As human populations increase worldwide, there are growing efforts to measure the impacts of anthropogenic activity on wildlife. In particular, anthropogenic linear features (ALFs; e.g. roads or fences) are constructed at high densities in regions with otherwise light human footprints [1, 2]. In these regions, ALFs may be the only anthropogenic feature many animals encounter regularly, making it critical to understand how ALFs affect individual space use and, consequently, their fitness. While some animals are attracted to ALFs such as roads because they are associated with specific resources [3] or because they allow for efficient movement over long distances [4, 5], there is consensus in the literature that ALFs have negative effects on many species of wildlife [6–8]. Besides lethal effects associated with, for example, vehicle collisions on roads [1], predation on seismic lines [4], or entanglement in fences [2, 9], ALFs can also exert non-lethal fitness costs on wildlife by hampering an animal’s ability to access resources [2, 9–13] and avoid risk [4, 14–17]. Measuring such non-lethal effects of ALFs, however, is challenging because fitness costs accumulate through time and are difficult to disentangle from other co-occurring causal factors.
Home ranges can be used as proxies for quantifying non-lethal effects of ALFs on wildlife fitness. The home range is the area an animal uses during its normal, daily activities [18] and manifests from the animal balancing risks and rewards while moving in space [19, 20]. Theory establishes a direct link between space-use decisions, home range properties (relative size and shape in geographic space and composition in environmental space), and an individual’s fitness [21–23]; thus, there is strong theoretical support for the use of home range properties as indirect indicators of behavioral optimality and, thus, fitness. Deviations from theoretical optimality in home range properties should correspond to a proportional decrease in fitness and can provide insight into the non-lethal effects of ALFs on wildlife.
Home-range size is a property emerging from balancing the energy required for an animal’s survival and available energy on the landscape [24]. Theoretically, and all else being equal, smaller home ranges arise in habitats with a high resource quantity and quality compared to habitats without [19, 25–28]. If animals behave optimally, home ranges should be as small as possible to encompass the minimum resources the animal needs and the costs associated with obtaining those [19]. Factors that affect an animal’s ability to move through the landscape and encounter resources can force animals away from optimal behaviors and inflate home-range size. Thus, when ALFs decrease habitat quality, animals may be forced to move more and farther to acquire the resources they need, and home-range size increases as a result. ALFs that are semi-permeable may also increase home-range size, as animals would be devoting time and energy attempting to cross the barrier, meaning they need to spend more time and move farther to acquire resources to make up for the energy spent [25]. However, when ALFs fully block movement, they constrain animals within smaller areas and may, conversely, decrease home-range size. Thus, the effects of ALFs on home-range size are context-dependent, and positive/negative relationships between ALF density and home-range size do not automatically indicate positive/negative (or vice versa) effects of ALFs; inferring on ALF effects based on home range properties requires multiple lines of evidence.
Home-range shapes are determined by the distribution of resources across the landscape and the balance of risk and reward. Theory predicts that home ranges would be completely circular and compact in a landscape with a homogeneous distribution of resources [29]. As animals use space within their home range non-randomly, certain parts of the home range are used at a higher intensity than others [30, 31]; a compact home range helps minimize movement costs between high-intensity use areas. Thus, a compact home range indicates a high intake o resources and a low expenditure of energy to access these resources. When the landscape becomes more heterogeneous or more difficult to move through, home-range shapes are expected to expand, elongate, and become less compact in response [29]. Partially or fully impermeable ALFs may force animals to move in an elongated and complex pattern and deviate from the optimal circular home-range shape, which increases movement costs and leads to less efficient access to core areas.
Characteristics of a home range include not only its geometry bu also its environmental composition, which reflects an animal’s habitat selection. Habitat selection is a functional response whereby the strength of selection for (or avoidance of) conditions, resources, and risks depends on their availability on the landscape [32–37]. Generally, animals are expected to avoid ALFs because ALFs fragment habitat and can pose direct mortality risks [1, 8, 11]. However, as ALF density increases, ALF avoidance is expected to decline because animals are unable to avoid features that are pervasive on the landscape [38]. The pervasiveness of ALFs on the landscape likely also impacts how animals select for other environmental characteristics [39]. For instance, according to the risk-disturbance hypothesis, animals may perceive anthropogenic disturbances as risky and react similarly as they would to an actual predator [15]; because animals balance trade-offs of risks and reward, an abundance of risks could compromise an animal’s normal habitat selection behaviors.
Building on the existing theory, this study aims to understand how the shape and size of and the habitat selection within the home range of two sympatric ungulate species change in response to ALF density–specifically roads and fences. We generated hypotheses based on previous literature on ALF effects on wildlife, and we developed predictions based on theoretical expectations of how deviations from behavioral optimality should manifest through home range characteristics. While ALFs have been shown to affect wildlife across taxa, responses to ALFs are taxon- or even species-specific [7]; thus, we developed our hypotheses based on the literature on ungulates. We tested four hypotheses: (H1) roads are permeable barriers that reduce access to resources by fragmenting and degrading habitat [10, 40]. Therefore, we predicted that roads force ungulates to move more and further in search of foraging opportunities, thus we hypothesized home range size to increase in response to increased road density. In contrast, because fences are less permeable and more likely to block movement [13, 41], we predicted home-range size to decrease in response to increasing fence density (Fig. 1a). (H2) Due to the semi-permeable to permeable nature of ALFs, we predicted home ranges to become more complex, elongated, and less compact as ALF density increases (Fig. 1b). (H3) Many animals, and most ungulates, avoid ALFs [10, 11, 13, 42], but may become habituated as they become more common (the ‘relaxed avoidance’ hypothesis, sensu [34, 43]). We predicted that ungulates would avoid ALFs at a low ALF density, but as ALF density increases on the landscape, avoidance behavior would erode because ALFs become so pervasive that they are impossible to avoid (Fig. 1c). (H4) As ALF density increases, the animal’s selection for or avoidance of other habitat characteristics should become compromised. We predicted that selection strength for habitat attributes would approach zero (i.e. no selection nor avoidance) in response to increasing ALF density (Fig. 1d), marking a deviation from natural behavioral patterns. Through the combination of these four hypotheses (multiple lines of evidence), rather than each hypothesis on its own, we will be able to draw conclusions of the impacts of ALFs on ungulate space-use and home-range behavior.