Conceptually the inverse of wildlife corridors, fences aim to disconnect. They are built to separate people across national borders, livestock from predators, to delineate property lines, and even to protect wildlife conservation reserves. Globally, fences are ubiquitous, more prevalent even than roads, and proliferating. Yet their ecological impact is relatively unstudied.
Fences are often framed as a management tool rather than a globally significant ecological feature, and they are a notable omission from efforts to map global infrastructure, including the human footprint [McInturff 2020: 971].
This analysis reviews 446 studies starting from 1948 on various types of fencing to assess impacts; however, most of the studies focus on the effect of fencing on particular species (specifically, those the fencing is meant to protect), rather than on multiple species, communities or ecosystems.
Conservation and restoration fences, for example, have support within the literature for their beneficial effects for wildlife and sensitive plant species for which they are built, making such species “winners” in the fencing game. On the other hand, there is a critical lack of information on species that are not the targets for which fences are built, and our review shows that only 10.8% (48 of 446) of studies focus on nontarget species [McInturff 2020: 975].
While fences aiming to protect particular species usually achieve that goal, they inevitably hurt other species.
… often the clearest winners because of fencing are the species that humans value most, whereas losers are inevitable but may remain invisible [McInturff 2020: 975].
Broadly speaking, fences favor generalists and disturbance specialists, many of which are invasive, as well as small and small-ranged, nonmigratory species. Fences therefore heavily restrict what makes a species a winner [McInturff 2020: 975].
The deleterious effects of fences include: impeding migration, reducing gene flow between populations, restructuring community composition and obstructing interspecies interactions, such as between predators and prey. These community-level changes can have ripple effects in the ecosystem. For example, livestock fences effectively excluding dingoes in Australia led to this large predator’s eradication. “Without dingoes, researchers have tracked a continental-scale mesopredator [mid-level predator] release that has altered biodiversity and habitats over enormous areas of Australia” [McInturff 2020: 979].
While fences limit certain interspecies interactions, they concentrate others:
Even where conservation or restoration fences seemingly protect whole habitats, research still points to differential outcomes for constituent species. In addition, pathogens and parasites may spread more rapidly where species interactions are concentrated within reserves. In central Kenya, for example, smaller fenced reserves produced higher gastrointestinal parasite infection rates among impala [McInturff 2020: 977].
The authors recommend a greater research focus on the cumulative ecological effects of fencing, policy that limits fence building and encourages fence removal or fence design that is more wildlife-friendly. They caution that fencing is among the major drivers of anthropogenic change.
As fencing continues to rapidly proliferate, there is potential for a dangerous future in which fences simultaneously alter ecological processes at multiple scales, likely producing more losers than winners, and potentially resulting in ecosystem state shift or collapse [McInturff 2020: 977].
Livestock fences effectively excluding dingoes in Australia led to this large predator’s eradication. “Without dingoes, researchers have tracked a continental-scale mesopredator [a mid-level predator] release that has altered biodiversity and habitats over enormous areas of Australia” [McInturff 2020: 979].