Size range and habitat

The ferns are extremelydiverse inhabitat, form, and reproductive methods. In size alone they range from minute filmy plants only 1–1.2 cm (0.39–0.47 inch) tall to hugetree ferns 10 to 25 metres (30 to 80 feet) in height. Some are twining and vinelike; others float on the surface of ponds. The majority of ferns inhabit warm, damp areas of theEarth. Growing profusely in tropical areas, ferns diminish in number with increasingly higher latitudes and decreasing supplies of moisture. Few are found in dry, cold places.

Some ferns play a role inecological succession, growing from the crevices of bare rock exposures and in openbogs andmarshes prior to the advent offorest vegetation. The best-known fern genus over much of the world,Pteridium (bracken) is characteristically found in old fields or cleared forests, where in most places it is often succeeded by woody vegetation.

Distribution and abundance

Geographically, ferns are most abundant in the tropics.Arctic andAntarctic regions possess few species. On the other hand, a small tropical country such asCosta Rica may have more than 900 species of ferns—about twice as many as are found in all ofNorth America north ofMexico. The finest display of ferndiversity is seen in the tropicalrainforests, where in only a few hectares more than 100 species may be encountered, some of which may constitute a dominant element of the vegetation. Also, many of the species grow asepiphytes upon the trunks and branches of trees. A number of families are almost exclusively tropical (e.g.,Marattiaceae,Gleicheniaceae,Schizaeaceae, Cyatheaceae,Blechnaceae, andDavalliaceae). Most of the other families occur in both the tropics and the temperate zones. Only certain genera are primarily temperate and Arctic (e.g.,Athyrium,Cystopteris,Dryopteris, andPolystichum), and even these tend to extend into the tropics, being found at high elevations onmountain ranges andvolcanoes.

Ferns are uncommon asinvasive species outside of their native ranges, although a few occur. The mostnotorious isbracken (Pteridium), which spreads quickly by its underground ropelikerhizome, rapidly invading abandoned fields and pastures in both temperate and tropical regions. One species of water spangles (Salvinia auriculata) became a major pest inIndia, blocking irrigation ditches and rice paddies. Another species (S. molesta) within three years covered 520 square kilometres (200 square miles) of the artificialLake Kariba in southern Africa, cutting off light and oxygen and thus killing otherplant life and fish. Some fern species have been introduced into tropical or subtropical areas (e.g., southernFlorida andHawaii) and in some cases have become naturalized and have spread into the native forest. Examples include the giant polypody (Microsorum scolopendrium),climbing ferns (Lygodium japonicum andL. microphyllum), greencliff brake (Cheilanthes viridis), silver fern (Pityrogramma calomelanos), Japanese holly fern (Cyrtomium falcatum), rosy maidenhair (Adiantum hispidulum), Cretan brake (Pteris cretica), and ladder brake (P. vittata). Two Old World species (Cyclosorus dentatus andMacrothelypteris torresiana) were introduced into tropical America beginning about 1930 and now are among the most common species even in some remote areas.

Britannica Quiz

Plants: From Cute to Carnivorous

Because of their ability todisperse byspores and their capacity to produce both sex organs on the samegametophyte and thus to self-fertilize, it would seem logical to assume that ferns possess higher powers of long-distance dispersal and establishment than doseed plants. Although genetic tests have shown that many, if not most, fern species tend to have an outcrossing breeding system, some other species are involved in the case of ferns with remote disjunctions—separated growing regions. There are interisland and intercontinental disjunctions, east and west, as well as wide north-south disjunctions including species found in the Northern and Southern hemispheres that skip the tropics. Some disjunctions seem to follow the pattern ofprevailing winds; the main centre of distribution of a species often may lead to downwind groups consisting of one or a few small populations sometimes hundreds or thousands of kilometres away. Examples of species exhibiting west-to-east transcontinental disjunctions in North America are Wright’s cliffbrake (Pellaea wrightiana), mountain holly fern (Polystichum scopulinum), and forked spleenwort (Asplenium septentrionale); all of these ferns are well known in the westernUnited States, and they exist as tiny populations in the mountains of the eastern states as well. Some species are disjunct between continents, such as betweenNew ZealandandSouth America (Blechnum penna-marina andHypolepis rugosula) orSouth Africa and Australia and New Zealand (Todea barbara). Some disjunct patterns, such as similar plants growing inAsia and in eastern North America, are not the result of long-range dispersal but rather are the remnants of an ancientcontinuous flora, the intervening areas having been changed over time.