This species is listed as Endangered because it has a restricted range (extent of occurrence is approximately 4,400 km²), it is known from only a few locations (less than five), and there is a continuing decline in the extent and quality of its habitat due to invasive grasses, fire, and urban development. Surveys are required to determine the population size; if there are found to be fewer than 50 mature individuals as in 2003, it would once again qualify for listing as Critically Endangered.

Leptocereus quadricostatus is found in Puerto Rico and in the British Virgin Islands, on the island of Anegada (Hunt et al. 2006). It occurs at or near sea level. There are two locations of this species in Puerto Rico (one of which is a small area in the dry southwest region of Guánica) and one in Anegada. The Anegada subpopulation is restricted to an area of less than 100 m x 100 m on one of the limestone cays of the Western Salt Ponds.

There are no current data on global population size for this species, but it is thought to be rare and in decline.
The Anegada subpopulation was surveyed in about 2002/2003 and at that time was found to comprise 20-25 individual clumps. The dense clumping nature of this species makes a precise determination of number of mature individuals very difficult, but 20-25 is a reliable estimate. Pedro Acevedo (pers. comm. 2003) said that the Puerto Rico subpopulation was less than 10 giving a global population size of approximately 35 mature individuals. This number might now be higher as there appears to be a second locality in Puerto Rico.

An erect or arching shrub found in coastal dry forests and shrubland. The Anegada subpopulation occurs on open limestone with very little soil. No study of the breeding biology has been undertaken, but individuals have been seen in flower and fruit on Anegada.

The main threats to this species are invasive grasses, fire, and tourism development.
Anegada is under severe development pressure resulting in both loss of habitat to residential and tourism infrastructure, and further fragmentation is expected due to upgrading and construction of new roads. However, the location of L. quadricostatus is in an area unlikely to be damaged from this type of development.

The highest point on Anegada is approx. 10 m above sea level. Most of the habitat for L. quadricostatus is <2 m above sea level and so global climate change will reduce the quality and area of habitat available to L. quadricostatus. Natural disasters are a current and on-going threat e.g., hurricanes, coastal inundation and earthquakes.
The Puerto Rican subpopulations are in areas zoned for residential development which may threaten the surviving plants.

It is not known if this species is used or traded.

This species is present in two protected areas in Puerto Rico (Guanica State Forest and Cabo Rojo). The Anegada subpopulation occurs on a limestone cay within the Western Salt Ponds Ramsar site (declared in 1999). The species is listed on CITES Appendix II.