Lehua Islet is a small, rocky, volcanic island 1.2 kilometers north of Niihau and 31 kilometers west of Kauai. The volcanic tuff crater that formed Lehua 4.9 million years ago is now highly eroded and nearly half submerged, forming a steep, crescent-shaped island with an area of 117 hectares, a length of 2 kilometers, and a maximum elevation of 213 meters. The upper slopes of the inner crescent are composed of parallel strata that have eroded at different rates, producing a series of weathered ledges 1?2 meters wide and 1?2 meters high. The lower slopes of the inner crescent are eroded into chasms and fissures. The outer slopes are smoother, but have several eroded gullies that widen near the shore. Cliffs up to 55 meters high occur on the eastern and western points. The shoreline is rocky and often washed by large waves, especially in the winter. Lehua is in the rain shadow of Kauai and is very dry, especially during the heat of summer. Much of the island is bare rock, eroded sediment has collected only in gully bottoms, ledges, and small caves. Vegetation is sparse but many plants have a growth spurt after winter rains. Rabbits and rats introduced over 70 years ago severely altered the islet's ecosystem by decimating native plants, allowing alien plants to dominate, and impacting smaller seabird species. The eradication of feral rabbits in 2005 and the planned eradication of rats in 2008 could cause major shifts in ecology of the island and a monitoring program is in place to track these changes. Lehua is owned by the U.S. Coast Guard and managed by the State of Hawaii as a seabird sanctuary. There is no evidence of permanent human habitation, but several stone structures built by ancient Hawaiians are present, and Hawaiians probably visited Lehua to fish and to harvest seabirds, feathers, and eggs. The federal government built a lighthouse on Lehua in 1932 that was replaced by a solar powered light in 1989, which is maintained by the Coast Guard.
Lehua Islet is important for the number and diversity of breeding seabirds it supports and for the presence of several seabird species that are rare or have restricted breeding ranges. Recent surveys documented over 25,000 pairs of 11 seabird species nesting or attempting to nest on Lehua. Wedge-tailed shearwaters are the most numerous species on the island, with an estimated 23,000 pairs. The Brown Booby colony on Lehua is the largest in the Hawaiian Islands with 521 breeding pairs, and the Red-footed Booby colony is one of the two largest in the Hawaiian Islands, with 1294 pairs and approximately 4288 total individuals. The colonies of Laysan Albatross (28 pairs, 93 total individuals) and Black-footed Albatross (16 pairs, 53 total individuals) are small but appear to be growing and are important because these species have restricted breeding distributions and Lehua is one of the few high islands where they nest. Other species nesting on the islet include Christmas Shearwater, Bulwer?s Petrel, Red-tailed Tropicbird, and Black Noddy. Newell?s Shearwater, which is listed as threatened under the U.S. Endangered Species Act, and Band-rumped Storm-Petrel, a candidate for listing, attempt to nest on Lehua in small numbers, but predation by alien rats makes it difficult for these small seabirds to nest successfully. These species appear to be declining in Hawaii and may be difficult to manage on the larger Hawaiian Islands. Offshore islets such as Lehua may become increasingly important in the conservation of these species because their small size makes it more feasible to eradicate predators and manage other threats.
The most serious threat to seabirds on Lehua Islet is predation by introduced Polynesian rats. Plans are underway to eradicate rats, which should improve nesting success for smaller seabirds in particular, and possibly allow additional species to colonize the island. Feral rabbits formerly occurred on Lehua but were recently eradicated, which will benefit seabirds by preventing loss of plants that provide nesting platforms and reducing erosion that could bury seabird burrows. Smaller seabirds are also vulnerable to predation by introduced Barn Owls, which visit Lehua regularly from nearby Niihau. Introduced Cattle Egrets that nest on Lehua and commute from Niihau may prey on seabird chicks and compete for nest sites with Red-footed Boobies. Seabird eggs and small chicks are sensitive to overheating, and flushing of adults from nests by humans can cause nests to fail. Lehua is not often visited by people because of its relatively remote location, wave-washed rocky shoreline, and steep terrain, but human disturbance is known to occur. Occasional visits by Coast Guard helicopters for maintenance of the navigation aid are also known to cause nest failure, but methods to minimize the impacts of such disturbance are being negotiated with the Coast Guard. Another possible threat is the introduced big-headed ant, which occurs in large numbers and is suspected to prey on native insects, plant seeds, and possibly seabird chicks. Because it is a high island, Lehua Islet is less threatened by rising sea level than the Northwestern Hawaiian Islands, many of which are low coral atolls.
Lehua Islet is completely owned by the U.S. Coast Guard, but it is managed by the State of Hawaii as a seabird sanctuary. Permission to land on the island must be obtained from both the Coast Guard and the State of Hawaii.
Lehua Islet is hot, dry, and rocky, and supports sparse vegetation consisting primarily of coastal shrubland, herbland, and grassland that is dominated by non-native species. The steeper areas support little vegetation, with only occasional plants clinging to small pockets of soil or growing in cracks in the bare rock. The more level benches and gully bottoms that retain some eroded sediment support mixed shrubs, herbs, and grasses. Non-native buffel grass (Cenchrus ciliaris) is the most abundant plant and dominates many slopes. The benches on the inner crescent and gully slopes on the outer crescent support low thickets of the introduced Indian fleabane (Pluchea indica), and these provide the main nesting platform for most Red-footed Boobies. The non-native shrub Abutilon grandifolium occurs in gulch bottoms on the outer crescent, and these are also used for nesting by Red-footed Boobies. The most common native plants include the endemic morning glory pa?uohi?iaka (Jacquemontia ovalifolia) the endemic grass hakonakona (Panicum torridum), and ilima (Sida fallax). Much of the vegetation dies back during the hot, dry summer months. Winter rains trigger a greening of the island, and many plant species may suddenly appear at this time. Water percolating down through the rock forms several small but apparently perennial seeps on the lower slopes of the outer crescent, and these support algal mats and clumps of the indigenous sedge Cyperus javanicus.
All of Lehua Islet is managed as a seabird sanctuary by the State of Hawaii. Conservation efforts are underway to restore populations of native plants and animals and remove alien species. Several tour companies on Kauai offer day trips that visit offshore waters of Lehua for snorkeling, whale-watching, and sight-seeing. Landing is allowed on the shoreline, but access to the rest of the island is prohibited without a permit. The Coast Guard maintains a solar-powered, lighted navigational aid at the summit of the island, which is serviced occasionally by helicopter.