Don't stop me now: Managed fence gaps could allow migratory ungulates to track dynamic resources and reduce fence related energy loss

Abstract

In semi-arid environments characterized by erratic rainfall and scattered primary production, migratory movements are a key survival strategy of large herbivores to track resources over vast areas. Veterinary Cordon Fences (VCFs), intended to reduce wildlife-livestock disease transmission, fragment large parts of southern Africa and have limited the movements of large wild mammals for over 60 years. Consequently, wildlife-fence interactions are frequent and often result in perforations of the fence, mainly caused by elephants. Yet, we lack knowledge about at which times fences act as barriers, how fences directly alter the energy expenditure of native herbivores, and what the consequences of impermeability are. We studied 2-year ungulate movements in three common antelopes (springbok, kudu, eland) across a perforated part of Namibia's VCF separating a wildlife reserve and Etosha National Park using GPS telemetry, accelerometer measurements, and satellite imagery. We identified 2905 fence interaction events which we used to evaluate critical times of encounters and direct fence effects on energy expenditure. Using vegetation type-specific greenness dynamics, we quantified what animals gained in terms of high quality food resources from crossing the VCF. Our results show that the perforation of the VCF sustains herbivore-vegetation interactions in the savanna with its scattered resources. Fence permeability led to peaks in crossing numbers during the first flush of woody plants before the rain started. Kudu and eland often showed increased energy expenditure when crossing the fence. Energy expenditure was lowered during the frequent interactions of ungulates standing at the fence. We found no alteration of energy expenditure when springbok immediately found and crossed fence breaches. Our results indicate that constantly open gaps did not affect energy expenditure, while gaps with obstacles increased motion. Closing gaps may have confused ungulates and modified their intended movements. While browsing, sedentary kudu's use of space was less affected by the VCF; migratory, mixed-feeding springbok, and eland benefited from gaps by gaining forage quality and quantity after crossing. This highlights the importance of access to vast areas to allow ungulates to track vital vegetation patches.

Effects of fences and fence gaps on the movement behavior of three southern African antelope species

“…We selected individuals from different dry season groups to cover the movement of many individuals per tracking device. Details on the capturing procedure can be found in Hering et al (2022) (a summary is provided in the Supplementary Material Section 1.1). Animal handling permits were issued by the Namibian National Commission on Research Science and Technology, certificate number RCIV00032018 (authorization numbers: 20190602 and AN20190808).…”

“…All analyses were performed with R (version 4.2.0, R Core Team, 2022) in R-Studio (v. 2022.02.2). We used fence interaction identification and categorization as described in detail in Hering et al (2022) (a summary is provided in the Supplementary Material Section 1.3). Note that we assigned consecutive localization to a fence interaction event when they were not more than 50 m away from a fence line.…”

“…Therefore, improved knowledge of the direct effects of fences and passable fence sections (from here on referred to as "fence gaps") on ungulate movement becomes essential. Fence and fence gap planning, as well as policymaking, require a fundamental understanding of how ungulates interact with fences, how locomotion is affected, and what ultimate consequences emerge, especially concerning the energetic loss faced by the animals (e.g., Hering et al, 2022;Jones et al, 2022) and consequences for the populations (e.g., Van Moorter et al, 2020). One step towards disentangling the complexity of fence effects on wildlife is to evaluate the behavior of ungulates at fences and fence gaps.…”

“…Thus, such gaps provide an ideal setup to study the direct effects of fences and fence gaps on animal movements across spatial and temporal scales. While largescale migratory movements of ungulates triggered by seasonal changes in food availability lead to periodic peaks in wildlifefence interactions (e.g., Xu et al, 2021;Hering et al, 2022), many interactions occur daily when home ranges contain fences (e.g., Laguna et al, 2022). At the home range scale, larger herbivores (>1 kg) are severely affected since many species travel long distances to reach forage and water resources (e.g., Bartlam-Brooks et al, 2011;Naidoo et al, 2022).…”

“…Studies on ungulate-fence interactions have focused on crossing behaviors such as passing under or jumping over (e.g., Palmer et al, 1985;Harrington and Conover, 2006;Burkholder et al, 2018), on time taken to cross (e.g., Segar and Keane, 2020), and on classifying interaction types (e.g., Wilkinson et al, 2021;Xu et al, 2021;Hering et al, 2022). For example, in a camera trap study at Lake Nakuru National Park, Kenya, Wilkinson et al (2021) observed most ungulate-fence interactions during the day and distinguished four behaviors at fences: "cross", "straddle" (individuals on opposite fence side), "grazing/drinking", and "vigilance".…”