A Life-or-Death Crossing for Green Island's Land Crabs: How Can Roadkill Rates Be Halved During the Breeding Season?

The coastal road encircling Green Island offers visitors scenic views of the island, but for land crabs inhabiting its coastal forests, it can become a dangerous obstacle during their breeding migrations. Each breeding season, some female land crabs must travel from their terrestrial habitats to the shoreline to release their newly hatched larvae into the sea. Along the way, passing motorcycles and cars pose a deadly threat, potentially killing females before they have completed reproduction.

Green Island covers an area of only about 15 square kilometers. Its peak summer tourist season coincides with the breeding season of many land crab species. The section of the island’s western coastal road between Shilang and Guiwan has long been identified as a roadkill hotspot for land crabs. Previous surveys have documented numerous crabs crushed by vehicles, particularly Metasesarma aubryi. This species typically inhabits vegetated coastal environments, where ovigerous females migrate toward the shoreline when conditions are suitable for larval release.

Locations of the two culverts on Green Island, Taiwan. The black circle indicates the control site, and the gray circle indicates the experimental site（Image source：Chen RT et al. (2025)， CC BY 4.0 ）

To reduce roadkill, two underground culverts were installed beneath this section of road as early as 2006, providing potential passages for land crabs to cross safely. However, the presence of a culvert does not necessarily mean that animals will find its entrance. Crabs following their usual migration routes may still crawl directly onto the road. A subsequent study therefore addressed a practical question: Could additional guidance facilities installed around existing culverts encourage more land crabs to use these passages and reduce their risk of being crushed by vehicles?

Land crab crossing passages and an ovigerous female on Green Island. (a) Mountainside entrance of the culvert at the experimental site; (b) seaside entrance of the culvert at the control site; (c) road and underground culvert at the control site without additional guidance facilities; (d) an ovigerous Metasesarma aubryi preparing to migrate to the shore for larval release（Image source：Chen RT et al. (2025)， CC BY 4.0 ）

The researchers compared two culverts located approximately 60 meters apart. One was left in its original condition as the control group, while the other was equipped with PVC pipes and plastic barriers as the experimental group. Both culverts had entrances measuring 40 centimeters in diameter and were approximately 8 meters long. At the experimental site, PVC pipes with longitudinal slits were installed along the roadside, together with plastic barriers measuring 10 meters in length and 18 centimeters in height. These structures were designed to guide migrating land crabs toward the slits, allowing them to enter the pipes and follow them into the underground culvert. On the opposite side, adjacent to a concrete retaining wall, halved PVC pipes were installed to direct crabs downward toward the culvert entrance.

Schematic diagram of the culvert and guidance facilities at the experimental site（Image source：Chen RT et al. (2025)， CC BY 4.0 ）

The study was conducted from May to September in both 2022 and 2023, with surveys scheduled around the end of each lunar month, when land crab breeding activity is relatively high. A total of 54 surveys were completed. During each survey, researchers observed the study sites for 20 minutes in either the early morning or evening, recording land crabs within 30 meters on either side of each culvert. Individuals were classified according to whether they had been killed by vehicles, were using the culverts or guidance facilities, were near culvert entrances, or were found elsewhere within the survey area.

Field photographs of the land crab guidance facilities at the experimental site. (a, f) PVC pipes and plastic barriers installed along the grassland; (b) PVC pipe with longitudinal slits; (c) main PVC pipe and branch pipes; (d) end of a PVC pipe connected to a plastic barrier; (e) transparent pipe leading to the culvert entrance; (g, h) halved PVC pipes installed along the retaining wall（Image source：Chen RT et al. (2025)， CC BY 4.0 ）

Across the two years, the researchers recorded 1,833 observations of land crabs and terrestrial hermit crabs. Metasesarma aubryi was the most frequently recorded species, followed by Coenobita brevimanus. In 2022, the overall roadkill rate was 26.8% at the control site, compared with only 8.5% at the experimental site. In 2023, the corresponding rates were 32.1% and 12.1%. Thus, during both breeding seasons, the proportion of recorded individuals killed by vehicles was more than 50% lower at the site equipped with guidance facilities than at the control site.

The terrestrial hermit crab Coenobita brevimanus（Image source：Robert Webster， CC BY-SA 4.0 ）

The two dominant species differed considerably in their vulnerability to roadkill. Metasesarma aubryi accounted for more than 90% of all recorded roadkill individuals. In 2022, its roadkill rate reached 41.7% at the control site but was only 19.4% at the experimental site. In 2023, the corresponding rates were 48.6% and 31.7%. By contrast, Coenobita brevimanus already had relatively low roadkill rates at the control site, at 4.5% in 2022 and 4.3% in 2023. These rates declined further to 0.9% and 0.4%, respectively, at the experimental site. The differences between the two species may be related to the timing of their breeding migrations and their likelihood of encountering passing vehicles.

Another observation revealed an important limitation of the existing conservation facilities. In May 2023, the roadkill rate of Metasesarma aubryi at the experimental site reached 85.2%, exceeding the 67.2% recorded at the control site. The researchers confirmed that the individuals killed during that month were juvenile crabs returning inland from the shore, traveling in the opposite direction to the ovigerous females that the guidance facilities were primarily designed to accommodate. After excluding these juveniles, the 2023 roadkill rates for M. aubryi were 45.8% at the control site and 22.8% at the experimental site. This finding highlights the need to consider migration in both directions. Even if the mountainside entrance successfully guides adult crabs toward the sea, the seaside entrance must also be accessible to juveniles returning to their terrestrial habitats.

Land crab roadkill rates on Green Island from May to September in 2022 and 2023. (a) Monthly overall roadkill rates at the control site (light blue) and experimental site (orange); (b) comparison of monthly roadkill rates between Metasesarma aubryi and Coenobita brevimanus（Image source：Chen RT et al. (2025)， CC BY 4.0 ）

Compared with relying solely on warning signs, guiding land crabs through pipes toward underground passages may offer a more effective way to reduce their exposure to traffic during the breeding season. Potential improvements include strengthening the connections between the guidance facilities and culverts, improving ventilation within the pipes, and preventing fallen leaves and other debris from obstructing the passages. Special attention should also be given to juveniles migrating inland from the shore. Whether land crabs can cross a coastal road safely depends in large part on whether road infrastructure is designed with their natural migration routes in mind.

Author: Shui-Ye You

Reference:

Chen RT et al. (2025). Using innovative guidance facilities to reduce roadkill rates during land crab breeding season in Green Island, Taiwan. Global Ecology and Conservation.