cycad - Origins, Taxonomy, History, Distribution, Speciation, Extinction, Conservation, Horticulture

A tropical or subtropical gymnosperm, palm-like in appearance, trunk usually unbranched, armoured with old leaf bases or scar-like remains, with a crown of tough, feathery leaves; flowers borne in separate male and female cones, the female very large. Cycads are considered to be the most primitive living seed-plants, appearing in the late Palaeozoic era, and thought to be related to seed-ferns, a fossil group dominant during the Triassic period. The slow-growing trunks have a large, starchy central pith which in some species yields sago. (Family: Cycadaceae, 100 species.)

iCycadophyta
Leaves and female cone of Cycas revoluta
Scientific classification
Kingdom: Plantae Division: Cycadophyta Class: Cycadopsida Order: Cycadales Dumortier
Families
Cycadaceae cycas family Stangeriaceae stangeria family Zamiaceae zamia family

Cycads are an ancient group of seed plants characterized by a large crown of compound leaves and a stout trunk.

Cycads are found across much of the subtropical and tropical parts of the world.

Origins

The cycad fossil record dates to the Early Permian, 280 mya. There is controversy over older cycad fossils that date to the late Carboniferous period, 300 -325 mya.

The family Strangeriaceae, consisting of only three extant species, is thought to be of Gondwanan origin as fossils have been found in Lower Cretaceous deposits in Argentina, dating to 70 – 135 mya. Cycadaceae is thought to be an early offshoot from other cycads, with fossils from Eocene deposits (38 – 54 mya) in Japan and China, indicating that this family originated in Laurasia. Cycas is the only genus in the family and contains 99 species, the most of any cycad genus. The current distribution of cycads may be due to radiations from a few ancestral types sequestered on Laurasia and Gondwana, or could be explained by genetic drift following the separation of already evolved genera.

Taxonomy

There are currently 305 described species, in 10-12 genera and 2-3 families of cycads (depending on taxonomic viewpoint).

The number of species in the clade is low compared to the number of species in other plant phyla, excluding Ginkgo biloba, the only remaining species in the phylum Ginkgophyta. Fossil evidence shows that structural diversity in Mesozoic cycad pollen "considerably exceeds that seen in surviving genera today". The disparity in molecular sequences is very high between the three main lineages of cycads, implying that genetic diversity in the clade was once high, but this fact has led to major disagreements about the divisions within the Cycadales.

The number of described cycad species has doubled in the past 25 years, mostly due to improved sampling and further exploration. Diversity hotspots also occur in Australia, South Africa, Mexico, China and Vietnam, which together account for more than 70% of the world’s cycad species.

Cycad systematists reject the biological species concept, as clearly defined cycad species can interbreed and produce fertile offspring; The phenetic species concept, which states that a species is defined based on overall similarities with other individuals of the same species combined with a significant gap in variation with other species, is also rejected. Most cycad taxonomists agree on a modified version of the evolutionary species concept, termed the ‘morphogeographic’ species concept, which recognises the combined effects of geographical isolation and morphological disparity. Thus the presence of large geographical gaps in cycad distribution has greatly affected the way cycads are classified.

Suborder Cycadineae Family Cycadaceae Subfamily Cycadoideae Suborder Zamiineae Family Stangeriaceae Subfamily Stangerioideae Subtribe Macrozamiinae

History

Modern knowledge about Cycads began in the 9th century with the recording by two Arab naturalists that the genus Cycas was used as a source of flour in India. The first report of cycads in the New World was by Giovanni Lerio in his 1576 trip to Brazil, where he observed a plant named ayrius by the indigenous people;

Cycads belonging to the genus Encephalartos were first described by Johann Georg Christian Lehmann in 1834.

Throughout the 18th-19th centuries, discoveries of several species were reported by numerous naturalist researchers and discoverers traveling throughout the world. His 15 years of travel throughout Africa, the Americas and Australia to observe cycads in their natural habitat resulted in his 1919 publication of The Living Cycads which remains a flowing and data-rich volume, and which remains current in its synthesis of taxonomy, morphology and reproductive biology of cycads, most of which was obtained from his original research.

Distribution

World distribution of Cycadales

Overall species diversity peaks at 17˚N and 28˚S, with a minor peak at the equator. Thus the distribution pattern of cycad species with latitude appears to be an artefact of the geographical isolation of cycad genera, and is dependent on the remaining species in each genus that did not follow the extinction pattern of their ancestors. Cycas is the only genus that has a broad geographical range and can thus be used to infer that cycads tend to live in the upper and lower tropics.

Speciation

There are no documented cases of sympatric speciation in cycads and allopatry appears to be the most common form of speciation in the group. Parapatric speciation may also have occurred, especially as cycads are pollinated by insects rather than by wind. If sympatric speciation has occurred in cycads this would most likely be because of a host shift in pollinators, due to the very fact that cycads are uniformly dioecious.

Extinction

The probable former range of cycads can be inferred from their current global distribution. However, the cycad fossil record is generally poor and little can be deduced about the effects of each mass extinction event on their diversity. It is likely that cycad diversity was affected more by the great angiosperm radiation in the mid-Cretaceous than by extinctions. Very slow cambial growth was first used to define cycads, and because of this characteristic the group could not compete with the rapidly growing, relatively short-lived angiosperms, which now number over 95,000 species, compared to the 947 remaining gymnosperms . The ability of cycads to survive in relatively dry environments where plant diversity is generally lower, and their great longevity may explain their long persistence.

Conservation

In recent years, many cycads have been dwindling in numbers and may face risk of extinction because of theft and unscrupulous collection from their natural habitats, as well as from habitat destruction.

23% of the 305 extant cycad species are either critically endangered or endangered, 15% are vulnerable and 12% near threatened. Thus 38% of cycads are currently on the IUCN Red List (2004), and the other 62% are in the Least Concern or Near Threatened category (i.e. Cycads are long-lived with infrequent reproduction, and most current populations are small, putting them at risk of extinction from habitat destruction and stochastic environmental events. African cycads are rare and are thought to be naturally decreasing due to small population sizes, and there is controversy over whether to let natural extinction processes act on these cycads.

All cycads are in the CITES appendix appearing under the heading Plant Kingdom and under three family names, Cycadaceae, Stangeriaceae and Zamiaceae.

All cycads are CITES APPENDIX II except the following, in APPENDIX I:

Cycas beddomei Stangeria eriopus All Ceratozamia All Chigua All Encephalartos Microcycas calocoma

Cycad seeds are not CITES regulated.

A 1997 the International Union for the Conservation of Nature and Natural Resources (IUCN), now known as the World Conservation Union, reported a list of over 150 threatened Cycad Species throughout the world that are indeterminate, rare, vulnerable, endangered or extinct.

Horticulture

Cycads can be cut up into pieces to make new plants, although the most environmentally responsible method is by direct planting of the seeds.

Some insects, particularly scale insects, some weevils and chewing insects can damage cycads, though the pests are susceptible to insecticides such as the horticulture soluble oil white oil.

While the cycads have a reputation of slow growth, it is not always well-founded and some actually grow quite fast, achieving reproductive maturity in 2 to 3 years (as with some Zamia species), while others in 15 years (as with some Cycas, Australian Macrozamia and Lepidozamia).