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The Cycad Pages
Ecology and Conservation

Cycad plants are long-lived and slow-growing, with slow recruitment and population turnover. The fleshy and starch-rich stems are highly susceptible to fungal attack, and almost all species grow in well-drained soils or sites. Habitats range from closed tropical forests to semideserts, the majority in tropical or subtropical climates in regions of predominantly summer rainfall. Cycads often occur on or are restricted to specialised and/or localised sites, such as nutritionally deficient sites, limestone or serpentinite outcrops, beach dune deposits or precipitously steep sites.

Contractile roots are present in all cycads, particularly in juvenile plants. These draw the sensitive growing apex of seedlings below the soil surface, affording protection against drought and the fires that are a frequent feature of many cycad habitats.

Coralloid roots also present in all cycads host symbiotic cyanobacteria, which fix atmospheric nitrogen and contribute to the nutrient needs of the plant. This provides an advantage in the nutritionally deficient soils of many cycad habitats.

Reproduction

The cycads have been generally thought to be wind pollinated. However, several recent studies in different regions indicate that cycads are mostly insect pollinated, often by weevils that are closely dependent on the cycads. This contrasts with both Ginkgo and the conifers (the other primitive seed plants), all of which are wind pollinated. Chemistry of the pollinator-attractants in cycads is markedly different from that of any flowering plants, suggesting that insect pollination has evolved independently in the two groups. Distribution is consequently constrained by the requirements of the pollinating organisms in addition to those of the plants themselves.

Dispersal

Cycad seeds are large, with a fleshy outer coat (sarcotesta) over a hard, stony layer (sclerotesta). The fertilised embryo develops slowly but continuously until germination, with short-term chemical inhibition of germination by the sarcotesta but no real dormancy. This makes seeds relatively short-lived and subject to damage by desiccation.

The fleshy sarcotesta attracts animals, mainly birds, rodents, small marsupials and fruit-eating bats, which serve as dispersal agents. In most cases, the fleshy coat is eaten off the seed and the entire seed is not consumed. Dispersal is consequently limited to the usually short distance that the animals can carry the seed.

Cycas subsection Rumphiae has seeds with a spongy endocarp not seen elsewhere among the cycads, which gives a potential for oceanic dispersal, and it has been demonstrated that seeds maintain viability after prolonged immersion in sea water. Subsection Rumphiae is the only subgroup of the genus to occur on oceanic islands, and is widely distributed through the Indian and western Pacific oceans, as well as all non-mainland parts of South-east Asia.

Conservation

Although a small group in overall numbers, the cycads attract a disproportionate interest for the following reasons:

This interest in cycads places pressure on the species in the wild, and creates threats to their continued survival. Threats are different in different parts of the world, and the IUCN Cycad Specialist Group is at the moment preparing a cycad action plan defining the different threats and proposing actions to counter them and ensure the continued survival of the cycads.


The Cycad Pages
1998 Royal Botanic Gardens Sydney
Written and maintained by Ken Hill
Please send comments and corrections