Thorny shrub from tropical America rapidly spreading in northern Australia and Thailand. In Australian floodplains it forms self-regenerating monotypic stands.
Spreading thorny shrub usually up to 2 m tall but occasionally to 6 m. Maximum age is about 5 years.
There are two varieties, var. pigra and var. berlandieri which differ slightly in pod pubescence and spine morphology. Only the former has spread around the world.
Flowering may start within a year of germination. The bee-pollinated inflorescences, containing up to 100 flowers, are spherical, pink and last one day. On main branches one inflorescence is produced daily for five months during the rainy season. In Australia it has been suggested that most of the seeds are produced by autogamy, although wind pollination can not be ruled out. The hairy pods are up to 15 cm long and contain 8 to 24 seeds (each seed is ca. 5 x 2.4 mm and weighs 0.09 mg). The fruits ripen in ca. 3 months and when mature fragment into indehiscent one-seed segments. The bristles covering the pods facilitate floating and enhance dispersal along river systems. In evergreen forests a few flowers and fruits are found throughout the year. In Australasia in permanently moist sites flowering occurs all year round and on average < 5% of flower buds produce seeds. Anthesis takes place about 8 days after bud formation and another month is necessary for the production of mature seed pods. In northern Australia the soil seed banks can reach up to 12 000 seeds m-1 and seeds remain viable for more than two years. Seeds generally germinate when they are first wetted and the germination rate is high.
In seasonally flooded areas fibrous adventitious roots develop around the base of the multiple stems. Mycorrhizae have been found associated with few strains of Rhizobium. However these associations are rare and it is not clear how important a source of nitrogen they are to populations of M. pigra. Following natural fires the plants resprout freely. The shrub does not naturally reproduce vegetatively.
Although adapted to seasonally flooded habitats M. pigra can regenerate under some degree of canopy cover.
It has been used as green manure and cover crop and is used as firewood and bean poles in Thailand. Utilized against snake bites.
Native to Mexico, Central and South America
Dry to seasonally wet.
In Costa Rica the M. pigra natural habitats are the banks of large rivers and marsh edges. Currently occupying similar habitats such as road sides and marshy spots in open pastures.
In Costa Rica M. pigra is probably much more common now than prior to European colonisation. Its seeds appear to be spread by road construction equipment and thus the plant is typical of roadsides.
Leaflets fed upon by a number of beetle species but avoided by cattle and horses. In Costa Rica, seeds are heavily predated by beetles.
M. pigra is widely distributed in Africa but unclear how it got there from Tropical America. Introduced, as an ornamental or seed contaminant, to the Darwin Botanic Gardens of Australia's Northern Territory prior to 1891. The spreading population is thought to be the result of a single introduction. During the second part of this century M. pigra has spread dramatically to form monotypic stands in floodplains. Introduced to Thailand in 1947 as green manure and cover crop and being prickly it was thought it would restrict access to water banks as thus reduce erosion. Now it covers large areas of standing waters and water banks. Also spreading in Indonesia and peninsular Malaysia. Long-distance dispersal is possible when cattle are transported and in Thailand road transportation spread seeds along highways.
Although introduced to Australia last century M. pigra remained an occasional nuisance around Darwin. It started to spread in the late 1950 when it reached the open floodplains and started to spread to increase dramatically in the 1970s. The time-lag resulted from the inability of the species to reach suitable habitats. The rate of population increase is high (r of ca. 0.66 per year). Once established as monotypic stands, M. pigra can regenerate under its own canopy. In these stands the half life of plants taller than 20 cm varies between 13 and 22 months depending on soil types. M. pigra can also regenerate under canopy of Melaleuca fringing the floodplains.
M. pigra is found in tropical regions with > 750 mm annual rainfall unless growing around dams and watercourses. It does not appear to have any soil type preferences, but is more commonly found in moist situations such as floodplains and river banks.
It invades sedgeland and grassland communities on open floodplains and Melaleuca forest fringing the floodplains. It is not found in tropical rain forest areas (rainfall > 2250 mm).
Ungulates have little impact on M. pigra and some post dispersal seed predation occurs.
M. pigra completely alters floodplain and swamp forest of northern Australia. Its main impacts are: fewer birds and lizards, less herbaceous vegetation and hinders tree regeneration. If the spread of M. pigra is not halted it may affect the touristic value of the Kakadu National Park as many visitors come to see the wetlands birdlife. In Thailand it interferes with irrigation systems (sediment accumulation) and access to electric power lines and is a safety hazard along roads. It also grows in fallow rice paddies making reclamation more expensive. It is a source of firewood and bean poles and its foliage can be used as an animal feed.
In Australia M. pigra poses a threat to the cattle industry as it invades buffalo pasture. It also restricts access to waterways for humans, particularly fishermen.
In Australia Neotropical biological control agents are being actively investigated as classical introductions while chemical control is used to contain the spread and eradicate new infestations. Aerial spraying of gelled gasoline followed by fire kills stands of M. pigra and soil surface seeds, however it enhances buried seed germination. Effective control is difficult because of the large soil seed bank.
In the floodplains of northern Australia there are no woody plants present.
The main difference appears to be habitat availability. In northern Australia large treeless floodplains exist which doe not appear to be the case in tropical America. A comparison of phytophagous insect fauna between Australia and Mexico showed that in the invaded region there were fewer insect species, more species were polyphagous and fewer feed externally. In Mexico many insects feed on new growth and inside the reproductive structures but such damage was not observed in Australia.