|Scientific Name:||Euphorbia origanoides|
Euphorbia berteriana Balb.
Euphorbia origanoides Bertero
|Red List Category & Criteria:||Critically Endangered B1ab(ii,iii,iv,v)+2ab(ii,iii,iv,v) ver 3.1|
|Assessor/s:||Gray, A., Stroud, S., Lambdon, P.W., Nissalo, M. & Renshaw, O.|
|Reviewer/s:||Hilton-Taylor, C. & Scott, J.A.|
The previous red list assessment of Euphorbia origanoides listed this species as Critically Endangered B1ac(iii,iv)+2ac(iii,iv) (Gray 2003). Significant new material pertinent to IUCN red list assessment has now been evaluated for this present assessment, including up to date census data from Ascension Conservation and all relevant published data including Gray et al. (2009).
This reassessment suggests no significant change to the red list category for E. origanoides, although, additional evidence from literature sources and personal observation suggest that some additional criteria are also likely to be met e.g. continuing decline in the area, extent and/or quality of the habitat. The extent of occurrence and area of occupancy have declined since numbers have been monitored over the last decade, and historical evidence shows the number of subpopulations have declined since 1900. This observed decline is also supported by published evidence (Gray et al. 2009). The major population at South Gannet Hill has declined in area by 40% since 2003 due to a combination of decreased rainfall leading to lower seedling recruitment and the release of the major herbivores, rabbits and mice, from predation by cats as a result of the cat eradication programme.
There are also fewer large mature individuals (defined as individual plants greater than 1 m in diameter) in the population as a whole than in 2003. This suggests that plants may not be living as long or that there has been a shift in ecological conditions or both. Since larger plants make a greater contribution to the seed bank (Gray et al. 2009) this may therefore have repercussions for longer term population dynamics.
|Range Description:||This species is known only from Ascension Island, South Atlantic. Subpopulations are located at Cotar Hill, Letter Box, Mars Bay, Round Hill, Sisters Peak, South Gannet Hill, along Spire Beach to Hummock Point and Wig Hill. A single plant was also found at Comfortless Cove in 2009 (known as a historical site) but died after being grazed by sheep. The extent of occurrence is approximately 67 km². As the species is extremely rare within this zone, the area of occupancy is estimated to be less than 2.5 km².|
Native:Saint Helena, Ascension and Tristan da Cunha (Ascension)
|Range Map:||Click here to open the map viewer and explore range.|
Following an annual census conducted in 2008, the total population was estimated to be approximately 4,850 mature plants. Classification of E. origanoides into mature and juvenile individuals is somewhat subjective, because in harsh conditions, they may flower whilst still only a few cm high. Furthermore, these smaller individuals are subject to large fluctuations in numbers between years as a result of grazing, drought and the instability of the substrate they grown on.
It is important to note that the South Gannet Hill population which has been the major population in recent times has reduced by 40% in area since the last assessment (Gray 2003). It is common in the South Gannet Hill, Cross Hill, Round Hill areas which contain approximately 90% of the island population. The Mars Bay population has almost disappeared in recent times though this has been bolstered by restoration activities.
Subpopulations are fragmented across the island and cross pollination between the easterly subpopulations and those found in more westerly locations may be difficult due to the strong continual southeast trade winds which may make it difficult for pollinators to cross from one side of the island to the other; especially weak flying flies and Hemiptera.
|Habitat and Ecology:||
Euphorbia origanoides is found on the dry lava plains of Ascension Island from almost sea level to 310 m. An assessment of E. origanoides ecology was published in 2009 and the information summarised below is largely taken from Gray et al. (2009); a more complete account can be found therein.
The mean annual rainfall at Georgetown (which is within the E. origanoides habitat zone) between 1899 and 2009 is 128 mm yr-1, during this period the largest amount recorded was in 1924 (471 mm yr-1) and the lowest in 1970 (22 mm yr-1). Rainfall events in the areas that E. origanoides inhabits on Ascension are therefore episodic and there are months where little or no rain falls; hence periods of drought are not uncommon.
The mean monthly minimum and maximum temperature data from the Meteorological Office at the Ascension Airhead (also within the E. origanoides habitat zone) suggest a Continentality Index of 8.8. This indicates that E. origanoides can be classified as a Hyperoceanic; barely hyperperoceanic species (using the criteria of Rivas-Martínez et al. 1999). Using the UNEP index of aridity (AIu) (UNEP 1992) (again from the Airhead data), AIu = 0.056. This suggests that the E. origanoides habitat almost qualifies for the upper range of Hyperarid.
Most populations are found between sea level and 150 m, the highest population is currently on the Sisters Peak range at 330 m. Strict altitudinal limits remain unknown and E. origanoides may have had a wider altitudinal distribution historically. Germination has been observed at slightly higher altitudes than recorded in the wild during recent re-introduction trials on Green Mountain (approximately 400 m), though survival is limited by competition with introduced animal and plant taxa (Gray and Stroud unpublished data). The current population distribution therefore appears to be limited as much by competition from introduced taxa as by climatic factors.
E. origanoides is a characteristic of the low lying lava plains of Ascension Island where soils are composed of weathered volcanic scoria. Soil chemistry data are variable, sites have medium to high soluble salt content, generally high phosphorus, magnesium, sodium and pH but with correspondingly low values for organic matter (and hence estimated nitrogen release) and calcium. There are extremely high levels of soluble salts at the historical English Bay site perhaps reflecting the coastal nature of the site. However, Comfortless Cove, also a coastal site, has lower salinity levels. It may be possible that there are historical effects from the deposition of guano at bird colonies in the English Bay though this remains speculative (see Gray et al. 2009). There does not seem to be any preference for particular volcanic substrate, E. origanoides being found on most of the volcanic deposits except those found on Green Mountain. However, as previously indicated, absence in this area may be more to do with increased competition than volcanic substrate.
When damaged, E. origanoides produces copious amounts of whitish latex from both stems and leaves. However, latex production does not appear to constitute an effective grazing deterrent. Historically, E. origanoides may have been browsed by goats (see Osbeck 1752), and although goats were eradicated in 1944, Ashmole and Ashmole (2000) hypothesize that this may partly account for the disjointed nature of present populations. In the lower xeric zone inhabited by E. origanoides, rainfall is sporadic; food availability for herbivores is therefore also likely to be periodic. A perennial species such as E. origanoides represents a potential food item in times of food scarcity, and although the nutritional quality is unknown, herbivores do consume stems, leaves, flowers and fruits; sheep, rabbits and mice have all been observed as eating E. origanoides. Field observations suggest that unless the entire plant is repeatedly defoliated recovery can occur.
Euphorbia origanoides exhibits some morphological differences between populations e.g. plant height and diametre. There are undoubted site environmental differences but there is at present no evidence to determine whether this variation is genetic or phenotypic, however, molecular research is currently being undertaken.
Euphorbia origanoides flowers are monoecious, and typical of the Euphorbiaceae composed of one female and a few male flowers together forming cyathia resembling true flowers. Typical pollinators include Diptera and members of this family are common among E. origanoides flowers; other likely pollinators include members of the Hemiptera which can be found in large numbers amongst plants at all sites. Controlled pollinations have indicated that E. origanoides is self-compatible. Normally three seeds are produced per fruit though fruits with four have been observed. Fruiting occurs throughout most of the life of the plant and the number of fruits found sheltered beneath plants can be related to the size of the plant. The Millennium Seed Bank reports the mean 1,000 seed weight for E. origanoides as 1.78 g. However, the four values reported are very variable; 0.21 g 0.66 g, 1.56 g and 4.67 g (Kew 2008). The large value of 4.67 g seems extreme, however, it is uncertain from the available data whether reported values include for example, minor covering structures, debris, the surrounding fruits or whether fresh or dry seeds were weighed (Kew 2008). A mean value of 0.81 g excluding this extreme value would seem a more cautious estimate. It is not known whether animal taxa are responsible for dispersal, though the lack of an elaiosome suggests that ants are unlikely to disperse seeds. Other main dispersal agents are likely to be wind and water. Ripe fruits are easily transported by the wind and the hemispherical dome life-form appears to offer shelter for detached fruits and seeds; the number of fruits found in soil drops extremely sharply with distance from plant.
The presence of invasive species is a major threat to the survival in the wild of E. origanoides. These include many species regarded as the worst invasive species in the world (IUCN 2008). Most of these species are in higher vegetation zones, however Gray et al. (2005) record the presence of at least six species which they consider capable of dominating vegetation in E. origanoides areas and hence threatening the survival of this species in the wild, these are: Heliotropium curassavicum L., Nicotiana glauca Graham, Leucaena leucocephala (Lam.) de Wit, Melinis minutiflora Beauv., Prosopis juliflora (Sw.) DC and Psidium guajava L.. The expansion of P. juliflora in much of the lower lava plains on Ascension since the 1980s is testament to the rapid expansion and domination that some species can attain (Ashmole and Ashmole 2000, Gray et al. 2005) and this species is very efficient at commanding available ground water with a root depth of over 10 m. The distribution of P. juliflora at present in the E. origanoides areas is limited. The presence of introduced scale insects and mealy bugs on a number of plants also presents a threat to some populations. Introduced herbivores also have some impact notably at the South Gannet Hill area where the population has decreased by 40% in area occupied.
Hybridisation with closely related non-native species may also be a pervasive threat to the E. origanoides gene pool. Plants that appear to be intermediate for certain characters have been recorded at the Airhead and Comfortless Cove areas. Although hybridisation in the subgenus Chamaesyce is rare, further research is urgently required to ascertain whether hybridization is currently occurring.
In the arid habitat where E. origanoides occurs, climate change represents a major threat to the population. However at present, it is difficult to predict changes to rainfall on Ascension with any degree of confidence. In contrast Ascension’s land temperature closely correlates to ocean temperature; current predictions suggest that ocean temperature will rise in the immediate and long term future (IPCC 2007). Gray (2009) also examined temperature data from 19 Global Circulation Models (GCM) for the Ascension grid square and found a close correlation with past and present temperature data and the modelled data for the period under study. All of the 19 GCMs currently predict a future rise in temperature for the Ascension grid square. This evidence strongly suggests that temperature is likely to rise on Ascension. A likely consequence of higher temperatures may be a reduction in available soil water in the habitat that E. origanoides occupies since temperature is a significant controlling factor of evapotranspiration
Euphorbia origanoides is currently present in the Hummock Point Nature Reserve though there are proposals to bring all of the areas where E. origanoides is currently distributed in the wild under legislation. Populations are also under cultivation and surveillance by Ascension Conservation and a restoration project has been underway at Mars Bay since 2006.
The Centre for Ecology and Hydrology (CEH) in Edinburgh has a collection of living plants and seed material from all extant populations. CEH are also currently researching the genetics of E. origanoides.
The species Action Plan for E. origanoides has also recently been updated and recommends a programme of extensive habitat restoration to ensure the sustainable conservation of this species in the wild.
|Citation:||Gray, A., Stroud, S., Lambdon, P.W., Nissalo, M. & Renshaw, O. 2012. Euphorbia origanoides. In: IUCN 2012. IUCN Red List of Threatened Species. Version 2012.2. <www.iucnredlist.org>. Downloaded on 25 May 2013.|
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