Cabomba Biology
Growth
Cabomba is a fast growing plant. Growth rates of 5 cm per day have been recorded in Queensland and are likely to occur in the NT. Growth generally increases with increasing light, temperature, nutrients and dissolved inorganic carbon. However, growth rates may decline above 27°C due to increasing respiration. Growth is also best in slightly acidic waters (pH 4-6) and is significantly inhibited above a pH of 8. In Townsville, high growth rates have been recorded in April and May. At Darwin River, growth also seems high during September.
Flowering and Seed Biology
The cabomba infestations in the NT are the first in Australia to produce seed. Although flowering occurs in other states, seed production is not evident. This inconsistency is yet to be fully explained, but may be related to climatic variables or to variations in DNA.
At Darwin River, flower production is possible throughout the year. However, sexual reproduction may be impeded during the wet season. Although high growth rates can initially match rising water levels, flowers may eventually be drowned. Also, increased water flow creates drag on plants, which can result in stem damage and a reduction in plant height. It is likely that cabomba can produce seed for 7-12 months of the year.
 Cabomba flowers in high density. (jpg 116Kb) |
Mature infestations are capable of producing a large number of flowers and seeds. Individual flowers are short-lived, emerging from the water for just two days (but falling back into the water during the intervening night). After these two days they no longer emerge. If the flowers have been pollinated, the stem of the flower will coil and the carpels of the flower will begin to swell up. After a few weeks the carpels will detach from the flower stalk, sink and decompose, exposing mature seeds. Where plants are well established, as many as 50 flowers can be produced per m 2 per day. Each flower is capable of producing as many as 9 seeds, although average production at Darwin River has not been assessed.
Pilot seed viability tests suggest that at one month of age, 100% of seeds produced are capable of germinating. This drops to 76% after 4 months. Other studies suggest that a small percentage of seeds will remain viable for at least 2 years. Despite the high rates of seed viability, few seeds have actually germinated during shade-house experiments. This outcome suggests that the seeds are dormant. The conditions required to break seed dormancy are not known. Research is currently endeavouring to improve our knowledge of seed germination and longevity.
Spread
The main vector for the spread of cabomba is humans. Cabomba has been spread around the world by the aquarium plant trade and this is how it was introduced into the NT. It is also likely that humans are responsible for introducing cabomba to Marlow Lagoon and Darwin River, either deliberately or accidentally. Now that it inhabits a river community, other mechanisms of dispersal are also present. However, humans remain the most likely vector for further expansion.
The distribution of cabomba will increase if a propagule is transported to an area that represents suitable cabomba habitat. To understand the potential for spread, we need to consider how many propagules are produced, and whether transportation is likely.
 Cabomba fragment. Fine roots grow from leaf nodes and attach to the soil (jpg 75Kb) |
 Cabomba seed 3 mm long (jpg 29Kb) |
 Cabomba attached to trailer after entering infested waters in Queensland (jpg 140Kb) |
A propagule is any part of a plant capable of producing a new plant. In the case of cabomba, the propagules are stem fragments and seeds. Stem fragments as small as 1 cm and with only one pair of leaves can take root and grow into new plants. Infestations are generally abundant with stem fragments because of the large amount of plant biomass present and the fact that the stems break up easily when disturbed. Anything that moves through the water can break up plants, including fishing lures, fishing traps, humans and other animals. Outboard motors are capable of producing thousands of individual fragments just by themselves. Large infestations are also able to produce vast quantities of seed. Therefore, it is evident that cabomba is able to produce a massive amount of propagules, especially if infestations are disturbed. This trait helps to make it a highly invasive species that can quickly take over affected water bodies.
Propagules can be transported in any number of ways. Stem fragments and perhaps detached carpels can float and can therefore be moved by water currents and by wind across the water surface. Because seeds do not float, movement by these methods is not likely to be significant, expect perhaps during periods of high water velocities. Propagules can also be transported by humans and other animals. If a propagule is attached to or contained within something that comes out of the water, such as a canoe, a net, an animal or some soil, this may be transported to another water body. Cabomba fragments are sensitive to desiccation and may die during transportation. However, if the propagule is not sufficiently desiccated, it may establish in this new water body. Seeds appear to be less sensitive to desiccation and are therefore more likely to survive transport. It should be noted that seeds do not have a specialised method of attachment such as bristles, and are thought to die during passage through an animal’s gut.
Despite some shortcomings that may help to limit spread, the large number of propagules produced and the availability of transport vectors suggests that the potential for spread is significant.
Habitat
Cabomba requires permanent freshwater and will not survive extended desiccation. It tends to establish in slow moving or stagnant water bodies with silt soil and is less common in fast flowing areas with a sand base. It may also require the water to have a pH of less than 8, which is generally the case in the Top End, except in some rivers fed by dolomite aquifers during the dry season. It can establish in water bodies greater than 3 m deep, although depth suitability will also be influenced by the amount of light reaching the water and water turbidity. Cabomba is generally quite tolerant of shade.
Competitiveness
The ability to establish in an area of suitable habitat often depends on the level of competition for resources such as light and nutrients. It is more difficult to establish in areas already covered with plant life. However, competition can be reduced when the plant communities are disturbed, which can happen when wet season floods wash competitors from the soil. The ability to successfully re-colonise these areas will increase with increasing propagule frequency and increasing growth rates. Cabomba infestations typically exhibit both of these traits and will therefore offer strong competition for native species. Cabomba may even be able to displace well established native populations through allelopathic activity that inhibits the growth of neighbouring plants. Also, cabomba may have higher tolerance levels to low light and high water velocities compared to some native species. Therefore, it may be able to grow in areas where there is little competition.
Predators
Cabomba has very few predators. The only organisms known to eat cabomba are exotic grass carp, which are also a pest, manatee and possibly some waterfowl. It seems that most invertebrates require the plants to senesce before they can feed on it.
