|Scientific Name:||Asplenium ascensionis|
|Taxonomic Notes:||Very close to Asplenium erectum, which occurs in tropical Africa and on the island of St. Helena 800 miles to the south. Asplenium ascensionis frequents drier, more rocky habitats, and never attains the size and luxuriance A. erectum, although in one small area of St. Helena (known as “the Barn”), A. erectum grows in a diminutive form on dry rocks, where it is extremely close in appearance to A. ascensionis. A genetic study is needed to assess the true level of separation between the species.|
|Red List Category & Criteria:||Vulnerable B1ab(iii)+2ab(iii) ver 3.1|
|Assessor/s:||Lambdon, P.W., Stroud, S., Gray, A., Nissalo, M. & Renshaw, O.|
|Reviewer/s:||Hilton-Taylor, C. & Scott, J.A.|
It is not unreasonable to regard all Asplenium ascensionis sites as part of a single meta-population. Ascension is a very small island where all subpopulations could easily be recolonized by spores. Due to the similarities of climate, vegetation, micro-habitat and edaphic conditions across all known localities, they are likely to be affected jointly by catastrophes such as severe weather, volcanic activity, disease outbreaks etc.
There is little historical documentation of the distribution of ferns on Green Mountain, so any supposed changes to distribution are largely inferred from observations of the authors. Ascension Island has been colonized by numerous invasive species over the past 150 years, and these species now dominate the landscape. The upper slopes of Green Mountain are almost completely clothed in thickets of dense, non-native species, and this has undoubtedly resulted in massive declines in the cover of native vegetation, principally through competitive exclusion. A strong reduction in the abundance of Asplenium ascensionis has certainly occurred since the mid 19th century, when it was described by Joseph Hooker as forming a carpet on the ground under other ferns and shrubs. It is now almost completely excluded from such habitats and confined to rock faces. Even in these refuges, many apparently suitable areas are now overwhelmed by weeds. There is no evidence to suggest that this advance has ceased, and the key competitor species are almost certainly continuing to spread to new areas of the mountain. In view of the potential threat this poses to Asplenium ascensionis, it seems realistic to assume a slow, long-term decline.
A detailed annual population census started in 2002, although new patches have been found every year since then and it is not possible to distinguish these finds from genuine increases or declines in numbers. However, it is now thought that most of the major areas have now been identified and mapped, and thus the latest counts represent a reasonably accurate assessment of the population size. It seems likely that numbers have been moderately stable over the past decade, but small declines at the edges of the strongholds are likely to have occurred and would not have been easily detected using existing methodology.
Given that an ongoing decline is highly likely, it seems appropriate to consider this species at a higher level of priority than Near Threatened, as formerly listed in 2003. Under this assumption, Asplenium ascensionis could qualify as Critically Endangered, as a result of its extremely restricted global range. However, since the decline is slow, and the population is reasonably secure in the short term this seems excessively cautious. As a precautionary measure, a shift to Vulnerable is recommended. In order to make an accurate assessment in future, detailed long term monitoring is required to establish the rate of decline.
|Range Description:||This species is known only from the central part of Ascension Island, Saint Helena. The extent of occurrence is approximately 3.95 km², and encompasses Green Mountain and the surrounding region, stretching from Middleton’s Ridge in the west to White Hill in the east. However, most of the population occurs in a belt between 550 m and 770 m asl on Green Mountain, where it is locally common on rock faces. The largest parts of the population are found around Breakneck Valley and several of the other valleys draining the south side of Green Mountain, the Old Marine Barracks and Bishop’s Path. With the exception of a narrow gulley on the north side of Cricket Valley where it is also very locally abundant (at 430 m, the lowest altitude recorded), all other outlying localities are extremely small and fragmented. The total area of occupancy is likely to be under 0.5 km².|
Native:Saint Helena, Ascension and Tristan da Cunha (Ascension)
|Range Map:||Click here to open the map viewer and explore range.|
|Population:||The total population is probably close to 3,500 mature individuals. As the plants are small and densities occasionally reach 100 per m², this does not cover a large area. The distribution on Green Mountain is patchy, and largely confined to areas of suitable humid rocks which have not yet been invaded by competitors. However, the overall population may be considered reasonably continuous as occasional plants are found between the core areas. Away from the upper part of Green Mountain, several hundred plants occur in very sheltered gulleys in Cricket Valley. Other populations are physically more isolated and generally number fewer than 50 very stunted mature individuals.|
|Habitat and Ecology:||The majority of the population occurs on rock faces. Deep shade and/or moderate levels of moisture appear to be necessary for germination, and most of the gametophytes and sporelings occur in deep crevices or very sheltered microhabitats. The moisture is mainly derived from incoming cloud arriving from the southern and eastern sides of the mountain, but plants are also present around a few wet seepages elsewhere (although rarely so, as they are usually overwhelmed by invasive species in such localities). Where found on arid, outlying hills such as Weather Post and White Hill, individuals are entirely restricted to deep crevices and are often very small, with fronds usually 2 cm long or less. However, in less extreme environments, adult plants may colonize drier rock-faces via proliferous plantlets which are produced from near the frond tips. If spreading via this means, small areas of continuous cover can build-up. Very rarely, patches are found growing on soil in dark, sheltered places, and a healthy colony occurs on the old stone walls of the now derelict Old Marine Barracks on Green Mountain.|
Probably the most serious competitors are the maidenhair ferns, Adiantum cappilus-veneris and A. raddianum. The former was first recorded by Eric Duffey in 1958, although as there were few other botanical visits in the preceding 50 years, it is difficult to identify the exact date of arrival. It now occurs over large areas of the south side of Green Mountain, at similar altitudes to those occupied by Asplenium ascensionis. Adiantum raddianum was not recorded until collected by John Packer in 1967. It was apparently common by then, but assuming it to be a relatively recent introduction, has spread very rapidly to occupy a large extent of all the moister banks on the north side of the mountain. In addition, other weedy chasmophytes have invaded suitable habitat, particularly Begonia hirtella and Clidemia hirta. It is almost certain that the advance of such aggressive colonists has lead to substantial out-competition of the Asplenium population, which is almost never observed to co-exist with either Adiantum species.
Since A. ascensionis appears to be dependent on humid, shady places for germination and does not establish easily on dry, open rock faces, climate change could pose further threats in the future if Ascension were to become significantly drier. However, this possibility has not been adequately assessed, and the true effects on the species have not been evaluated.
Green Mountain was declared a National Park in 1996. Due to the low population density on Ascension, there is little human interference in the area and legislation is not a priority.
Some management has been conducted in recent years by the Conservation Department in order to clear areas of suitable rock face from weeds. Most of the effort has been focused on other, rarer endemic species, and only a small proportion has helped to maintain Asplenium ascensionis patches. Spores have recently been germinated with the aim of maintaining a safeguard stock in cultivation, and a spore bank has been established.
|Citation:||Lambdon, P.W., Stroud, S., Gray, A., Nissalo, M. & Renshaw, O. 2012. Asplenium ascensionis. In: IUCN 2012. IUCN Red List of Threatened Species. Version 2012.2. <www.iucnredlist.org>. Downloaded on 18 May 2013.|
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