A new species of critically endangered edible yam endemic to northern Madagascar, Dioscorea irodensis (Dioscoreaceae) and its conservation
Morphological character data are used to show that a distinct morphotype of Dioscorea L. from the Irodo valley (East of Sadjoavato) in Antsiranana Province of Madagascar is an undescribed species, differing in its leaf arrangement, pubescence form and male inflorescence structure from all other taxa. It is described as Dioscorea irodensis Wilkin, Rajaonah & Randriamb., illustrated and a distribution map and ecological information provided. It is known from three sites, but is likely to have been eradicated from one of them. The population that has been studied in the field contains a very low number of adult plants. Tubers have been extracted for use as food at a level that appears to be unsustainable. Thus its provisional IUCN conservation status assessment is that it is critically endangered (CR). Its vernacular name in Irodo is Bemandry.
Key Wordsconservation critically endangered distribution edible ex situ in situ Madagascar morphology new species yam
Ny toetra sy ny bikan’ny oviala dia azo ampiasaina hanehoana fa miavaka ny oviala iray, izay hita tao anatin’ny lohasahan’Irodo (Atsinanan’i Sadjoavato, faritanin’Antsiranana) ao Avaratr’i Madagasikara, ary tsy mbola voasoritra ny mombamomba azy. Mampihavaka azy amin’ny hafa ireto toetra manaraka ireto: ny fipetraky ny ravina, ny fisian’ny volo, sy ny firafitry ny tahom-bony lahy eo aminy. Nosoritana ho Dioscorea irodensis Wilkin, Rajaonah & Randriamb. izy ity, nampiratiana an-tsary ny momba azy, namoahana saritany ny toerana misy azy, ary nomena ireo vaovao ara-ekolojika mikasika azy. Vondrona oviala miisa telo fara-faha-betsany no fantatra fa misy azy, ary efa ho fongana ny iray amin’ireo. Ny vokatry ny fanadihadiana izay natao tamin’ny vondrona oviala misy no nahitana fa vitsy ny fototra oviala lehibe afaka mamoa, ary maro ireo nohadiana ho sakafo, izay mamaritra sahady fa tsy hateza io karazana io. Noho izany, ny sata fiarovana vonjimaika IUCN dia manombana fa zavamaniry tena tandindomin-doza (CR) io karazana oviala io. Bemandry no fiantso io karazana oviala io any Irodo.
The taxonomic and nomenclatural history of Dioscorea in Madagascar was summarised in Wilkin & Randriamboavonjy (2012). Since the publication of that paper, field research in the North of the country has been undertaken by the authors of this paper under the project “Conserving Madagascar’s yams through cultivation for livelihoods and food security”, funded by the UK Darwin Initiative. This led to the discovery of a morphotype collected from three sites east of the RN6 near Sadjoavato in Antsiranana Province that potentially represented a new taxon. The morphotype has extensive and sometimes dense pubescence and vegetative stem leaves in clusters on short shoots, traits it shares with D. ovinala Baker. Two taxa of the D. sambiranensis R. Knuth complex (Wilkin et al. 2009) are also pubescent (D. sambiranensis and D. namorokensis Wilkin) but their leaves are always borne singly in an alternate arrangement. The first specimen collected of the morphotype was misidentified as D. sambiranensis, which has similar indumentum but in addition to the leaf arrangement, differs in having a long-pedicellate male flower (Wilkin et al. 2009), flat discoid floral torus and thicker leaf blades. Elements of the D. soso Jum. & H. Perrier complex, found further south, also have similar indumentum, however these taxa also have solitary, alternate leaves and short petioles in relation to blade length, by which they differ from D. ovinala and the plants from East of Sadjoavato. Thus this entity was compared with D. ovinala to assess its taxonomic status.
Materials & Methods
The hypothesis that the morphotype from East of Sadjoavato represents a distinct taxon was tested by morphological comparison with existing species of Dioscorea from Madagascar as defined in the introduction. The most detailed comparison was made with D. ovinala. Knowledge of the systematics of the yams of Madagascar is based on comparative morphological study and a database of 1346 specimens from the following herbaria: B, BM, G, K, MO, P, TAN, TCD, TEF, UPS and WAG (herbarium acronyms follow Thiers (2012)) and in the field. The specimens examined in the study of the new species are cited below. Floral dissections were carried out and measurements made using a Leica MZ95 with a measuring eyepiece. Vegetative and inflorescence characters were measured with a dial caliper.
Results and Discussion
A comparison of the principal characters differentiating Dioscorea irodensis from D. ovinala.
Tuber habit and size
Vertical in substrate, 50 – 100 × 10 – 15 cm, fusiform
Horizontal in substrate, to c. 80 × 15 cm, cylindric
Often dense, velutinous and soft to touch, especially on leaf lower surface and inflorescences
Sometimes dense but coarse rather than soft and velutinous
Solitary to in clusters of up to 4 on short herbaceous branches
Usually in clusters of up to 7 on short herbaceous branches, especially on primary vegetative shoots, sometimes solitary towards shoot apices
Chartaceous to thickly so, not translucent even when dry
Thinly chartaceous to membranous, sometimes translucent when dry
1 – 4 flowers per node, dense, sessile or rarely with a pedicel to 0.5 mm long
Dense cymular clusters of (1 –) 2 – 8 flowers on 0.2 – 1.1 mm long pedicels
Male flower habit and tepal shape
Tepals spreading, making flower rotate, tepals ovate with obtuse apices
Tepals ascending with recurving apices, making flower shallowly bowl-shaped, ovate to elliptic or broadly so with rounded to acute apices
Fruit dimensions and outline shape
26 – 40 × 17 – 30 mm, obovate to suborbicular
23 – 28 × 15 – 19 mm, obovate to oblong-obovate
Fruit wing form during development
Each wing c. 4 – 5 mm thick and fleshy when immature, when dry with clearly differentiated exocarp
Immature wing thickness not exceeding 1 mm, not fleshy, when dry wing single-layered.
GeoCAT yielded figures of 7.3 km2 for EOO CR and 12 km2 for AOO based on a user defined cell width of 2 km, as recommended by IUCN Standard and Petitions Subcommittee (2014). Both the EOO and AOO are below the thresholds for categorisation as EN; the EOO is also below the threshold for categorisation as CR (IUCN 2001).
Dioscorea irodensis Wilkin, Rajaonah & Randriamb., sp. nov. Type: Madagascar, Antsiranana: Diana, Antsiranana II, Anivorano, Irodo, ala fady S of village towards Irodo river estuary, 12°39'6.3"S 49°31'38.2"E, ♀ fr. 8 Feb. 2015, P. Wilkin, J. A. Kennerley, F. Rakotonasolo, M. Hamido & M. Tsaratiana 1675 (holotype TAN!; isotype K!).
recognition. Tuber horizontal in soil with a curved apex (derived from digging up two plants and inferring a similar position in others from the shape of extraction holes and the Malagasy vernacular name). Leaves in clusters of up to 7 on short herbaceous branches, clustered particularly towards bases of main vegetative stems (Fig. 1A, 2B), blades thin in texture, densely pubescent below and when immature but coarse and never forming a tomentum as in Dioscorea ovinala. Plants at the locality near Irodo represented by Wilkin et al. 1674 and 1675 often have white spotting on their leaf blades which has the appearance of pathogenic infection (see Fig. 2A). Male inflorescences (Fig. 1 E, F) with irregularly spaced, spirally arranged dense cymules of 2 – 8 pedicellate flowers or rarely solitary, vs flowers (sub)sessile, or rarely with a pedicel to 0.5 mm long and solitary or in groups of 2 – 4 (D. ovinala). Fruit not inflated and fleshy during development but thin-lobed and capsular throughout, single layered at dehiscence. Restricted to a small area East of Sadjoavato in Antsiranana Province.
distribution and habitat. Endemic to the Irodo river Valley and Sahafary forest in Antsiranana Province (Map 1) at altitudes from 30 – 230 m. It is possible that the three collections from South of the village of Irodo represent two subpopulations on either side of the river as opposed to two distinct populations (the term population is used here in an ecological sense, except in the Conservation Status section), but the specimen from Sahafary forest is spatially isolated. At Irodo, it is found in semi-deciduous forest with a canopy to 10 m and a clear shrub layer, the principal canopy trees being Colvillea and Tamarindus, with Pachypodium in the shrub layer. Soil a brown sandy loam, possibly alluvial, or red sand over limestone bedrock. The Sahafary Forest specimen states that it was from scrub forest on red sand.
specimens examined. madagascar. Antsiranana Province: Sahafary Forest, Analakay Forest, E of National Route #6, 12°35'03"S 49°25'08"E, ♂ fl. & ♀ immat. fr., 8 Nov. 2006, Z. S Rogers, R. Ranaivojaona, C. Davidson, S. R. Christoph & F. Ratov 1149 (K!, MO); Analamera, bank of Irodo R., near Irodo camp, 12°40'25"S 49°32'40"E, ♀ immat. fr 6 Jan. 2002, P. De Block, F. Rakotonasolo & T. Randriamboavonjy 1079 (BR, K!, K000523804); Diana, Antsiranana II, Anivorano, Irodo, ala fady S of village towards Irodo river estuary, 12°39'6.2"S 49°31'37.9"E, old ♂ fl. 8 Feb. 2015, P. Wilkin, J. A. Kennerley, F. Rakotonasolo, M. Hamido & M. Tsaratiana 1674 (K!, TAN!).
conservation status. The specimen data cited above and field surveys to date suggest that Dioscorea irodensis occurs at no more than three localities. D. irodensis is only known to be extant at one locality south of Irodo village on the banks of the Irodo River, where it was found in 2015 and 2016 (see Map 1). It has not been possible to cross the Irodo River during field activity in 2015 and 2016 to survey the subpopulation represented by the 2002 specimen (De Block et al. 1079). In March 2016, a field team spent a day locating the coordinates of the Sahafary forest specimen (Rogers et al. 1149) and searching other suitable habitat in Sahafary forest. The specimen was collected in highly degraded vegetation on the edge of a Eucalyptus plantation. No plants of D. irodensis were seen anywhere in Sahafary. If present they would certainly have possessed above ground organs in March, thus the subpopulation has either been eradicated through habitat degradation and loss or there is an error in the GPS coordinates, perhaps caused by use of an older GPS model. For the present we must assume the former. It is likely that the subpopulation represented by De Block et al. 1079 is extant given its isolated location, so we recalculated EOO and AOO excluding only the Sahafary forest data point. This gave an EOO of less than 1 km2 and AOO of 8 km2 based on a standard cell width of 2 km. Both are below the CR threshold.
In addition to habitat loss and restricted EOO/AOO, Dioscorea irodensis appears to be threatened by limited population size and extraction for use as food. At the site where it known to be extant, 10 adult plants were seen in 2015, of which 4 were female, and 15 in 2016, of which 5 were female. There is a limited reservoir of juvenile plants and a much more substantial one of seedlings with one or two leaves. A conservative estimate of generation length is 10 years; this is based on the authors’ experience of cultivating wild Dioscorea species from Madagascar from seed. Thus it could be 8 or 9 years before seeds are produced by plants that are currently juveniles or seedlings. We are in the process of acquiring data on the abundance of these life history stages via plot surveys. Approximately 80 seeds were collected for banking by the Millennium Seed Bank Partnership in March 2015. The sacred forest in which the subpopulation is found has an estimated 100 yam extraction holes, of which at least 20 appear to have been made within the last 12 months. Whilst timber extraction from the forest is culturally forbidden, removal of non-timber forest products is not. It is likely that any undiscovered subpopulations face a similar level of extraction which appears to be unsustainable. The village of Irodo has recently agreed to join the Darwin Initiative-funded project “Conserving Madagascar’s yams through cultivation for livelihoods and food security". The principal shared goal will be to conserve D. irodensis and D. maciba Jum. & H. Perrier through cultivation alongside the winged yam D. alata L. The productivity of the latter should drive down demand for wild-harvested yams. We will also collect further seed for banking and duplicate living accessions elsewhere as far as possible.
In accordance with IUCN (2001) criteria, these data and observations suggest Dioscorea irodensis should be designated as CR A2acd+4acd; B1ab(i, ii, iii, iv, v)+2ab(i, ii, iii, iv, v); C1+2a(i); D (Kennerley & Wilkin, in prep.).
etymology. Named for the village of Irodo and the Irodo River valley where this species is found.
vernacular name. Bemandry in the village of Irodo. This name appears to be applied particularly to species with horizontal tubers, for example the element of Dioscorea soso Jum. & H. Perrier sensu lato with the same tuber habit.
uses. Tuber edible cooked. Rather watery following cooking and thus not fully satisfying the appetite. Dioscorea maciba, known as batatala in Irodo, is the more sought after species of the two that are found in the Irodo valley and surrounding areas. Not believed to be sold in markets but consumed usually in March/April.
notes. The specimen de Block et al. 1079 was included within Dioscorea sambiranensis subsp. bardotiae Wilkin by Wilkin et al. (2009) as an atypical element. Its removal to D. irodensis restricts the known distribution of D. sambiranensis subsp. bardotiae to the Montagne des Français (just one specimen) and the Réserve spéciale d’Ankarana and surrounding limestone outcrops. The resulting reduction in EOO and AOO is likely to increase the threatened status of that subspecies from Vulnerable (Wilkin et al. 2009) to Endangered and this is confirmed by recent unpublished EOO and AOO results obtained from GeoCAT.
Our principal acknowledgement is to the Darwin Initiative as funders of project 22-005, “Conserving Madagascar’s yams through cultivation for livelihoods and food security”. We also acknowledge The Lord Faringdon Charitable Trust for funding fieldwork in February 2015 that enabled confirmation that this species was distinct and undescribed. We also wish to thank the people of the Irodo Base-Community for their help and hospitality, especially M. Paul, M. Moustafa and Mme Berthine. We welcome the community joining those engaged in the Darwin Initiative project with a view to conserving Dioscorea irodensis (Bemandry) and D. maciba (Batatala) together. Thanks are also due to Madagascar National Parks in Anivorano, especially M. Hamido, Mr Ramiandrarivo Liva Hariniaina, Directeur des Aires Protégées Terrestre (DAPT) — Ministère de l’Environnement, de l’Ecologie, de la Mer et des Forêts (MEEMF) for issuing collecting permits, and Mr Zafimandimby, Chef Cantonnement in Ambilobe for their help. Finally, we thank Steve Bachman for his input into the conservation status assessment and Dr Hélène Ralimanana, Dr Franck Rakotonasolo, Landy Rajaovelona, Hajatiana Rakotoarimanana, Roger Rajaonarison and Joel Rakotonirina for their facilitation of this research, and the anonymous reviewers who helped to improve the manuscript
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