|Scientific Name:||Ptisana purpurascens (de Vriese) Murdock|
Marattia purpurascens de Vriese
|Red List Category & Criteria:||Critically Endangered B1ab(iii)+2ab(iii) ver 3.1|
|Assessor(s):||Niissalo, M., Stroud, S., Gray, A., Lambdon, P.W. & Renshaw, O.|
|Reviewer(s):||Scott, J.A. & Hilton-Taylor, C.|
A single population of Ptisana purpurascens exists, which extends across the highest parts of the Green Mountain, Ascension Island, less than 0.5 km² in area. The number of mature individuals is known to be under a thousand. Some of these plants are large and consist of several stems, but most only consist of a single growth point.
The quality of the habitat, as well as the number of individuals of this species, is inferred to have declined in the past 20 years and this decline is projected to continue. Green Mountain was originally populated by a relatively impoverished native plant community, consisting principally of a small number of fern species, bryophytes and a few small grasses (Duffey 1964). Only a very small and decreasing area of open vegetation now remains on Green Mountain. This particular situation holds the majority of P. purpurascens in the population. Invasive species such as Buddleja madagascariensis, Rubus spp. and grasses are spreading in this part of the population. In other parts the invasive vegetation has largely replaced the vegetation type that supported this species, only leaving scattered plants or groups of plants.
Of more immediate influence is the spread of various herbaceous invasive plants, particularly grasses such as Paspalum scrobiculatum. Mature plants are capable of growing in a variety of conditions, and will not necessarily be readily outcompeted by species such as Buddleja. However, very little regeneration has been observed. Gametophytes and small sporophytes have been observed almost exclusively in the very few minor landslides which have not been quickly grown over by invasive herbaceous plants. These sites are restricted to a few square metres altogether at most, in places where surrounding thick growths of Alpinia zerumbet has stopped the spread of invasive grasses. There is also some evidence of regeneration in a bamboo thicket which has invaded the summit area of the mountain, however this is a minor part of the population, and the reproduction is inferred to be threatened by several invasive weeds which comptete with it in this habitat, e.g. Clidemia hirta, a very vigorous invasive shrub. The lack of recruitment was not known during the 2003 assessment. Young individuals of P. purpurascens are also browsed by rabbits, which are abundant in the area.
Ptisana purpurascens is a long-lived species which will only show fluctuations and trends in population size very slowly, but as the only population is heavily disrupted by invasive species and the most populous areas could degenerate rapidly as the older individuals are lost, it is important that the critical state of this species is acknowledged.
Due to very small area of occupancy P. purpurascens is also susceptible to catastrophic events such as landslides and unintended introduction of pests or disease.
|Previously published Red List assessments:|
|Range Description:||This species is known only from the central part of Ascension Island, South Atlantic Ocean. Ptisana purpurascens only occurs in Green Mountain above approximately 700 m. The majority of the population is in a very small area on steep, south and east facing slopes where it is exposed to the prevailing winds. These bring reasonably high amounts of moisture from incoming mists. The area has retained the last vestiges of a fern-dominated community similar to that observed by early visitors to the island, although it is increasing invaded by non-native weeds (AEIUO 2010). A few isolated patches are scattered in the highest parts of the SW slopes of the Green Mountain, where it is very rare. A small number of individuals have also established in the adjacent bamboo thicket at the summit, and under trees nearby. The total extent of occurrence is approximately 12 ha, and the area of occupancy less than 4.5 ha.|
Native:Saint Helena, Ascension and Tristan da Cunha (Ascension)
|Range Map:||Click here to open the map viewer and explore range.|
|Population:||The population has been subject to an annual survey (AEIOU, 2010). From these counts it has been estimated that the wild population consists of some 6-800 mature plants. Some of these plants, particularly at Buddleja Ravine, are large and consist of several crowns, which have been counted as separate individuals in surveys, but are best considered as large, mature individuals.|
|Current Population Trend:||Unknown|
|Habitat and Ecology:||“Ptisana purpurascens is found on the weather side of Green Mountain growing in the open grassland and amongst the dense stand of bamboo at the summit, from an altitude of approximately 700 to 859 m. Ptisana purpurascens is physically the largest of all the endemic species on Ascension and appears to be able to compete favourably with other exotic species, possessing a degree of phenotypic plasticity that may contribute to its survival (Cronk 1980). This plasticity is evident when comparing individuals amongst the bamboo near the summit with those on the more open grassland. The former tends to be smaller and darker, whereas the latter are lighter in colour and larger.” (quoted from Gray 2003)|
The reproductive ecology of P. purpurascens is poorly known. Ex-situ attempts to grow this species from spore have failed unless in vitro techniques with sugars have been utilised . Though inconclusive, this might suggest that the species requires or benefits from a symbiotic fungal partner. In-situ, very little reproduction has been observed underneath mature plants. Young plants have been seen in few recently opened land slide areas. These have been very small in total area, and restricted to localities without fast-growing seed-distributed invasive plants. Young plants are sometimes found amongst rich native bryophyte communities
|Use and Trade:||This species is not utilized.|
Habitat loss, invasive species and catastrophic events are the main threats to this species. The population is no longer in a natural state (Duffey 1964). Evergreen trees have been planted in much of the area where this species occurs. Under these only a small number of P. purpurascens survive. The areas not planted with trees are seriously degraded due to invasive vegetation. The degradation has continued until this date and is projected to continue in the future. Both invasive shrubs and herbaceous plants contribute significantly to the decrease in quality of habitat of P. purpurascens, particularly interfering with reproduction by removing bare ground which is needed for gametophyte establishment.
Ptisana purpurascens is susceptible to unpredictable events due to its very small area of occupancy. The main part of the population is in a single narrow ravine, where for example a land slide could do serious damage.
The entire population is included in Green Mountain National Park. It is surveyed by the Ascension Island Government Conservation Department, including an census of plant numbers conducted annually.
A propagation protocol for P. purpurascens has been created. The species is grown in a local nursery setting. Introduced plants have been planted out in small numbers, and it is hoped that this programme will be expanded in the future. Ptisana purpurascens is held in ex situ conservation at the Royal Botanic Gardens, Kew.
|Citation:||Niissalo, M., Stroud, S., Gray, A., Lambdon, P.W. & Renshaw, O. 2012. Ptisana purpurascens. The IUCN Red List of Threatened Species 2012: e.T43922A2990240.Downloaded on 24 September 2017.|
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