Fences erected at borders across continental Europe are keeping out more than just human beings. The nearly 30,000 kilometers of barriers that have been placed along the borders of countries in Eastern Europe and central Asia are a major threat to wildlife, BBC reports. Researchers say the fences, which are a response to the 2015 refugee crisis and have been growing since 9/11, endanger the lives of animals that get caught in them. The study, published in PLOS Biology, also found the fences could isolate animal populations, prevent access to resources, and disrupt migrations. Although conservation fences have been successful in some cases, such as protecting animals from areas with poachers, the researchers maintain that in Europe, the temporary barriers could become permanent and recommend technology-monitored openings to allow animals to pass through. Until then, they remain a cause for concern for conservationists, keeping out more levels of the food chain than anticipated.