RIO GRANDE CITY, Texas— The U.S. Fish and Wildlife Service today protected the prostrate milkweed as endangered. Only 24 populations of the plant survive, in south Texas and northern Mexico, where they serve as an important food source for pollinators like bees and imperiled monarch butterflies.

The Service also protected 661 acres of critical habitat for the plant in eight south Texas units in Zapata and Starr counties. Recent border-wall construction degraded another 20 acres of habitat that were proposed for protection last year to the point that they were unsuitable for the plant and withdrawn from designation. All populations of the milkweed in the United States are within nine miles of the border, making it one of hundreds of species threatened by wall construction.

“Protecting prostrate milkweed is a big deal for the monarch butterflies who lay their eggs on these plants as they fly through Texas after spending the winter in Mexico,” said Tierra Curry, a senior scientist at the Center for Biological Diversity. “For the sake of the milkweed and all the pollinators who rely on it, it’s a relief that this important native plant finally has the safeguards of the Endangered Species Act.”

Construction and maintenance for roads, utilities, and the oil and gas industry also destroy the prostrate milkweed, and additional border-wall construction on the Lower Rio Grande National Wildlife Refuge threatens to uproot more of them. These activities and livestock grazing foster the spread of invasive buffelgrass, which is planted as livestock forage. Buffelgrass displaces native plants and is very difficult to control.

Under natural conditions the prostrate milkweed is thought to be able to persist at low densities. It produces so much nectar that far-flying pollinating insects such as tarantula hawks and large bees are so juiced up after visiting it that they can fly farther and pollinate other relatively distant prostrate milkweed populations. But as prostrate milkweed numbers and densities have declined, the plant is also imperiled by lower reproductive success and loss of genetic diversity.

Just 24 populations of prostrate milkweed remain in Starr and Zapata counties in Texas and in Tamaulipas and eastern Nuevo León in Mexico. Nineteen of those populations are rated in low condition, the remaining five are in moderate condition and none are in high condition — indicating acute imperilment.

The Endangered Species Act has been successful in keeping more than 99% of species under its protection from going extinct. But long delays in adding animal and plant species to the endangered list have heightened the perils and made recovery more difficult and expensive. For example, the Service must decide by the end of 2024 whether to protect monarch butterflies as threatened, 10 years after a petition seeking to protect them under the Endangered Species Act was filed.

The prostrate milkweed listing comes in response to a Center lawsuit to gain final decisions on protection for 241 plant and animal species threatened with extinction, including the prostrate milkweed and more than 35 others in Texas. The prostrate milkweed was the subject of a 2007 protection petition by WildEarth Guardians.

The prostrate milkweed’s low and sprawling leaves and stem wilt during droughts. But the plant’s subterranean tuber stays alive and after soaking up moisture from occasional tropical storms sends up stalks and pink and yellow flowers.