|Scientific Name:||Sporobolus caespitosus|
|Taxonomic Notes:||Although Sporobolus as a genus is prone to hybridization and uncertainty over species boundaries, S. caespitosus is not known to hybridize with the introduced S. africanus, which grows abundantly around many of the subpopulations.|
|Red List Category & Criteria:||Critically Endangered 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 Sporobolus caespitosus sites on Green Mountain as part of a single meta-population, as all are very small and occur within 500 m of each other. The subpopulation on Weather Post very small and isolated from this main locality.
There is no direct data to indicate changes in habitat quality, but a general decline is inferred from observations of the authors. The extant localities are gradually becoming encroached by invasive weeds and are currently kept open only by management. Numbers are also maintained by the replanting of individuals into the wild. These activities could cease if the Conservation Department’s work is discontinued in the future, and the unusual political and financial situation on Ascension (the island’s inhabitants have no right of residency and a human population is only maintained to service military and communications bases) make such a scenario a possibility. Also, it is known that the subpopulations are very vulnerable to bank slippages, which could lead to a substantial proportion of the overall numbers being lost at any one time. Small catastrophes are likely to occur periodically, and are probably more regular than colonization of new sites, which is a rare occurrence due to the lack of suitable locations, thus leading to overall losses. This species is therefore listed as Critically y Endangered.
|Range Description:||This species has an extremely restricted world distribution. It is known only from the central part of Ascension Island (Saint Helena), South Atlantic Ocean. The main part of the population is restricted to sparsely-vegetated banks and rock ledges on the eastern side of Green Mountain, between 700 and 750 m altitude. A second, small population centre was recently relocated on the cliffs of Weather Post, 2 km northeast (altitude close to 600 m). All subpopulations are very small and both the extent of occurrence and area of occupancy are likely to be smaller than 1 km².|
Native:Saint Helena, Ascension and Tristan da Cunha (Ascension)
|Range Map:||Click here to open the map viewer and explore range.|
|Population:||Due to the discovery of several new sites over the past year, the total population count has increased from approximately 460 mature individuals to 700-800, spread over six to seven major subpopulations. Scattered individuals probably remain unaccounted for, but no new large patches are likely to remain unknown. The totals include a small number of plants reintroduced to existing sites, as these were not differentiated in the survey. It is estimated that including these reestablished individuals, numbers have remained relatively constant in recent years, although declines could be expected if management efforts were reduced. Densities may reach 65 mature tussocks per m².|
|Habitat and Ecology:||As no records of S. caespitosus were made before the mid 1800s, nothing is known of its original habitat preferences. Plants can form tussocks up to 40 cm across in good conditions and may have occurred in sparse grasslands at lower altitudes on the mountain than they do today. Now, almost no plants exist in the wild above 20 cm in diameter. Most are very small, and restricted to sparsely-vegetated cinder banks and cliffs on windy, exposed mountain ledges. These habitats are clearly refugial. The almost constant association with wind-exposed locations may partly be due to the greater quantities moisture from incoming mist, but is also likely to be partly because invasive weed species are less vigorous in such places. Colonies are usually associated with the endemic fern Xiphopteris ascensionense and a rich community of native bryophytes (many of these species are also severely threatened). S. caespitosus flowers throughout much of the year and produces numerous seedlings, many of which die in dry conditions, but a reasonable number survive to maturity.|
Sporobolus caespitosus is very vulnerable to encroachment by invasive weed species at its remaining locations. The most serious invaders are grasses such as Sporobolus africanus and Paspalum scrobiculatum, which form continuous ground cover and thus remove potential germination sites. Broadleaved weeds such as Clidemia hirta and Begonia hirtella also constitute a significant threat. Without regular management, suitable sites would certainly be lost.
Sheep and rabbits will readily graze plants if accessible, and usually kill plants because the weak roots are easily torn-out. For this reason, most plants are on steep ledges and difficult to access.
Landslips occur regularly on the soft cinder banks, and one in 2009 probably destroyed close to 50 plants. Such disturbance events are also valuable as they create new open habitat areas, but since the existing populations are so small, the losses are damaging because they affect a significant proportion of the population.
The leaves of the tussocks are often infected with a black, smut-like fungal infection. This is common on other plants in the area and appears to be epiphytic. It is not known whether any damaging effects result.
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 seed orchard established near the summit of Green Mountain. Recently a seed bank has been established.
Further efforts to protect the distinct subpopulation on Weather Post should be established.
|Citation:||Lambdon, P.W., Stroud, S., Gray, A., Nissalo, M. & Renshaw, O. 2012. Sporobolus caespitosus. In: IUCN 2013. IUCN Red List of Threatened Species. Version 2013.2. <www.iucnredlist.org>. Downloaded on 20 April 2014.|
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