Palmyra’s terrestrial habitats support one of the largest remaining stands of Pisonia beach forest in the Pacific and several other native varieties of ferns and shrubs. The Atoll is also home to the world’s largest landbased invertebrate, the coconut crab, so named because of its ability to crack open a coconut with its huge claws.
The land crabs at Palmyra are considered one of the most unique species groups at the atoll. Six species of terrestrial crabs have been reported at the Palmyra Atoll. The most abundant of the terrestrial crab species is the red hermit crab (Coenobita perlatus). It occurs in high densities around the atoll, with the largest concentration in the coconut palm forest type. The other common hermit crab is purple (Coenobita brevimanus) and largely resides in the Indian almond forest. Crabs are omnivorous, generally nocturnal scavengers that feed primarily on land plants and on detritus along beaches.
The two species of burrowing land crabs are the orange land crab (Cardisoma carnifex) and the purple land crab (Cardisoma rotendum). These land crabs are restricted to the low lying and shoreline areas where they excavate large burrows. They are significant players in the food web, consuming a wide assortment of items ranging from vegetation and coconuts to fallen eggs and chicks.
Significant changes to the vegetative structure have been noted throughout the years at Palmyra. Of the 117 distinct vascular plant species identified on Palmyra Atoll, approximately 27 are determined to be native, 45 are considered naturalized, and the remainder are vegetables or ornamentals purposely introduced. Three main vegetation types occur on the islands including: 1) native wet Pisonia forest, 2) introduced coconut forest, and 3) native beach naupaka.
Pisonia trees are considered the redwoods of the Pacific. Mature Pisonia trees range between 70 to 100 feet in height, form characteristic buttress-like roots, dense spreading canopies, and an absence of undergrowth in mature stands.
Increased human population and development have caused Pisonia forests to degrade or disappear throughout their historic range. Palmyra had one of the largest remaining undisturbed Pisonia forests in the Pacific. Since 2001, Pisonia grandis has continually shown signs of canopy loss at Palmyra. This decline is attributed to a severe infestation by a Caribbean scale insect. It is unknown exactly how the scale promotes canopy loss. One theory is that the high scale density on the leaves inhibits photosynthesis. Another theory is that the scale insects are a secondary cause due to a weakened condition of the trees.
The decline of Pisonia has caused dramatic changes in the ecosystem at Palmyra Atoll NWR. The Pisonia forests are extremely important to Palmyra’s terrestrial and marine ecosystems. Providing nesting and roosting sites for numerous seabirds, they also promote guano fall fron the seabirds roosting in Pisonia that serves as a fertilizer source and supply food and nutrients supporting life in the understory.
Tournefortia and naupaka occur mainly around the shoreline areas on nearly all the refuge islets. Tournefortia is more common in exposed areas on the ocean edge of the islands, while naupaka is more dominant in the protected areas such as the outer borders on the lagoon side. This vegetation type provides nesting and roosting sites for the red-footed booby and the great frigatebird.
Coconut Forest (Cocos) distribution has continued to increase at Palmyra Atoll NWR because of previous tree planting as part of a failed copra enterprise at Palmyra between 1979 and 1987, historic World War II construction activities, and nonnative plant introduction. Cocos occur in very dense groves on Palmyra and are interspersed with occasional native tree species. Cocos are extremely prolific and an aggressive colonizer. An estimated 138,000 Cocos seedlings are growing throughout the atoll.