Cycads are gymnosperms arising from an ancient lineage, and are considered to be the most primitive of the living seed-bearing plants (Norstog 1997, Jones 2002). Certain aspects of their biology and life cycle, are combining with human activities to push them to the edge of extinction (Norstog 1997, Jones 2002, Terry 2008).
Cycads have been around for at least 250 million years (Raven 1999), but they have been rapidly declining in numbers in recent years. The cycads belong to a large class of trees known as the gymnosperms, which are non-flowering seed-bearing plants with “naked seeds” in their ovules that are directly exposed to pollen grains (Norstog et al. 1997, Everett). The gymnosperms are comprised of eight orders, with three extinct (Cordaitales, Bennettitales and Glossopteridales) and five still living (Gingkoales, Cycadales, Pinales, Taxales and Gnetales) (Jones 2002). The nearest relatives are thought to be the long extinct Pteridospermales (seed ferns) (Norstog 1997). At first glance, they can appear similar in form to palms, and in fact, in America are sometimes called sago-palms (Everett 1968). But, they are more closely related to ferns than to palms (Everett 1968).
Cycads flourished during the Mesozoic era (Snow 2007), reaching the peak of their evolution by the Jurassic (Norstog 1997). There are now believed to be upwards of 300 species (Donaldson 2004, Terry 2008) with 10 subspecies in 11 genera (Jones 2002).
The following are biological factors that predispose cycads to extinction: habitat degradation and fragmentation, obligate insect pollinators, large seed size, and a dioecious life cycle.
Habitat Degradation and Fragmentation:
The world distribution for cycads is mainly tropical, subtropical to warm temperate regions of both the Northern and Southern hemispheres, with the region of greatest diversity occurring in Central America (Jones, 2002). They are also usually restricted to areas within roughly 30 degrees on either side of the equator (Norstog 1997), but can occur in a variety of habitats, such as moist rainforests, deciduous mesophyll and evergreen sclerophyll forests (Jones, 2002).
However, cycads are usually restricted to particular climates and soil types (Jones 2002). For example, Zamia Montana, is a mist forest cycad restricted to mist forests in Columbia (Jones 2002). Therefore, any degradation or fragmentation to these habitats is troublesome. For example, habitat degradation can lead to an increase in genetic drift, inbreeding, and can negatively impact interactions with pollinators and dispersers (Lopez-Gallego 2010, Terry 2008). This can be particularly problematic for rare species, such as Dioon sonorense from Sonora, Mexico, due to high genetic differentiation, low diversity and limited to no recruitment of juveniles (Gonzalez-Astorga 2009).
Insect Pollination and Seed Dispersal:
It was long thought that cycads were predominantly wind pollinated (Norstog 1997, Jones 2002), but recent studies have shown that they are almost exclusively insect pollinated (Jones 2002, Terry 2008). It was also found that insect pollinators, such as beetles, weevils and thrips, are often involved in obligate mutualisms with their host cycads (Jones 2002, Terry 2008), and that these relationships are ancient, dating back more than 200 million years ago (Jones 2002). However human activities, including the use of pesticides, overcollection, habitat degradation and fire are leading to the loss of these host-specific pollinators (Jones 2002, Terry 2008, IUCN 2003).
Cycads also have large seeds when compared to other gymnosperms, with the largest cycad seeds belonging to Macrozamia species (Jones 2002). In these cases, faunal extinctions can signal the loss of important seed dispersal mechanisms, as in the case for Macrozamia lucida (Snow 2007).
Another paper looked at Macrozamia platyrhachis of Queensland, Australia (Terry 2008). This species has a long juvenile stage of 5 to 20+ years, along with low seedling establishment and localized seed dispersal. This is another reason why habitat destruction and disturbance is problematic, as it can take a long time to re-establish viable adult populations.
Dioecious Life Cycle:
Cycads are generally dioecious (Raven 1999, Jones 2002, Norstog 1997), meaning that both male and female plants have separate forms. This is beneficial for genetic outcrossing (Norstog 1997), but has negative implications for conservation for various reasons.
In the dioecious life cycle, both male and female plants need to be within the same vicinity for genetic outcrossing to occur. Therefore, at least two separate dispersal events are required if establishment of a new colony is to be successful (Jones 2002).
The dioecious nature of cycads also makes them vulnerable to overcollection of one sex or another, which can lead to sex imbalances. For example, in the case of Cycas beddomei, of the Eastern Ghats in India, over-collection of male cones for medicinal purposes is threatening this species (Jones 2002).
Human Issues Affecting Conservation:
The biological aspects described above, are combining with human activities to further drive cycads toward extinction. Examples of human activities include commercialization of cycad products such as starch from stems and seeds, illegal poaching for the nursery trade, and traditional uses (such as medicinal and religious (Perez-Farrera 2006)). Habitat fragmentation and disturbance is due to agriculture and fire, the construction of highways and dams, mining and other activities. (Jones 2002, Donaldson 2004, Perez Farrera 2006, Norstog 1997).
Solutions and Conservation Management Strategies:
Many efforts are being made to slow or stop the extinction of cycads. These include networks and rescue schemes (such as “Operation Wildflower”) along with in situ and ex situ conservation strategies (Jones 2002, Whitelock 2002).
Other efforts include the creation and enforcement of local and international laws, such as CITES and IUCN red listing, which are designed to limit the import and export of rare and endangered cycads (Norstog 1997, Whitelock 2002, Jones 2002). IUCN red listing and taxonomy efforts increase public awareness of vulnerable species, and help to prevent illegal trade (Donaldson, 2004). Another important effort is educational programs for the general public and of indigenous peoples who do not know the value of the plants being destroyed (Whitelock 2002, Jones 2002).
In conclusion, cycads are beautiful, ancient plants that can offer public enjoyment and insight into an ancient era. However, their biology and human activities are combining to make them extremely vulnerable to extinction. Hopefully, with greater public awareness and recent conservation management strategies, cycads will be on earth for many more millennia to come.
Books and Journals:
- Alvarez-Yepiz, J., Dovciak, M., Burquez, A. (2011). Persistence of a rare ancient cycad: Effects of environment and demography. Biological Conservation, 144: 122-130
- Donaldson, J., (2004). Saving Ghosts? Concepts and Recommendations: Chapter: Saving Ghosts? The Implications of Taxonomic Uncertainty and Shifting Infrageneric Concepts in the Cycadales for Red Listing and Conservation Planning. In Osborne, T., Walters, R., Cycad classification: concepts and recommendations. Cambridge, MA: CABI Publishing.
- Everett, T.H. (1968). Living trees of the world. New York: Doubleday & Company
- Gonzalez-Astorga, J., Vovides, A.P., Cabrera-Toledo, D., Nicolalde-Morejon, F. (2009). Diversity and genetic structure of the endangered cycad Dioon sonorense (Zamiaceae) from Sonora, Mexico: Evolutionary and conservation implications. Biochemical Systematics and Ecology, 36: 891-899
- Jones, D.L. (2002). Cycads of the World: Ancient Plants in Today’s Landscape. Washington D.C.: Smithsonian Institution Press.
- Lopez-Gallego, C., O’Neil, P. (2010). Life-history variation following habitat degradation associated with differing fine-scale spatial genetic structure in a rainforest cycad. Popul Ecol, 52: 191-201
- Norstog KJ, Nicholls T. (1997). The Biology of the Cycads. Ithaca, New York: Cornell University Press.
- Perez-Farrera, M., Vovides, A.P., Bol.Soc.Bot.Mex. (2006) The commercial use of the threatened “Espadana” cycad (Dioon merolae, Zamiaceae) by a community of the central depression of Chiapas, Mexico, Bol.Soc.Bot.Mex.,78: 107-113
- Raven, P.H., Evert, R., Eichhorn, S.E. (1999) Biology of Plants, Sixth edition. Freeman, Worth.
- Snow, E.L., Walter, G.H. (2007). Large seeds, extinct vectors and contemporary ecology: testing dispersal in a locally distributed cycad, Macrozamia lucida (Cycadales). Australian Journal of Botany, 55: 592-600
- Terry, I., Forster, P., Moore, C.J., Roemer, R.B., Machin, P.J. (2008). Demographics, pollination syndrome and conservation status of Macrozamia platyrhachis (Zamiaceae), a geographically restricted Queensland cycad. Australian Journal of Botany, 56: 321-332
- Whitelock, L.M., (2002). The Cycads. Portland, OR: Timber Press
- “The Cycad Society, Inc.” www.cycad.org/conservation.htm (Publications section, February 2012). Publication: IUCN News Release, (2003), Modern Lifestyle Threatens Oldest Seed Plants on Earth.
- picture credit: http://www.iucnredlist.org/apps/redlist/details/41881/0