Thelypteris
Thelypteris dentata 
(Forsskål) E. St. John has the distinction of being one of the very first alien species to have naturalized in Hawai'i. W. E. Safford, botanist and lieutenant in the U.S. Navy, made a collection of it on O'ahu in 1887, and in 1897 Heller reported it to be common on that island (see Wagner 1950). It is now indeed common on all of the main islands, particularly in disturbed areas, most noticeably along roadsides and trails. It can also be found growing in large stands as a ground cover in disturbed forested areas. This species has become one of the most commonly encountered ferns along Hawaiian trails.
Thelypteris parasitica (L.) Fosberg evidently did not reach the islands until a couple of decades later; it was first collected in 1926. It is now widely found throughout the islands, often in association with T. dentata, and is now much more common than T. dentata. 
Wherever the two species occur together they commonly form hybrids. This hybrid was described by W. H. Wagner (1993) as Thelypteris xincesta W. H. Wagner, and at the same time he published a key to help in distinguishing the three taxa.
Thelypteris dentata also hybridizes with the Hawaiian endemic Thelypteris cyatheoides (Kaulfuss) Fosberg. The hybrid is triploid and forms large, widely scattered clones. It has been named Thelypteris xpalmeri W. H. Wagner (1993); while it resembles T. dentata it is larger and has pinnae with more lobes that are less deeply incised. It should be sought wherever the two species coexist.
