Research Article |
Corresponding author: Marcel P. Huijser ( mhuijser@montana.edu ) Academic editor: Clara Grilo
© 2022 Marcel P. Huijser, James S. Begley.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Huijser MP, Begley JS (2022) Implementing wildlife fences along highways at the appropriate spatial scale: A case study of reducing road mortality of Florida Key deer. In: Santos S, Grilo C, Shilling F, Bhardwaj M, Papp CR (Eds) Linear Infrastructure Networks with Ecological Solutions. Nature Conservation 47: 283-302. https://doi.org/10.3897/natureconservation.47.72321
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Florida Key deer mortality data (1966–2017) showed that about 75% of all reported deer mortalities were related to collisions with vehicles. In 2001–2002, the eastern section of US Hwy 1 on Big Pine Key (Florida, USA) was mitigated with a wildlife fence, 2 underpasses, and 4 deer guards. After mitigation, the number of reported Key deer road mortalities reduced substantially in the mitigated section, but this was negated by an increase in collisions along the unmitigated section of US Hwy 1 on Big Pine Key, both in absolute numbers and expressed as a percentage of the total deer population size. The data also showed that the increase in Key deer collisions along the unmitigated highway section on the island could not be explained through an increase in Key deer population size, or by a potential increase in traffic volume. The overall Key deer road mortality along US Hwy 1 was not reduced but was moved from the mitigated section to the nearby unmitigated section. Thus, there was no net benefit of the fence in reducing collisions. After mitigation, a significant hotspot of Key deer-vehicle collisions appeared at the western fence-end, and additional hotspots occurred further west along the unmitigated highway. Exploratory spatial analyses led us to reject the unmitigated highway section on Big Pine Key as a suitable control for a Before-After-Control-Impact (BACI) analysis into the effectiveness of the mitigation measures in reducing deer-vehicle collisions. Instead, we selected highway sections west and east of Big Pine Key as a control. The BACI analysis showed that the wildlife fence and associated mitigation measures were highly effective (95%) in reducing deer-vehicle collisions along the mitigated highway section. Nonetheless, in order to reduce the overall number of deer-vehicle collisions along US Hwy 1, the entire highway section on Big Pine Key would need to be mitigated. However, further mitigation is complicated because of the many buildings and access roads for businesses and residences. This case study illustrates that while fences and associated measures can be very effective in reducing collisions, wildlife fences that are too short may result in an increase in collisions in nearby unmitigated road sections, especially near fence-ends. Therefore it is important to carefully consider the appropriate spatial scale over which highway mitigation measures are implemented and evaluated.
Collisions, fences, fence-end, key deer, mitigation, net benefit, road ecology, roadkill
Most wildlife mitigation measures along highways are aimed at improving human safety, reducing direct wildlife mortality, and providing safe crossing opportunities for wildlife (e.g.
While a combination of fences and crossing structures is probably the most reliable and robust measure to improve human safety, reduce direct wildlife mortality, and provide safe crossing opportunities for wildlife, there is still much to learn on how to both make fences and crossing structures more effective and have the structures more readily accepted by different species (e.g.
Here we investigate the effectiveness of a wildlife fence and associated measures in reducing collisions with an endangered species, the Florida Key deer (Odocoileus virginianus clavium), on Big Pine Key, Florida, USA. We first explored the absolute Key deer road mortality numbers over the years and evaluated the spatial pattern in reported collisions with Key deer before and after the fence was constructed. Then we corrected the number of reported collisions for the Key deer population size for both the mitigated highway section and different potential control road sections for a Before-After-Control-Impact (BACI) analysis through which we evaluated the effectiveness of the fence. We also investigated potential differences in traffic volume between the mitigated and unmitigated highway section on Big Pine Key, and how traffic volume may have affected Key deer collisions along the unmitigated highway section. These exploratory analyses allowed us to find a suitable control for the BACI analysis. The careful consideration of different potential control road sections also allowed us to explore the net benefit of the wildlife fence on a larger spatial scale. The results help us to be more effective when designing wildlife mitigation measures along highways and to be more accurate when evaluating their effectiveness.
In 1957, the National Key Deer Refuge was established in the Lower Florida Keys, Florida, USA. It is one of four national wildlife refuges in the area. The refuges were established to protect the endangered Key deer along with other endangered species and the habitat they depend on. At the time, hunting had reduced the Key deer population to fewer than 50 individuals (
In 2001–2002 a 1.64 mi (2.64 km) section of US Hwy 1 on the east side of Big Pine Key was mitigated with a 2.4 m (8 ft) high fence, 2 underpasses, and 4 deer guards (similar to cattle guards) (
We used the existing database on Key deer mortalities between 1966 (first record 9 March 1966) through partway 2017 (last record 9 November 2017) to assess road mortalities versus mortality from other causes. We calculated the absolute number of Key deer road mortalities along all roads combined (1966–2016) and in the ten years before mitigation (1991–2000) and in the fourteen years after mitigation (2003–2016) along both the mitigated and unmitigated highway section on Big Pine Key. We also explored where Key deer collisions occurred before and after the mitigation measures were implemented. Exploration of the spatial pattern of Key deer road mortalities after the mitigation measures were implemented allowed us to identify locations where further efforts to reduce Key deer-vehicle collisions should be directed, should one choose to do so. In addition, the spatial patterns in Key deer collisions before and after the mitigation measures were implemented provided the first step in identifying a suitable control for a BACI analysis to calculate the effectiveness of the fence and associated measures.
We investigated where the highest greatest concentrations of Key deer roadkill were after the eastern section of US Hwy 1 was mitigated. For this hotspot analysis, we only selected roadkill records of Key deer for the most recent 10-year period (2007 through 2016, n=1,182), regardless of where they occurred i.e. both on and off Big Pine Key, both inside and outside the mitigated section of US Hwy 1. We chose to use this subset of records as a balance between having recent data that identify current hotspots, and having a robust sample size. To identify hotspots, we conducted a Kernel density analysis using ArcGIS 10.6.1 (
Wherever a fence ends, there is a possibility of a concentration of collisions just beyond the fence end; the “fence-end effect”. For example, after implementation of the fence, some Key deer may have walked alongside the fence until they reached one of the two fence-ends. They could then cross the highway at-grade at the fence-end where they are exposed to potential collisions with vehicles. If such fence-end effects are indeed present, and if such road sections would be included in the control, it would result in an overestimation of the collisions in the control section and, through the BACI analysis, it would then also overestimate the effectiveness of the mitigated road section. Therefore, for a control to be suitable, it should not be influenced by the mitigation measures, and potential fence-end effects should be excluded from the control. We explored the potential presence of a concentration of Key deer road mortality near the western and the eastern fence-ends through an optimal hot spot analysis (Getis-Ord Gi*) in ArcGIS 10.6.1 (
We investigated the net benefit of the mitigation measures by calculating Key deer road mortality as a percentage of the Key deer population size for all roads combined, US Hwy 1 on Big Pine Key (mitigated and unmitigated sections combined), and the mitigated road section of US Hwy 1 on Big Pine Key. We conducted the same analysis for different sections of unmitigated road sections of US Hwy 1 to identify a suitable control for the BACI analysis. The potential control sections that were evaluated included the unmitigated road section of US Hwy 1 on Big Pine Key up to the fence-end, the unmitigated road section of US Hwy 1 on Big Pine Key excluding a potential fence-end effect (see previous section), and the unmitigated road sections of US Hwy 1 west and east of Big Pine Key. This allowed for a second step in finding a suitable control for the BACI analysis as the analyses described above can detect evenly distributed increases in road mortality in different potential control road sections that are associated with the implementation of the mitigation measures.
To express Key deer road mortality as a percentage of the Key deer population size, we relied on historical population estimates. Unfortunately, total Key deer population size estimates were only available for certain years (Appendix
To investigate if traffic volume may have played a role in the increase in Key deer road mortality along the unmitigated section of US Hwy 1 on Big Pine Key after the mitigation measures were implemented, we summarized traffic volume on US Hwy 1 on Big Pine Key between 1994–2017 based on traffic counter data (
We investigated the effectiveness of the mitigation measures in reducing collisions with Key deer through a BACI analysis. We selected roadkill records of Key deer; 10 years before the implementation of the mitigation measures (1991 through 2000), and 14 years after the implementation of the mitigation measures (2003 through 2016). We searched for a suitable control section of US Hwy 1 through the analyses described in the sections above. The unmitigated section of US Hwy 1 on Big Pine Key, starting immediately adjacent to the fence-end, was not suitable due to a concentration of collisions just beyond the fence-end. Excluding the fence-end effect still did not result in a suitable control as Key deer road mortality expressed as a percentage of the Key deer population size was still elevated, presumably because of the nearby mitigated road section. However, the combined unmitigated road sections US Hwy 1 west (11.7 mi; 18.8 km) and east (2.7 mi; 4.3 km) of Big Pine Key seemed unaffected by the implementation of the mitigation measures on Big Pine Key. Therefore, we selected these road sections as the control for the BACI analysis (total length for the control was 14.4 mi; 23.2 km) (Fig.
There were 4,753 recorded mortalities of Key deer from 1966–2017. Overall, roadkill was the most common recorded cause of mortality (N=3,412, 71.8%), followed by “undetermined” (N=681, 14.3%), and disease (N=276, 5.8%). Drowning, predation by dogs, entanglement, intraspecies combat, poaching, humans (various causes), and physical impact of hurricanes each represented less than 5% of the recorded mortalities. Road mortality has consistently been the leading known cause of mortality since record keeping began in 1966. The average percentage of Key deer road mortalities out of all recorded Key deer mortalities for each year (1966–2016) per year was 75.5% (SD=10.2). While the absolute number of recorded Key deer road mortalities dropped substantially in the mitigated road section after the mitigation measures were implemented in 2001–2002, the number of Key deer road mortalities for all roads combined and for the unmitigated section of US Hwy 1 on Big Pine Key continued to increase (Fig.
The number of Key deer road mortalities per year along all roads combined (1966–2016) and during the ten years before (1991–2000) and the fourteen years after mitigation (2003–2016) along both the mitigated and unmitigated section of US Hwy 1 on Big Pine Key (BPK).
After the mitigation measures were implemented, Key deer road mortality was concentrated along the unmitigated western section of US Hwy 1 on Big Pine Key (Fig.
Before the eastern section of US Hwy 1 was mitigated, there was a significant concentration of Key deer-vehicle collisions at the eastern edge of Big Pine Key (Fig.
Key deer population size has grown exponentially since the 1940s (Fig.
Estimated Key deer population size. An exponential growth curve (y=e(0.04279*((year)-1853.976); R2 = 0.93) was fitted through data obtained from the literature (see Appendix
The percentage of roadkilled Key deer of the total population size for all roads combined was similar before (average 14.2%, SD=3.2) and after (average 14.9%, SD=1.8) the fence and associated mitigation measures were implemented along the eastern section of US Hwy 1 (Kruskal-Wallis One-Way ANOVA on Ranks, Chi2 = 0.109, p = 0.742) (Fig.
In 2017, US Hwy 1 on Big Pine Key had an Annual Average Daily Traffic Volume (AADT) of 18,590–19,600 vehicles per day depending on the location of the traffic counter (
Before the mitigation measures were implemented, the average number of Key deer roadkill per mile per year was 8.4 for the mitigated road section, and 0.4 for the control section (Fig.
Direct road mortality has consistently been the most common recorded cause of mortality for Key deer since record keeping began. Therefore, if the objective is to reduce unnatural mortality for Key deer, reducing direct road mortality should be explored first. After the mitigation measures were implemented, Key deer road mortality was concentrated along the unmitigated western section of US Hwy 1 on Big Pine Key. Therefore, this is the road section that should be prioritized if the objective is to reduce direct road mortality of Key deer.
Based on the BACI analysis, the wildlife fence and associated mitigation measures along the eastern section of US Hwy 1 on Big Pine Key were highly effective (94.8%) in reducing Key deer-vehicle collisions along the mitigated road section. However, when corrected for the population size, Key deer road mortality was similar before and after highway mitigation for all roads combined as well as for US Hwy 1 on Big Pine Key (mitigated and unmitigated section combined). Similar to the absolute numbers, the percentage of roadkilled Key deer in relation to the population size sharply decreased by 90.0% in the mitigated section of US Hwy 1 but substantially increased by 68% in the unmitigated section of US Hwy 1 and by 65% when the fence-end effect was excluded. The hypothesis that the continuing increase in Key deer-vehicle collisions after the mitigation measures were implemented may have been associated with an increase in Key deer population size must be rejected. Similarly, traffic volume can also not explain the increase in collisions. Traffic volume was, on average, lower after the implementation of the fence and associated mitigation measures, likely because of the lead-up to the economic crisis in 2008 and the gradual recovery afterwards. However, in general, higher traffic during certain hours of the night is positively correlated with an increase in collisions with Key deer (
The increase in Key deer road mortality along the unmitigated section of US Hwy 1 on Big Pine Key can be seen as a form of environmental leakage as the “extraction” was moved from a now protected area to a non-protected area rather than reduced (
It is important to bear in mind that the overall number of collisions is just one parameter associated with the presence of the mitigation measures along the eastern section of US Hwy 1. For example, even though the overall number of key-deer vehicle collisions along US Hwy 1 was not reduced after mitigation, the remaining collisions mostly occur along the section where the design speed and surroundings (side roads, entrances to businesses, pedestrians, cyclists) may encourage drivers to have lower operating speed and pay more attention to their surroundings compared to the mitigated section of US Hwy 1 (very few side roads, no buildings adjacent to the highway, wide right-of-way). Thus, there may be a lower likelihood of human injuries and human fatalities when hitting a Key deer in the western section of US Hwy 1. Another benefit of the mitigation measures is that the mitigated section of US Hwy 1 also provides safe crossing opportunities for Key deer through the underpasses (
While the mitigation measures along the eastern section of US Hwy 1 on Big Pine Key were highly effective in reducing Key deer-vehicle collisions, the data indicate that there was no “net benefit” of the wildlife fence in reducing collisions with Key deer along the entire section of US Hwy 1 on the island (mitigated and unmitigated road section combined). Measures that could be considered for the currently unmitigated western road section on the island include erecting a fence behind the businesses and residential properties that are adjacent to US Hwy 1. This would mean that the “first row” or “first block” of buildings would be included in the fenced road corridor, resulting in unhindered access to these buildings from US Hwy 1. A limited number of gaps in the fence, with wildlife guards, would allow for access to areas beyond the first row or block of buildings. In places where natural habitat remains adjacent to US Hwy 1, fenced corridors leading up to US Hwy 1 may be considered for wildlife, including Key deer. The fenced corridors would lead to underpasses (similar to the ones in the eastern mitigated road section) or at-grade crossing opportunities with wildlife guards or electrified barriers embedded in the travel lanes that encourage Key deer to cross the highway straight and to keep them from wandering off to the sides into the fenced road corridor. This measure can be expected to result in a reduction in Key deer collisions of about 95% (with underpasses, see this article) or 40% (with at-grade crossing opportunities, see
In order to substantially reduce the overall number of deer-vehicle collisions along US Hwy 1 on Big Pine Key, the entire highway section on Big Pine Key would need to be mitigated. However, the section of US Hwy 1 that remains unfenced has many buildings and access roads to businesses and residences. This means that there are many competing interests; implementing mitigation measures that are effective in reducing Key deer-vehicle collisions and that also provide safe crossing opportunities for Key deer and other wildlife species will affect other interests on and along US Hwy 1. This case study also illustrates that while fences and associated mitigation measures can be very effective in reducing collisions in the mitigated road section, wildlife-vehicle collisions in the larger area may not be reduced because the collisions can move to nearby unmitigated road sections, especially just beyond fence-ends. This phenomenon is not an indication that wildlife fences do not reduce wildlife-vehicle collisions. Instead, it is an indication that the fenced road section is too short. Therefore it is important to carefully consider the appropriate spatial scale over which highway mitigation measures are implemented and evaluated.
We thank the U.S. Fish & Wildlife Service for funding this project. Special thanks are due to Daniel Clark, Christine Ogura, and Kate Watts (all U.S. Fish & Wildlife Service) and to Fernando Ascensão, Edgar van der Grift, Clara Grilo, and Bethanie Walder for their help and review of different versions of this manuscript.
Year | Minimum (n) | Maximum (n) | Average (min-max) (n) | Source |
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1940 | ? | 50 | 50 |
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1952 | 25 | 80 | 52.5 |
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1974 | 300 | 400 | 350 |
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1990 | 250 | 300 | 275 |
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2001 | 453 | 517 | 485 |
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2005 | 555 | 619 | 587 |
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2014 | 987 | 1012 | 999.5 |
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