By Scott P. Egan, Mattheau Comerford, Glen R. Hood, Linyi Zhang and Kelly L. Weinersmith
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Predicting the biological implications of human-made barriers dividing natural populations is a critical and urgent challenge for scientists and policymakers worldwide. Inspired by the documented increase of ‘border barriers,’ or anthropogenic barriers along international borders, on a global scale, this report addresses the biological impacts of these barriers on natural populations, with a focus on evolutionary changes. While typically built for geopolitical and security reasons, border barriers can also have unintended consequences for biodiversity.
The construction of border barriers is not a new phenomenon. From the Great Wall of China, to the Berlin Wall, to the contemporary walls between the U.S. and Mexico, humanity’s history of erecting border barriers spans thousands of years. Current border barriers exist between Israel and the West Bank, Malaysia and Thailand, India and Pakistan, Iran and Iraq, China and Mongolia, and Botswana and Zimbabwe, among others. Given the global scale of border barriers, we know surprisingly little about their impact on ecological and evolutionary outcomes in natural populations. Moreover, there is no clear review directly summarizing the potential evolutionary impacts of border barriers, at a time when biodiversity is rapidly declining on a global scale.
There are many reasons for this lack of information, which is partially due to the fundamental characteristics of border barriers. First, border barriers are built between two countries, which have different laws and practices associated with conducting scientific research on the biological impacts of these structures. Second, border barriers are commonly built for security reasons, and there is often restricted access to these areas on either side as a result. Third, best practices and strategies for mitigating the negative effects of human-made barriers on wildlife could compromise the intended security purposes of border barriers. Fourth, environmental laws intended to protect biodiversity along border areas can be temporarily waived in the interest of national security and safety. Fifth, borders are often created along natural geographic boundaries, therefore border barriers can amplify the historical effects of natural barriers on biodiversity.
This report addresses the evolutionary impacts of border barriers. We summarize data directly from border barriers studies and provide supporting information from studies of other anthropogenic barriers, such as roads, dams, and game fences. To this end, we first provide a brief overview of what can be learned from geographic isolation under natural conditions. Second, we address changes to the movement of individuals and gene flow between populations. Third, we highlight three predicted evolutionary outcomes in response to border walls: (1) divergence due to natural selection and/or genetic drift as gene flow decreases; (2) immediate and long-term changes to effective population size, including population bottlenecks and changes to genetic variation; and (3) inbreeding and inbreeding depression. For each of these outcomes, we provide evidence from theory and previous studies on the biological consequences of human-made structures to show how local populations can respond to border barriers.
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