are to minimise the use of: (1) fence materials that could cause abrasion injuries, (2) dry substrates that could lead to desiccation, (3) geofabrics that could lead to entanglement, and (4) fence aprons that animals could easily become trapped under. This is likely to be a valuable guide for practitioners who are required to install amphibian fences and for policy makers who prescribe fences for mitigation. This guide is applicable for projects managing threatened native species, as well as invasive species, such as the cane toad (Rhinella marina).

On the fence: reevaluating the use of temporary amphibian exclusion fencing

“…Land development projects are likely to result in the permanent loss of habitat (Hamer & McDonnell 2008). This loss may be temporary if the site is undergoing remediation and new habitat replaces what was lost (Gould et al 2023a). Irrespectively, the site could possess resources of use for amphibians, even for a short period, prior to project completion.…”

“…If the project site is in an area known to be used by amphibians, there is a chance that individuals become trapped after exclusion fencing is installed (Gould et al 2023a). For example, the instinct of amphibian juveniles is often to disperse away from natal ponds into the terrestrial environment (Semlitsch 2008).…”

“…Fencing may impact populations by interrupting terrestrial dispersal and habitat connectivity (Matos et al 2019), and by isolating or excluding habitat features (Rathbun & Schneider 2001). Fences may also inadvertently cause harm to individuals or entrapment in areas with reduced cover, increasing predation, overheating, and desiccation risk (Boyle et al 2019;Gould et al 2023a). Additionally, fencing is ineffective at preventing unwanted movements if it is not impermeable (Gould et al 2023a).…”

“…Fences may also inadvertently cause harm to individuals or entrapment in areas with reduced cover, increasing predation, overheating, and desiccation risk (Boyle et al 2019;Gould et al 2023a). Additionally, fencing is ineffective at preventing unwanted movements if it is not impermeable (Gould et al 2023a). Yet, few studies report on amphibian ingress or conduct regular monitoring to detect its occurrence (Dodd 1991;Dodd et al 2004;Gould et al 2023a), which further puts into question the use of temporary exclusion fencing despite its recommendation (Calhoun et al 2005).…”

“…However, this may place a strain on smaller, isolated populations, particularly if individuals miss breeding in optimal years or if project delays restrict access over consecutive years (e.g. Gould et al 2023a).…”