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.
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.
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
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.
Although a small group in overall numbers, the cycads attract a disproportionate interest for the following reasons:
- They are an ancient group with a long history. There are few living survivors, and fossil records date back to over 250 million years old. As such, cycads have great intrinsic interest, both to the scientific community and to the general public, as a survivor from eras long past, and as a possible window on life in those times.
- Cycads are of cultural and religious significance to many different peoples around the world.
- Cycads have considerable economic importance in horticulture.
- Cycads contain unique and potent toxic compounds that have caused problems to people and livestock in contact with them, and are of interest to the pharmaceutical industry.
- The cycads are the sister group to all other extant seed plants. An understanding of the evolution of the cycads is important in understanding the evolution of all modern plants.
- Cycads fix atmospheric nitrogen, important in maintaining health and fertility of soils. The fixation mechanism is through a cyanobacterial symbiosis that is unique to the cycads.
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.