Emily Brown (Environmental Science and Geography)
Gabriella Saloio (Environmental Science)
Dylan Giles (Building and Construction Technology)
Robert Golden (Urban Forestry)
On November 16, 2016, a motion-detection camera from the U.S. Bureau of Land Management captured a rare image in the Dos Cabezas Mountains of Arizona. Displayed prominently in the foreground of the photograph was the unmistakable yellow and brown speckled coat of a jaguar. This jaguar sighting was the second in the last four months and the seventh since 1996, leading scientists to believe the population might re-establish in the U.S. after more than a century of decline (Davis, 2017, para. 10). Ironically, however, the photograph was taken one week after the election of President Donald Trump, whose promise of constructing a border wall between the United States and Mexico would prevent this endangered animal from entering the country (Davis, 2017, para. 18). Though the jaguar’s persistence in the U.S. is of concern to scientists, this is only one of many species threatened by the proposal to build an impermeable border wall (Lasky, Jetz, & Keitt, 2011, p. 673).
Immigration control has been a topic of concern for many years in the United States, and a cornerstone of President Drumpf’s election campaign (Fleischner & Sonneland, 2016, para. 3). Construction along the border first began in the 1990s following concerns of illegal immigration in busy border cities (Fleischner & Sonneland, 2016, para. 3). These concerns intensified as the number of immigrants living in the U.S. grew dramatically from 3.5 million in 1990 to 12.2 million in 2007, although the number has since stabilized (Krogstad, Passel, & Cohn, 2016, para. 3). Despite this stabilization, President Drumpf renewed the call to strengthen and expand border security, which will entail environment consequences.
A major reason why immigration control measures over the years have harmed wildlife is that there is little consideration of environmental impacts. According to the REAL ID Act of 2005, the Secretary of the Department of Homeland Security (DHS) has “authority to construct barriers across the entire border at any time without oversight of environmental regulatory law” (Lasky et al., 2011, p. 673). The current legislation therefore removes any imperative to protect and consider wildlife health while constructing border barriers. While border security is important for the U.S., research shows that a full scale border wall will cause significant habitat fragmentation, inhibit wildlife movement, and lead to reduced gene flow across wildlife populations.
Given the purpose of the proposed wall is to prevent the movement of people over the border, it will undoubtedly impede wildlife movement as well. There is clear evidence that wildlife in the U.S.-Mexico borderland region has difficulty crossing the current barriers. For example, the Pygmy-Owl is a non-migratory bird with a flight path averaging only 1.4 meters above the ground; as a result, only 23% of Pygmy-Owl flights are successful in clearing the 4-meter height of the current barriers (Flesch et al., 2010, p. 177). Wild felids including jaguars and pumas are also unable to cross impermeable sections of the border, and rather they rely on openings in the border as dispersal corridors (Grigione et al., 2009; McCain & Childs, 2008; McCallum, Rowcliffe, & Cuthill, 2014). Anthropogenic barriers pose not only a problem in the United States and Mexico, but also around the world. In the region between Mongolia and China, a border fence prevents gazelles and Mongolian wild asses from reaching their natural habitat on the other side of the barrier (Ito et al., 2013; Kaczensky et al., 2011). Research also suggests that anthropogenic barriers less intrusive than fences, such as railroad tracks and highways, still deter many species from crossing and prevent movement (Ascenso et al., 2016; Sawyer et al., 2013; Seidler, Long, Berger, Bergen, & Beckmann et al., 2015). Knowing semi-permeable barriers inhibit wildlife movement, a much higher and less permeable barrier along the U.S. border will further restrict movement of local wildlife.
If the Drumpf administration constructs the proposed wall and seals permeable sections of the border, there would be widespread habitat fragmentation. The current border fence intersects the ranges of 38 amphibian species, 152 reptilian species, and 113 mammalian species (Lasky et al., 2011, p. 677). The desert tortoise, Sonoran pronghorn, and jaguar are only a handful of the hundreds of wildlife species that would experience habitat fragmentation by an extension of the border wall (Flesch et al., 2010, 179). One mammal particularly threatened by an impermeable wall is the black bear, whose habitat in the desert Sky Island mountain range spans across Arizona and the Mexican state of Sonora. While there is an abundance of suitable habitat for black bears on the U.S. side of the border, there is little on the Mexican side; consequently, a border wall threatens to restrict the movement of the black bear population south of the border by cutting off its habitat range (Atwood et al., 2011, p. 2859).
With a wall in place, the resulting lack of wildlife movement across suitable habitat would disrupt gene flow among subpopulations. Scientist use the fixation index (FST) to measure genetic differences between populations, where an FST value between 0.05 and 0.15 indicates moderate genetic differentiation between subpopulations and a value less than 0.05 indicates little to no genetic differentiation (Balloux & Lugon-Moulin, 2002, p. 161). Looking more closely into the black bear’s predicament, the fragmented habitat already caused by the current border fence has led to genetic differences between black bear populations near the border and those further north in central Arizona. The FST value between these populations is moderately high at 0.113 (Atwood et al., 2011, p. 2859). Other species, such as the bighorn sheep, currently have significant gene flow between the U.S. and Mexico in areas where there is no physical barrier inhibiting movement (Buchalski, 2015; Flesch et al., 2010). The FST value between bighorn sheep populations in the U.S. and Mexico is less than 0.05, indicating high levels of gene flow (Flesch et al., 2010, p. 179). Based on a simulated border fence that proceeded to cut off all dispersal corridors between the U.S. and Mexican bighorn sheep populations, scientists believe a border wall impermeable to the bighorn sheep will prevent transboundary genetic exchange that occurs today (Flesch et al., 2010, p. 177). In the U.S., the jaguar is one of the most susceptible species to a reduction in gene flow. The proposed border will disrupt gene flow between the small population located in the southern U.S. and larger populations in Sonora, Mexico. If the U.S. constructs an impermeable barrier, it is likely the jaguar population will not persist in the United States given these populations only reproduce south of the border (McCain & Childs, 2008, p. 8).
Just as with most ecosystems, these species contribute to the biodiversity of the U.S.-Mexico border. Biodiversity is the variety of life in a particular area and is extremely important to the health and quality of an ecosystem (“What is biodiversity,” para. 4). In other words, removing one species from an ecosystem alters the entire ecosystem, therefore impacting the area’s biodiversity. The U.S.-Mexico border region is even more susceptible to biodiversity loss as it has the highest rate of species endangerment in the United States (Van Schoik, 2004, para. 1). Approximately 31% of species listed as endangered in the United States dwell in this region (Van Schoik, 2004, para. 1). An endangered species listing means a species is at risk of extinction in a significant area of their range, or all of their range (“Endangered Species,” 2015, para. 3). Species such as the jaguar and bighorn sheep are currently on the endangered species list in the United States, and they each contribute to the biodiversity of the U.S.-Mexico border and keep the area healthy (“Endangered Species,” 2015, para 3).
Just as biodiversity is important to ecosystems, it is important to humans and the economy. According to the United Nations Convention on Biological Diversity, biological resources account for at least 40% of the world’s economy (“Benefits of biodiversity,” 2015, para. 1). This estimates to be 33 trillion dollars per year in the U.S. economy (“Benefits of biodiversity,” 2015, para. 6). Many jobs are dependent on biodiversity, such as farmers, park rangers, and even pharmaceutical and personal care markets, which rely on chemicals extracted from diverse life forms. On the outdoor recreational side of the matter, the U.S. spends approximately $29 billion per year on fishing and $12 billion per year on hunting (Pimentel et al., 2017, p. 753). Given this, an impact on biodiversity holds clear economic ramifications for our country.
The proposed U.S.-Mexico border wall will inhibit wildlife movement, fragment habitat and decrease gene flow between separated wildlife populations; however, amending legislation and incorporating wildlife corridors in the barrier design can alleviate these problems. Creating a porous border that supports wildlife movement first entails amending the REAL ID Act (Lasky et al., 2011, p. 684). As legislation currently stands, the DHS waived thirty-seven federal policies under this act to construct the current border fence. Many of these are important environmental laws such as the Endangered Species Act (“Laws waived,” 2014, para. 1). To begin protecting wildlife and biodiversity in the borderlands, the DHS must consider environmental consequences of border constructions.
After the implementation of environmental regulations, we propose designating wildlife movement corridors throughout the border wall. Wildlife corridors are essentially strips of habitat that connect larger, separated habitat patches for species threatened by anthropogenic barriers (Beeland, 2009, para. 3). In the case of a border wall separating habitat, gaps in the barrier result in sufficient corridors to connect habitat. To maximize efficiency of these gaps, corridors should target umbrella species, which are large mammals like wolves and elk that have wide habitat ranges (Beeland, 2009, para. 5). Protecting and linking habitats of umbrella species simultaneously protects habitats of smaller species (Beeland, 2009, para. 5). Jaguars, such as the one recently photographed in Arizona, and other wild felids are umbrella species that exist in the borderland region. Establishing corridors for wild felids would simultaneously protect many sympatric species in the same geographic area (Grigione et al., 2009, p. 84).
In addition to targeting umbrella species, the design of wildlife corridors should target areas of high species richness, with richness referring to the number of species in an area. Species rich areas in the border region between the U.S. and Mexico correlate with sharp environmental gradients, such as mountain ranges. Given this, scientists have identified the California, Madrean, and Gulf ecoregions as threatened by a border wall that would bisect a large number of narrow habitat bands (Lasky et al., 2011, p. 682). Of the seven ecoregions along the border, these three have the greatest number of endemic species with small range sizes overlapping the border; therefore, placing dispersal corridors in these ecoregions will protect the greatest number of species in a relatively small area (Lasky et al., 2011, p. 682). Border openings in these regions should be at least 300 meters wide, which eliminates significant negative edge effects like noise, light, or human activity associated with border security (Beier & Spencer, 2008, p. 846). Scientists can use computer models to identify corridors for individual species through spatial layering of suitable habitat variables, such as resource availability or topography (Beier & Spencer, 2008, p. 841). Combining these simulated corridors would reveal the locations of corridors suitable for multiple species (Beier & Spencer, 2008, p. 846).
Dispersal corridors are successful methods of connecting wildlife populations fragmented by anthropogenic barriers, and they will likely increase connectivity for borderland wildlife fragmented by an impermeable wall. In South Carolina, the longest-running study of movement corridors revealed that of the twenty species studied, eighteen moved more frequently with a corridor and in some cases, ten times more frequently than those with no corridor (Robbins, 2011, para. 16). Another study conducted over three years in Banff National Park found that grizzly and black bears successfully disperse across movement corridors over the Trans-Canada Highway (Sawaya, 2014, p. 7). Not only did these structures allow bears to reach suitable habitat on the other side of the highway, but they encouraged gene flow; 47% of black bears and 27% of grizzly bears that used the movement corridors bred successfully (Sawaya, 2014, p. 1).
While there are legitimate concerns of border openings enabling human migration into the U.S., alternative methods such as remote sensing and motion detection can monitor human activity. Boeing Aircraft Manufacturing Company successfully created a virtual border fence prototype along the Arizona-Mexico border. In partnership with the DHS, Boeing created this virtual fence using 9 movable radio towers across a 28-mile area (Rotstein, 2008, para. 4). A string of 17 radio towers anchoring upgraded radar, cameras, and new microwave-based communications installed since then improve the fence’s efficiency and aid border patrol agents over long distance monitoring (Rotstein, 2008, para. 5). This fence currently enables DHS border security to remotely monitor areas along the border in areas of Arizona that are not fenced. A group of scientists from the Georgia Institute of Technology developed and tested another virtual wall success known as “BorderSense,” a hybrid wireless sensor (Sun et al., 2011, p. 468). “BorderSense” consists of wireless multimedia sensor networks and wireless underground sensor networks that can distinguish humans motion from other animals (Sun et al., 2011, p. 473). The recognition algorithms in these wire systems and sensored cameras detect human movement and have successfully done so in lab simulation runs (Sun et al., 2011, p. 473). Given virtual walls are capable of detecting human activity, they are an excellent method of monitoring the proposed wildlife corridors of the border wall.
There is no doubt that the impermeable border wall proposed by Donald Drumpf will lead to habitat fragmentation and reduced connectivity across many species found in the region. Although immigration control is necessary, an impermeable border wall is not the solution. We propose a porous border that will address issues of illegal immigration while allowing species to disperse between the United States and Mexico. This will involve careful consideration of wildlife movement corridors to create openings in the border secured virtually using sensing technology. It is vital that we take actions to protect wildlife species today more than ever as we experience a global decline in biodiversity (“What is biodiversity,” para. 7).
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