to be the prime reason for their decline. One of the major factors contributing to the habitat loss was the widespread planting, beginning in the early 1900s, of Chinese tallow trees (Triadica sebifera) to establish a soapmaking industry. Since that time, T. sebifera has proven to be an aggressive invader of the coastal grasslands, where it displaces the diverse native plant assemblage that was dominated by prairie grasses and forbs with dense, near-monospecific stands that significantly alter biotic and abiotic ecosystem processes. Urbanization further contributes to habitat loss. As a result of these changes over the last 100 years, the entire grassland ecosystem where Attwater's prairie chicken once thrived exists in small, scattered patches whose continued existence is threatened. Where once grazing plains bison and periodic wildfires due to lightning reduced brush, the birds now have difficulty making their way through thick undergrowth. Other, less-apparent changes in the ecosystem possibly have had an effect, as well. A 1937 study recorded about 8,700 Attwater's prairie chickens remaining in four Texas counties. Attwater's prairie chicken has been on the endangered species list since March 1967 when an estimated 1,070 birds were left in the wild. By 2003, fewer than 50 birds remained in the wild. In 1999, Nature Conservancy decided to permit new drilling close to primary breeding grounds on Texas land owned by the Conservancy. This has harmed the grouse and their habitat, according to the Wildlife Society. Drilling in sensitive areas is destructive and opposed by environmentalists. The number of Attwater's prairie chickens nesting on the land owned by the Conservancy fell from 36 in 1998 to 16 in 2003. Analysis by Nature Conservancy's Texas science director said that that new oil drilling project subjected the grouse to a "higher probability of death." In 2007, Nature Conservancy issued new permits to drill a new oil well on protected land. Attwater's prairie chickens have since disappeared from the site. In 2014, an estimated 260 birds remained, with about 100 living in the wild. Captive-breeding programs are underway at places such as Fossil Rim Wildlife Center, Abilene Zoo, and Caldwell Zoo (Tyler, TX). Through a partnership with the Houston Zoo, a captive-breeding flock is residing on the grounds of the NASA Lyndon B. Johnson Space Center near Clear Lake. In 2016, the population declined to 42 birds following heavy spring floods, which wiped out an entire generation of eggs. Hurricane Harvey in