|Scientific Name:||Pteris adscensionis|
|Taxonomic Notes:||Very close to Pteris dentata Forssk., a native of Africa and the nearby South Atlantic Island of St. Helena. Perhaps merely a form of this species, although Ascension plants shows some morphological and ecological differences from those on St. Helena, and further work is needed to establish the true taxonomic status.|
|Red List Category & Criteria:||Critically Endangered B1ac(iv)+2ac(iv) ver 3.1|
|Assessor/s:||Renshaw, O., Stroud, S., Gray, A., Lambdon, P.W. & Nissalo, M.|
|Reviewer/s:||Scott, J.A. & Hilton-Taylor, C.|
The number of fertile individuals probably remains only a few hundred, distributed across a few main localities, although this total does not include a modest scattering of isolated individuals which occur widely over 4.5 km of the southern slopes of Ascension. It seems likely that these outliers are short-lived and have germinated from wind-blown spores from one of the core areas. Most are not fertile at any one time, and the remainder are very small with limited chances of founding a new colony. However, since recently germinated sporelings may appear anywhere across the range, all individuals are best regarded as part of a single meta-population.
Although the recent total represents a substantial increase on previous estimates, since 2002, the total population size has fluctuated between 2-300 and 50-60 mature plants, which suggests that it is subject to substantial variation. Numbers would be expected to drop well below 250 plants in poor seasons, which ensures that the species qualifies at least as Endangered. However, these large fluctuations are an important further factor to be considered. The population has possibly been as low as 50-60 in recent years. This is a decline to 20% of the maximum, which is not “extreme” according to the IUCN definition. However, as P. adscensionis has only been monitored for a few years, it seems unlikely that the full extent of the fluctuations have yet been observed. It is known that plants are vulnerable to drought, rockslides and competition from invasive weeds, and current evidence suggests that individuals are relatively short-lived, so could well be vulnerable to larger crashes. Furthermore, judging from the long term declines over a century or more, it is probably safe to say that a continued decline would occur without further management.
Retaining the status of Critically Endangered seems a sensible precaution.
|Range Description:||This species is known only from the central part of Ascension Island, South Atlantic Ocean. It has been a very rare species over at least the past 100 years. A few descriptions of the original habitat and range are available from the mid 1800s, when it was widely-distributed across the drier, mid-altitude slopes of Green Mountain. Joseph Hooker recorded it as growing between 365 and 550 m altitude in 1843. Even by this stage, the vegetation on Green Mountain had been heavily modified by grazing animals and introduced species. The full original native range is unknown.
The extant population occurs predominantly in sheltered valleys on the southern and eastern sides of the mountain and its foot-slopes, scattered between approximately 370 and 697 m. The three largest centres occur in lower Breakneck Valley, an un-named gulley running due south down the mountain and joining with Breakneck, and in Cricket Valley, a crater 1 km to the southeast. Further individuals may be found widely dispersed across the dry slopes nearby, from Mountain Red Hill in the west, across Castle Hill to White Hill in the east. A very small colony comprising only approximately 10 mature individuals has also recently been identified in a deep crevice on the north side of Green Mountain, at approx. 620 m altitude.
The total extent of occurrence is approximately 5.4 km², but as this includes may wide-ranging outliers, the area of occupancy is much less, probably below 5 ha, and even this includes much habitat with very low densities.
Native:Saint Helena, Ascension and Tristan da Cunha (Ascension)
|Range Map:||Click here to open the map viewer and explore range.|
|Population:||According to an annual plant census conducted in 2008, there were thought to be close to 250 mature individuals present in the wild. The exact number is difficult to assess. Although in the 2008 census, a total of 1,246 plants were recorded, most of these were found at a single locality where a very large number of seedlings germinated in response to clearing of guava scrub. In 2009, a new, large population was surveyed in a remote ravine on the south side of Green Mountain. Ptisana purpurascens generally only produces spores at a relatively large size, and so then number of mature individuals is only a fraction of the total count. However, the recent finds probably increase the total estimate to over 250 mature plants.|
|Habitat and Ecology:||Apparently an early successional fern which is capable of dispersing efficiently by spores over a few km, and which may appear on a range of relatively dry, sparsely vegetated areas including cinder banks, rock crevices and open soil. In the driest, low altitude situations, individuals may produce a few sporangia at a small size and die after just one to two years. In moist, shaded situations they tend not to spore until relatively large, and may take two to three years to attain this state. From the high ratio of seedlings to mature plants observed in such localities, it seems likely that only a small proportion of sporelings survive to maturity in the wild. Furthermore, observations of cultivated plants grown at the Royal Botanic Gardens, Kew, suggest that even under favourable conditions, adults last only a few years. These factors indicate that the rate of population turn-over is high.|
Pteris adscensionis is very vulnerable to encroachment by invasive weed species at its remaining localities. The most serious invaders include grasses such as Melinis minutiflorus and Sporobolus africanus, which form continuous ground cover and thus remove potential germination sites. Broadleaved weeds such as Alpinia zerumbet, Psidium guajava, Lantana camara, Juniperus bermudiana and Spermacoce verticillata are responsible for in-filling large areas of suitable habitat, and Clidemia hirta, Robus rosifolius and Begonia hirtella are also a significant threat as they invade key refuges on banks and rocks. At the higher altitudes, there may be some competition with the maidenhair ferns Adiantum cappilus-veneris and A. raddianum. Such species have already overwhelmed most of the original suitable habitat areas which once existed on Ascension and have undoubtedly been responsible for an already massive decline. A site in Cricket Valley cleared of guava (Psidium guajava) in 2007 increased in numbers rapidly over the following two years, presumably mainly as a result of liberation from shading and a reduction in the deposition of sclerophyllous leaf litter. Without regular management, further suitable sites would probably be lost.
The other major threat is posed by introduced grazing animals, particularly sheep and rabbits. These are very common at mid-altitudes. Whilst ferns such as Pteris ssp. are not particularly palatable as adults, they are often grazed avidly grazed rapidly when young, and rabbits dig-up the root systems for unknown reasons.
Bank slippages and rock falls are a minor threat, although in most areas would only be likely to remove a small proportion of the population, which is distributed thinly. Sporelings can be prone to drought, particularly at lower altitudes.
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 further legislation is not a priority.
Regular weeding is conducted by the Conservation Department to keep invasive weeds in check. However, larger scale clearance to create new open habitat areas, and restoration of functioning native communities is needed to secure the long-term future of the species. Sheep and rabbit control should also be considered.
Plants have been grown in cultivation since 2004, and a small spore orchard established near the summit of Green Mountain. At least one individual is also in cultivation at the Royal Botanic Gardens, Kew, London, and recently, cryopreserved gametophytes and have been placed in long-term storage at the same institute.
|Citation:||Renshaw, O., Stroud, S., Gray, A., Lambdon, P.W. & Nissalo, M. 2012. Pteris adscensionis. In: IUCN 2013. IUCN Red List of Threatened Species. Version 2013.2. <www.iucnredlist.org>. Downloaded on 16 April 2014.|
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