The growth of border fences in Central and Eastern Europe in recent years is a "major threat" to wildlife, according to a new study.
Up to 30,000km (18,600 miles) of wire fences and walls have been built, some in response to 2015's refugee crisis.
Researchers say they can divide threatened species such as deer and bear, as well as increase mortality.
The study points out that many of these "temporary" structures may become permanent and have long-term impacts.
Conservation scientists believe that the 1980s and 1990s heralded a new understanding by governments that many species needed to be dealt with on a trans-border basis.
The fall of the Berlin Wall, the expansion of the European Union and the signing into law of a number of legal agreements giving greater protection to animals - all contributed to this idea.
According to the authors of the new study, conservation biologists took their eye off the ball when it came to the building of new border fences.
"We hypothesise that 9/11 was the main driver, when the risk of terrorism and drug dealers coming in meant that governments were closing their borders to reduce the risk while conservationists were driving for a more open system to allow wildlife to cross," said Dr Matt Hayward from Bangor University, UK.
"Certainly, there's a lot of high-profile fences that have been put up in recent times driven by the Syrian and refugee crisis."
The study says that between 25,000 and 30,000km of wire fences and walls surround many countries in Eastern Europe and Central Asia, and they are having significant impacts on species.
In November 2015, the Slovenian government decided to construct a razor wire fence along large parts of its border with Croatia, to stop refugees crossing their country.
The authors say that this barrier has unforeseen consequences for animals, as it separates bears, lynx and wolves from their core populations.
In other parts of Europe and Asia, a growing number of deer have been killed by barriers.
"Animals try to cross the fences and get stuck," said Dr Hayward.
"There's also the isolation of populations. So if those fences are effective, animals can't move from one side to another and the population is split and this reduces genetic diversity in the species and risks in-breeding."
The researchers involved in the study believe that many of the fences may have been built in contravention of the World Heritage Convention, the Convention on Biological Diversity or the Habitats Directive of the European Union.
However, they also acknowledge there may be times when the construction of fences actually benefits species.
They give the example of the Asiatic Wild Ass, or Khulan, on the Mongolian-Chinese border. A 4,700km fence prevents these animals from wandering into Inner Mongolia where illegal hunting remains a major problem.
"If you look at lion populations around Africa, the best way to conserve them is to have them behind fences in large-scale national parks," said Dr Hayward.
"So conservation fencing by itself is not a bad thing; it's where we don't consider the impacts or carry out environmental assessments with these border fences - that's where the damage is done."
The study argues that governments should think about opening fences at important times for animal migration and to look at different types of fencing that might be less harmful to species.
The authors argue that authorities should also consider leaving gaps in fences that could be policed by technology.
More than anything, the study asks that conservation biologists make their voices heard when governments are planning new constructions; especially as many of the temporary fences may become permanent.
"It seems a fair bet that many of these will be sticking around for a fair while," said Dr Hayward.
The study has been published in the journal, Plos Biology.