Fences Can Cause 'Ecological Meltdown,' Study Finds

While some species may benefit from fences, they still create a cascading impact on ecosystems.

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Fence On Field Against Sky During Sunset
There may be more fences than roads on Earth. Mostefaoui Sofiane / EyeEm / Getty Images

Fences don’t always make great neighbors. The combined length of fences on our planet may be greater than the global distance of roads, according to researchers who have released a report on these popular barriers. They say that fences are difficult to study but their impact can be harmful on ecosystems.

In their report in BioScience, scientists reviewed existing fence research and offered suggestions for future studies. The team reviewed 446 studies published from 1948 to 2018 and found that fences have measurable effects at every ecological scale, with both winners and losers. In fact, the same fence can be both beneficial and detrimental. For example, conservation fences in Africa may protect vulnerable species from poaching, but they may also prohibit those same animals from reaching watering holes that they need to survive.

Lead author Alex McInturff started his PhD working at a research site in Kenya where he saw conservation fences in place, but also the disastrous impacts of large veterinary fences on wildebeest migrations. He worked near large experimental fences that let animals of different sizes into different areas, but was surprised that none of the research ever studied how the fences themselves changed animal behavior.

Later, when McInturff moved to California, he noticed how black-tailed deer would make long detours around fences rather than jump over them. Field cameras showed how predators would use fences as “predator highways” to trap prey. Curious about how the effects of fences could cascade through entire ecosystems, he launched a systematic literature review on every paper on fence research.

McInturff, who was at the University of California, Berkeley, at the time of this research, talked to Treehugger about the impact of fences.

Treehugger: The research notes that fences are very difficult to study. Why is that?

Alex McInturff: If stretched end to end, the world's fences would likely stretch from Earth to the sun and back multiple times. They're so ubiquitous that they're easy to overlook...

Even though there has been research on fencing, our review connected the dots between a large number of diverse and siloed research projects. Taken individually, most of these studies tell us something very specific about a particular species, ecosystem, or fence type. However, taken together, we were able to discover broad, surprising, and frankly frightening consequences of an enormous global fence network.

With all that said, there are aspects of fences that are quite difficult to study, and this is reflected in sweeping trends in the literature. Most of the literature focuses on animal movement and examines processes one species at a time at relatively small scales. Studies of more complex ecological processes that include multiple species and large areas are rarer and harder to do, but our study suggests that this kind of research is sorely needed.

[One key difficulty for researchers studying fences, McInturff tells Treehugger, is that it's hard to accurately map them. Roads can be mapped using technology such as satellite imagery but fences are difficult to see from space. So there isn't much accurate data about fence locations throughout the world.]

Where people have tried to map them, they've found that their length can outstretch roads by an order of magnitude. We made a very conservative map predicting where fences occur in the western United States, and our results showed that a number of areas thought to be remote and unaffected by human activities and development are densely fenced, and likely undergoing ecological change as a result.

What ecological consequences can fences have?

Our review uncovered a huge range of ecological impacts of fences. They can act on very small processes, like changing how spiders build their webs or influencing where birds build their nests. There are famous examples of their effects on bigger animals, especially movement - migratory animals like wildebeest are particularly susceptible to the effects of fences. But fences can also work at very large scales. Rapidly expanding networks of fences are putting Africa's Mara ecosystem on the precipice of collapse, and Australia's dingo fences, arguably the longest man-made constructions in the world, have created chain reactions that have altered ecology at a continental scale. Putting all this together, one of the striking findings of our review is that fences have measurable effects at every ecological scale.

However, it is important to mention another broad pattern our review brought to light. Our study shows that fences are rarely, if ever, unequivocally good or bad. Instead, they reorganize species and ecosystems by creating "winners" and "losers." Who wins and loses varies a lot by context, but there are still some trends. Generalist species that can adapt quickly to disturbance tend to be winners, while more specialized species and ecosystems tend to be losers. This pattern tends to favor invasive species, for example, and amplify pressure on sensitive species that are already coping with many other risks. 

Another key point is that for every winner, fences tend to produce multiple losers. With a high enough density of fences, this can create ecological "no man's lands" where only a narrow range of traits can survive and thrive, and there is evidence that over time this can cause ecological meltdown.

In some cases, aren't fences helpful?

One of the goals of our paper is to change the way people talk about fencing. It's natural to want to parse the good fences from the bad, but the winners and losers framework tells us why it's not that simple: Even "good" fences will reorganize ecosystems by creating winners and losers.

Of course, there are contexts where fences might produce more winners than losers, or might serve a critical ecological or economic purpose. We aren't suggesting that all fences are bad! Instead, we hope that more caution and care will go into decisions about fencing. While an individual fence might be helpful for a particular goal, it might have costs when considered as part of a bigger landscape of fences. We hope this perspective can change the calculus about whether a fence is helpful and worth building or maintaining.

Did your research lead you to any good solutions to fencing?

Our research shows that any decision about fencing has to happen in context. This means considering not just the local ecological questions, but also how fences are entangled with society, economics, and politics. That said, our research does point to a few policy considerations that we hope can gain traction quickly. 

First, subtle changes to fence designs can provide big benefits. In places like Wyoming, agencies have been testing "wildlife-friendly" fences that reduce impacts on wildlife without affecting how well fences do their jobs. 

Second, fences are often built for short-term purposes and then abandoned. Removing derelict fences could produce a lot of ecological benefits without disrupting local economies. However, even when fences are removed, there is evidence that their "ghosts" haunt the landscape, continuing to influence animal movements and ecological patterns.

Because of this, our final recommendation is to think bigger before building a fence. The effects of fences are likely to be long-lasting, and their impacts part of a bigger landscape of ecological deterioration. We suggest managers look for alternatives to fencing that might be equally effective and consider the bigger ecological picture when making decisions about when and where to build.

[Related: Plant a Wildlife Hedge Instead of Building a Fence]