Countries of Occurrence:
Saint Helena - British Overseas Territory
P. Lambdon & A. Beard
S. Black & M. Bohm
Facilitators / Compilers/s:
This species is now confined to a single location, where the population may drop well below 1,000 individuals in poor years. Although the total remains too large to permit qualification as Critically Endangered under either of Criteria C or D, the tiny extent of occurrence (EOO) and area of occupancy (AOO) are easily under the range size thresholds for B1 and B2. At present, there is no evidence to suggest that the pronounced annual variations in numbers are sufficient to qualify as 'severe fluctuations' and due to the absence of long-term data it is not possible to directly assess a population trend. However, a decline seems likely, based on the well-documented demise of the host plant across St Helena, and the acute threats from invasive species which are likely to have a pronounced impact at the remaining extant sites in future years. This species is Critically Endangered.
Marion's Plume Moth is endemic to the island of St Helena, South Atlantic Ocean, where it is now believed to survive only in one area along the south coast. The estimated extent of occurrence (EOO), based on the area of a minimum convex polygon around known localities, is between 0.47-0.97 ha. The lower estimate includes only confirmed localities, whereas the upper estimate also includes potential localities which cannot be easily surveyed. The area of occupancy (AOO), based on a 2 x 2 km grid, is 4 © The IUCN Red List of Threatened Species: Agdistis marionae – published in 2015. http://dx.doi.org/10.2305/IUCN.UK.2015-4.RLTS.T67367669A67368887.en 1 km2. Following IUCN Red List Guidelines, the EOO is therefore increased to 4 km2 to match the AOO. The world population is almost certainly confined to the Man & Horse area on St Helena's southwest coast. This is the only locality where the larval food plant (St Helena Tea Plant, Frankenia portulacifolia (Roxb.) Spreng.) still exists in substantial numbers (Lambdon and Ellick in prep). Although small subpopulations of the host survive in other parts of the island, these now mostly consist of just a few scattered individuals, and extensive searches have revealed no signs of the distinctive caterpillars or pupae. Man & Horse comprises a large area of coastal cliff, in places arising almost vertically 590 m asl from the sea. Tea Plant is distributed sporadically across the escarpment but only reaches high densities in a few places, occupying a total habitat area of approximately 0.2 ha. Thus far, Marion's Plume Moth has only been detected in two of the major sites for its preferred host, which are separated by approximately 1.5 km. The first of these lies near sea level on coastal screes at Old Father Point, and contains some of the largest surviving Tea Plant individuals. The second locality lies on the cliff top around the western side of Joan Hill, the highest summit. Elsewhere on Man & Horse, Tea Plant colonies generally consist of few, stunted shrubs. There are some reasonably healthy and populous stands on ledges further west of the Joan Hill summit stronghold, and it is quite possible that additional moth survive in this area, which is largely inaccessible. However, the suitable habitat is extremely limited. The degree of cross colonization between the 'sites' is unknown, and is not clear whether they should be considered as distinct subpopulations. However, since the major threat is likely to be habitat degradation as a result of invasive species affecting the entire Man & Horse area, they almost certainly represent a single location.
The population was assessed during two recent summers. In 2010-11, the peak number of larvae at the two known sites combined was estimated at 2,307 (approximate 95% confidence range: 2,264 – 2,349). In 2013-14, following a particularly dry winter, the estimate was much lower, with a projected count of only 542 larvae (approximate 95% confidence range: 462 – 623). This represents a drop of 77%, and suggests substantial inter-annual fluctuations. Based on existing observations it appears highly likely that species has an annual life cycle, and thus there must be fewer adults maturing each year than there are larvae, but since the moths are rarely seen no direct assessment of the mature population size is available. As the species may be more widespread on the cliffs than currently recognized, the calculated projections may be somewhat conservative. By extrapolation, if it is assumed that Marion's Plume Moth is potentially present in all Tea Plant patches around Man & Horse, the maximum population estimate could be increased: as high as 3,720 in 2010-11 (confidence range: 3,642 – 3,797) and 817 in 2013-14 (confidence range: 710 – 925). However, as not all areas provide equally suitable habitat it is extremely unlikely that the true totals approached these levels. The larvae exist at very low densities and signs of them (living caterpillars, pupae or exuviae) require considerable effort to locate. Therefore, it was only possible to sample a relatively small number of plants during these assessments. Current Population Trend: Unknown
Marion's Plume Moth is almost certainly monophagous, feeding only on Frankenia, as no similar alternative hosts exist on St Helena. The Tea Plant is a small, halophytic shrub usually found close to the sea, although a few colonies were formerly know up to three kilometres inland. It generally inhabits barren, rocky places on weathered basalt which are exposed to salt spray and face the prevailing southeasterly winds. Insects which feed on it must be adapted to cope with high levels of salt ingestion, and tolerant of cool and exposed conditions. Furthermore, the caterpillars are remarkably well camouflaged amongst the small, sub-spherical leaves; The prothorax is enlarged into a 'hood' with dorsal ridges mimicking the relief of the foliage, and the entire body is covered in small granules similar to the host's minute salt glands. From the limited observations made thus far, it appears that the larvae grow and develop over the summer months (November – January), coinciding with the Tea Plant flowering period. Almost no signs of their presence remained during a visit in May. The larvae certainly consume flowers on occasions, although they also appear to feed on the foliage. They pupate on exposed twigs and emerge after 10-12 days. Adults have rarely been observed, and only in summer, flitting rapidly around and between the bushes. Up to 11 larvae have been detected on a single host plant, although they are typically dispersed very thinly and rarely more than three occur together. There appears to be a strong preference for eggs to be laid on large shrubs. Small Tea Plant specimens (under 40 cm in diameter) are almost totally avoided, and in 2013-14 only the largest host individuals (over 70 cm in diameter) were occupied. The reasons for this are unclear, but could be related to the greater ability of large plants to tolerate water stress, which undoubtedly affects food quality. However, at the more exposed Joan Hill site, Tea Plant grows in a low, wind-stunted 'heath' at much higher densities, and here there is a tendency to breed on smaller specimens.
The extremely constrained population size and the relatively low densities make the species inherently vulnerable to extinction. It is unlikely that there is much migration between Tea Plant colonies at Man & Horse due to the regular strong inshore winds at the site, which prevents dispersal lateral. Meanwhile, the host population at Old Father Point comprises just 932 mature plants, with 1,132 at Joan Hill and approximately 910 'available' elsewhere on the cliffs. Many of these are below the preferred size threshold. It seems reasonable to assume that Marion's Plume Moth was once much more widespread along St Helena's south-facing coasts, but has declined substantially with the loss of Tea Plant from previous strongholds. The initial declines were thought to have resulted from the introduction of Goats (Capra hircus L.) to St Helena by Portuguese mariners in the early 16th century. The herds soon multiplied into huge numbers and decimated the native flora of the coastal hills. Goats were largely eradicated between the 1950s and 1970s, but another introduced herbivore, the Rabbit (Oryctolagus cuniculus L.) persists in large numbers. Rabbits graze seedlings efficiently, thus inhibiting regeneration, and Tea Plant seedlings also suffer moderate levels of mortality from infestations of Mealybug (Pseudococcus spp. Westwood). Most subpopulations are now therefore confined to highly isolated refugee in inaccessible places, and comprise too few plants to sustain dependent herbivores. Small colonies of Marion's Plume Moth are inherently prone to extinction because the species feeds at low densities and experiences annual fluctuations in reproductive success. Although the host plant remains secure in the short-term at Man & Horse, even here if faces substantial longer-term threats to survival. Rabbits are present in the Joan Hill area and there have been recent signs of them at Old Father Point, despite the fact that the small headland, almost hemmed-in by rocks, has previously been considered a refuge from grazing animals. Seedling browsers such as Rabbits may not have an immediate impact on Marion's Plume Moth due to the preference for older plants, but could further fragment the spatial distribution, and there may be a sudden catastrophic decline as the current cohort ages. Furthermore, the spread of African fountain grass (Pennisetum setaceum (Forsk.) Chiov.) is a substantial concern. This relatively recent introduction has already swept across huge areas of the far west of St Helena and appears to be advancing at a rapid rate. Some patches have already been detected on Joan Hill. Once well-established, it forms extensive tussock grasslands where it outcompetes most other plant species.
Man & Horse lies within Sandy Bay National Park, part of a proposed Protected Areas Network which is currently being developed for St Helena. Both Marion's Plume Moth and its host plant will also be protected under the new Environmental Protection Ordinance, presently in the final stages of drafting and expected to be issued in 2015. However, there is little immediate prospect that legislation will alleviate the threats to the species, as the majority of the occupied habitat is already inaccessible to humans and there is little direct impact from exploitation. Three main approaches are needed to stabilize the important habitat in the Man & Horse area: (1) Control of mammalian herbivores. Efforts to address the spread of Rabbits is much needed, particularly at Old Father Point. Improved sheep fencing at Joan Hill is also required, to prevent livestock from straying onto the cliffs from the adjacent pastures. (2) Control of invasive plant species. At present, there is little possibility of developing a © 7.en sustained management initiative due to a lack of funding and manpower, the difficult and inaccessible terrain and the sheer scale of the problem. There is no prospect of successful widespread eradication for African Fountain Grass on St Helena. If the aggressive colonist becomes more widely established on Man & Horse cliffs then sustained and locally intensive management will be needed just to maintain the current Tea Plant strongholds in reasonable condition. (3) Habitat restoration. Given the severity of the issues described above, in the worst case, a programme of reintroducing Tea Plant to the wild in safe refuges, followed by managed translocations of larvae, could offer the only realistic chance of augmenting the precarious Marion's Plume Moth population. Unfortunately, Tea Plant is a difficult species to grow in the nursery and research is needed to determine successful cultivation techniques. It is also likely to be difficult to re-establish in natural situations due to the dry and remote localities it frequents.