|Red List Category & Criteria:
||Renshaw, O., Stroud, S., Gray, A., Lambdon, P.W. & Niissalo, M.
||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.
|Previously published Red List assessments:|
- 2003 – Critically Endangered (CR)
- 1998 – Endangered (E)
|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.
Saint Helena, Ascension and Tristan da Cunha (Ascension)
|♦ Lower elevation limit (metres):||370|
|♦ Upper elevation limit (metres):||697|
|Range Map:||Click here to open the map viewer and explore range.|
Pteris adscensionis is very vulnerable to encroachment by invasive weed species at its remaining localities. The most serious invaders include grasses such as Melinis minutiflora 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, Rubus 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.