Journal of Threatened Taxa | www.threatenedtaxa.org | 26 July 2018 | 10(8): 12056–12063

Conservation status of Mascarene Amaranth Aerva congesta Balf.F. Ex Baker (Eudicots: Caryophyllales: Amaranthaceae): a Critically Endangered endemic herb of the Mascarenes, Indian Ocean

Kersley Bruno Pynee¹, David Harold Lorence² & Poojanraj Khurun³

¹ The Mauritius Herbarium, Agricultural Services, Ministry of Agro-Industry and Food Security, Reginald Vaughan Building, Mauritius Cane Industry Authority Compound, Réduit, Mauritius

² National Tropical Botanical Garden, 3530 Papalina Road, Kalaheo, HI 96741, USA

³ The Forestry Service, Mauritius, Ministry of Agro Industry and Food Security, Head Office, Botanical Garden Street, Les Casernes, Curepipe, Mauritius

¹ kpynee@govmu.org (corresponding author), ² lorence@ntbg.org, ³ pkhurun@govmu.org

Abstract: Aerva congesta Balf.f. ex Baker (Amaranthaceae), an endemic of Mascarene Islands (Mauritius and Rodrigues) is assessed for its conservation status. Considering its local extinction in Rodrigues and very small population in Mauritius, in Round Island and at Gris Gris, south of Mauritius its area of occupancy (AOO) is estimated at 8km². Due to its small AOO and threats by increasing native vegetation and alien species, A. congesta is evaluated as Critically Endangered following the latest IUCN Red List criteria. The species needs an urgent well-defined restoration program and ecological studies to prevent its extinction.

Keywords: Amaranthaceae, conservation status, extinction, Mauritius, Rodrigues.

doi: http://doi.org/10.11609/jott.2402.10.8.12056-12063

Editor: F.B. Vincent Florens, University of Mauritius, Moka, Mauritius. Date of publication: 26 July 2018 (online & print)

Manuscript details: Ms # 2402 | Received 04 January 2017 | Final received 12 June 2018 | Finally accepted 20 July 2018

Citation: Pynee, K.B, D.H. Lorence & P. Khurun. (2018). Conservation status of Mascarene Amaranth Aerva congesta Balf.F. Ex Baker (Eudicots: Caryophyllales: Amaranthaceae): a Critically Endangered endemic herb of the Mascarenes, Indian Ocean. Journal of Threatened Taxa 10(8): 12056–12063; http://doi.org/10.11609/jott.2402.10.8.12056-12063

Copyright: © Pynee. et al. 2018. Creative Commons Attribution 4.0 International License. JoTT allows unrestricted use of this article in any medium, reproduction and distribution by providing adequate credit to the authors and the source of publication.

Funding: None.

Competing interests: The authors declare no competing interests.

Author Details: Mr. Kersley Bruno Pynee, is a Senior Technical Assistant at The Mauritius Herbarium, Agronomy Division, Agricultural Services of the Ministry of Agro-Industry & Food Security. His main research interests are botanical survey, population survey of Critically Endangered plants, plant identification and conservation of plant species. He is holder of a BSc in Botany with specialisation in Environmental Management. He has 16 years of experience in botanizing and identifying the local flora. David H. Lorence, Ph.D., is Director of the Science and Conservation Department and Curator of the Herbarium at the National Tropical Botanical Garden. His research specialties are systematics and floristics of tropical plants, specifically the Rubiaceae and Monimiaceae families and ferns of the Pacific and Indian Ocean islands. Mr. Poojanraj Khurun is Deputy Conservator of Forest in the Forestry Service of the Ministry of Agro-Industry & Food Security of Mauritius. He is holder of a MSc in Forestry Management. He has 17 years of experience in forest management and biodiversity conservation.

Author Contribution: KBP: participated in botanical survey at Gris Gris where he collected and identified the plant; contributed also in taxonomic and editorial expertise. DHL: contributed taxonomic and editorial expertise. PK: organized and participated in field survey at Gris Gris and contributed in editorial expertise

Acknowledgements: We thank the Sir Seewoosagur Ramgoolam Botanic Garden Trust, Pamplemousses, Ministry of Agro-Industry & Food Security (MoAIFS) for organizing the field trips, the staff from the National Parks & Conservation Service (MoAIFS) and the Mauritian Wildlife Foundation for sharing information on the species. Comments from Cláudia Baider greatly improved the manuscript.

Introduction

The genus Aerva Forssk. (Amaranthaceae) consists of 21 species (The Plant List 2013) distributed mostly in the arid or semi-arid regions of the world. Bojer (1837) noted three species of Aerva on Mauritius, A. lanata (L.) Juss., A. caudata Bojer (nomen nudum), and A. chenopodifolia Bojer (now Nothosaerva brachiata (L.) Wight), which are considered as native. Later, Baker (1877) described three species of Aerva on Mauritius, two perennials, A. lanata and A. congesta Balf. f. ex Baker, and one annual, A. brachiata Mart. More recently, Townsend (1994) listed three species, the Mascarene endemic A. congesta, and two alien species formerly known only from cultivation, A. lanata and A. sanguinolenta (L.) Blume.

Referring to Townsend (1994) and Sukhorukov (2013), Aerva congesta is one of the two known endemic species of the Amaranthaceae in the Mascarene Islands (South West Indian Ocean) (Image 1). It is known from both Mauritius (Image 2) and Rodrigues (Image 3), but is not recorded on Réunion Island. The first specimen was collected by Balfour in 1879 where he indicated that the species grew as a small compact herb present on coralline limestone, in association with Abrotanella rhynchocarpa Balf.f. (= Rhamphogyne rhynchocarpa S. Moore) and Oldenlandia sieberi Baker var. congesta. According to Strahm (1989) A. congesta has not been seen or collected on Rodrigues since Balfour, and it is probably extinct there; extensive surveys made on mainland Rodrigues (e.g., Anse Quitor) and some islets (e.g., Ile Gombrani, Ile Chat, Ile Crabe) were unfruitful (Wiehe 1949; Cadet 1972, 1975; Guého 1980; Smith et al. 2004a,b). On Mauritius, Strahm (1989) quoted that there were recent collections from one locality on mainland Mauritius, but only samples from Round Island, an outer islet northern of Mauritius (Image 2), were accessed at The Mauritius Herbarium.

Material and Methods

In June 2013, while carrying out a preliminary population survey of the endemic fern, Ctenitis maritima (Pynee & Khurun 2013) at Gris Gris, in the village of Souillac, south coastal region of Mauritius (-20.52526⁰S and 57.53086⁰E, 6m elevation) (from Google Earth), the authors (KBP & PK) collected an interesting small prostrate herb, 0.8–3.2 cm tall, on border of the cliffs. Two small patches of A. congesta sized 1.8 x 2 cm and 6 x 12.5 cm with a distance of 2.2m between them and consisting of around 10 plants were found. Through closer study and comparison with herbarium specimens and using the ‘Flore des Mascareignes’ at The Mauritius Herbarium (MAU), the sample collected was identified as Aerva congesta Balf.f. ex Baker. The herbarium specimen is deposited at The Mauritius Herbarium, Department of Agricultural Services for the Ministry of Agro Industry and Food Security (Image 4). Significant morphological characters, locality, absolute location, elevation, phenology, other associated species and distribution data were included in the herbarium label.

The actual collection at The Mauritius Herbarium (MAU 0014824) of this small herb is the first sample from mainland Mauritius accessed for this herbarium. The small dimensions of the species, coupled with the steepness of cliffs at Gris Gris (Image 5), are likely to have contributed towards it not being recently collected in this locality.

All MAU available collections before 2014 were from Round Island, with only one voucher (Strahm s.n., MAU 0016304) giving details of its habitat or ecology, showing that the species was restricted to an area near a big gully and below the helipad (-19.85668⁰S and 57.78629⁰E; 63m elevation) (from Google Earth), and found occasionally in cracks on bare rocks, similar to where the species was found at Gris Gris.

Results

Aerva congesta in the Mascarenes is extant only on Mauritius and is believed to be extinct in Rodrigues. This halophyte species grows in open areas and bare rocks, within the salt-sprayed region (Strahm 1986; Bullock et al. 2002; Lavergne 2007; Khadun et al. 2008). On Round Island it grows with native species of grasses, forming the only native dominated community in this islet (Johansson 2003) and includes a subpopulation of around 19 individuals (A. Gungadurdoss, pers. comm. 21 June 2016). At Gris Gris, the species shows a similar ecology; it is found rooted in rock crevices of exposed cliffs in association with the native grasses and sedges, including (Zoysia matrella (L.) Menill, Stenotaphrum dimidiatum (L.) Brongn. and Fimbristylis cymosa R.Br.), as well as other native species (Dichondra repens J.R. Forst. & G. Forst., Ctenitis maritima (Cordem.) Tardieu and Selaginella obtusa Spring). In Gris Gris, the fire prone alien invasive grass Heteropogon contortus (L.) P.Beauv. ex Roem. & Schult. is also present.

Evaluation of current conservation status (IUCN 2001)

The whole population of the species is considered under ‘2 locations’ comprising two subpopulations. The extent of occurrence (EOO) of the species cannot be calculated from two localities and is deemed to be small. Taking the topographic limitations throughout its geographic range, the Area of Occupancy (AOO) was measured as 8.0km² (800ha) and the number of mature individuals was 29.

Criterion B: B1: The EOO of the species cannot be computed from two localities and is deemed to be very limited. Of the three conditions to be fulfilled under B1, the species is found to qualify for two: (a) severely fragmented and (b) continuing decline observed under the sub criteria: (iii) area, extent and or quality of habitat and (v) number of mature individuals. B2: The AOO is 8.0km² and since this estimate is less than 10km², and fulfils two of the three conditions: (a) severely fragmented, and (b) continuing decline observed under the sub criteria: (iii) area, extent and or quality of habitat, and (v) number of mature individuals; the species qualifies under ‘Critically Endangered’ category.

Criterion C: C2: Based on observed, estimated, projected and inferred continuing decline, the species is ‘Critically Endangered’ as it qualifies under C2, fulfilling one of the two conditions: (a) (i) number of mature individuals in each subpopulation comprises less than 50 mature individuals.

Criterion D: Since the estimated population comprises less than 50 mature individuals it qualifies under ‘Critically Endangered’ category.

Final assessment: CR B1ab(iii,v) & B2ab(iii,v); C2a(i); D (Appendix 1).

Discussion

The species has been closely monitored on Round Island for nearly four decades and has always occurred in low density (Strahm 1986; Bullock et al. 2002; Khadun et al. 2008), with an estimated population of less than 100 individuals (Page 1995). From 1975 to 1996 the density and distribution remained unchanged (Bullock et al. 2002) even after eradication of invasive alien goats and rabbits, indicating that grazing by alien species seemed not to have been a threat. This could be related to the fact that the species might be adapted to grazing as both Mauritius and Rodrigues had two species of giant tortoises each. On the other hand, the species has declined by 50–60 % in more recent times and it was mentioned that increasing native vegetation cover could be the main threat on Round Island (Khadun et al. 2008). Indeed, it is known that grazing can promote species co-existence by reducing interspecific competition between plants and thereby reducing likelihood of competitive exclusion of one plant by another (Begon et al. 2006). In 2007, an analogue species of extinct tortoises of Mauritius, the Giant Aldabra Aldabrachelys gigantea and the Madagascan Radiated Tortoise Astrochelys radiata, were introduced to Round Island to help restore ecological function such as grazing and seed dissemination (Griffiths et al. 2013). The number of individuals of the species on Round Island after introduction of these tortoises has, however, decreased from an estimated population of less than 100 individuals (Page 1995; Bullock et al. 2002) to a subpopulation of 16 individuals (A. Gungadurdoss, pers. comm. 21 June 2016).

As a safeguard conservation action, the species is successfully propagated at the nursery of the Nature Reserve of Ile aux Aigrettes since over a decade. The propagated plants were used for both augmenting the population of Round Island (Khadun et al. 2008), and also trying to establish a new population on Ile aux Aigrettes. Of the 50 individuals planted on Ile aux Aigrettes in 2013, however, none survived the first year (M. Goder, pers. comm. 24 July 2014).

The discovery of a new population at Gris Gris reduces the species’ chances of extinction. However, the newly discovered population in Gris Gris is very small. Furthermore, there are signs of natural erosion of the cliff around where the species is growing, decreasing the number of suitable sites. The presence of alien invasive weeds, as well as fast-growing native species, like Stenotaphrum dimidiatum has potentially negative impacts on A. congesta at Gris Gris through interspecific interactions detrimental to native plant populations as shown elsewhere on the island (Baider & Florens 2011). To conserve the recently located population, we suggest setting up ex situ propagation by the institutions concerned like the National Parks & Conservation Service and the Forestry Service; avoiding mixing the plants from the two populations to preserve their eventual genetic distinctiveness. A minor weeding would minimize the negative effect posed by alien species as well as the fast growing grasses like Z. matrella and S. dimidiatum; however, this needs to be well planned to minimize soil erosion. A restoration program should also be implemented to increase native cover of the site that contains other threatened native species like the fern Ctenitis maritima and the endemic liana Cissus anulata Desc., which is known only from this region of the island; taking into account not to create too much shade over the plant which requires substantial exposure to thrive.

Considering the failed augmentation on Round Island since 2004 and the failed introduction on Ile aux Aigrettes in 2013, we suggest setting up systematic research to better understand the ecological requirements of the species. Finally, it is advisable to survey similar habitats in Mauritius, Rodrigues and Réunion to try to locate eventual new populations.

Specimens examined: Mauritius: Round Island, 22.vii.1968, Michel, s.n. (MAU 0015990); August 1975, Bullock et al., s.n. (MAU 0015991); August 1978 Gardner et al. s.n. (MAU 0016305); 2.viii.1986, Strahm s.n. (MAU 0016304); Gris Gris, 23.vii.2013, -20.525277S; 57.530555E, 9m elevation, Pynee et al. s.n. (MAU 0014824); Ile aux Aigrettes, 24.vii.2014, -20.420833S; 57.730833E, 8m elevation, Pynee s.n. (MAU 0016024; cultivated).

Rodrigues: no loc., August–December 1874, Balfour s.n. (holotype K 000243711; isotype M).

References

Baider, C. & F.B.V. Florens (2011). Control of invasive alien weeds averts imminent plant extinction. Biological Invasions 13(12): 2641–2646; http://doi.org/10.1007/s10530-011-9980-3

Baker, J.G. (1877). Flora of Mauritius and the Seychelles: A Description of the Flowering Plants and Ferns of Those Islands. L. Reeve & Co. London, UK, 557pp; http://doi.org/10.5962/bhl.title.139960

Begon, M., C. Townsend & J.L. Harper (2006). Ecology: From Individuals to Ecosystems. Blackwell Publishing. ISBN : 978-1-405-11117-1

Bojer, W. (1837). Hortus Mauritianus ou énumération des plantes, exotiques et indigènes, qui croissent à l’île Maurice, disposées d’après la méthode naturelle. Aimé Mamarot et Compagnie, Mauritius, 456pp.

Bullock, D.J., S.G. North, M.E. Dulloo & M. Thorsen (2002). The impact of goat and rabbit eradication on Round Island, Mauritius. In: Veitch, C.R. & M.N. Clout (eds.) Turning the Tide: The Eradication of Invasive Species. Proceedings of the international conference on eradication of island invasives. Occasional Paper of the IUCN Species Survival Commission 27: 53–63.

Cadet, T. (1972). La végétation des régions calcaires de l’Ile Rodrigues. Cahiers du Centre Universitaire Réunion: 2: 1–6.

Cadet, T. (1975). Contribution à l’étude de la végétation de l’Ile Rodrigues. Cahiers du Centre Universitaire Réunion: 6: 5–29.

Griffiths, C.J., N. Zuel, C.G. Jones, Z. Ahamud & S. Harris (2013). Assessing the potential to restore historic grazing ecosystems with tortoise ecological replacements. Conservation Biology 27: 690–700.

Guého, J. (1980). A survey of the vegetation of the lagoon islets of Rodrigues. Revue Agricole Sucrière de l’ Ile Maurice 59: 17–48.

IUCN (2001). IUCN Red List Categories and Criteria. Version 3.1. Gland, Switzerland.

Johansson, M.C. (2003). Vegetation Monitoring and Change on Round Island, Mauritius. MSc Thesis, University of Birmingham, Birmingham, UK.

Khadun, A., V. Bachraz, E. Dulloo, R. Atkinson, V. Tatayah, C. Griffiths & J. Mauremootoo (2008). A Management Plan for the Restoration of the Round Island, Mauritius (2008–2012), 36pp.

Lavergne, C. (2007). Les palmiers menacés de Maurice et de ses ilots. Technical Report. 45pp. http://palmeraie-union.com/pdf/rapport_Maurice.pdf

Page, W. (1995). Conservation management strategies for the endangered flora of Mauritius. Unpublished report submitted to the National Threatened Plant Technical Committee. Ministry of Agriculture, Food Technology and Natural Resources. January 1995.

Pynee, K. & P. Khurun (2013). A preliminary population inventory of Ctenitis maritima (Cordem.) Tardieu (Aspidiaceae) at Gris Gris, southcoast of Mauritius. Cahiers Scientifiques de L’Océan Indien Occidental 4: 39–44.

Smith, M.A.K., J. Hills, F.B.V. Florens, I. Sotheran & D. Daby (2004a). Proposed management plan for Gombrani Island. Ministry of Agriculture, Food Security and Natural Resources, Mauritius, 33pp.

Smith, M.A.K., J. Hills, F.B.V. Florens, I. Sotheran & D. Daby (2004b). Proposed management plan for Crabe Island. Ministry of Agriculture, Food Security and Natural Resources, Mauritius, 42pp.

Strahm, W.A. (1986). Botanical Report, Round Island 1986. Unpublished Report to WWF/IUCN.

Strahm, W.A. (1989). Plant Red Data Book for Rodrigues. IUCN-WWF Plants Conservation Programme. Konigstein, West Germany, 241pp.

Sukhorukov, A.P. (2013). A new Atriplex species from the island of Mauritius. (Amaranthaceae). Skvortsovia 1(1): 75–80

The Plant List (2013). Version 1.1. Published on the Internet; http://www.theplantlist.org/ (accessed 1st January 2015).

Townsend, C.C. (1994). Amaranthacées. In: Bosser J., T. Cadet, J. Guého & W. Marais (eds.). Flore des Mascareignes - La Réunion, Maurice, Rodrigues. 81. Rosacées a 89. Callitrichacées. IRD/MSIRI/ Kew., Paris, 32pp. ISBN 2709914115.

Wiehe, P.O. (1949). The vegetation of Rodrigues Island. Bulletin of the Mauritius Institute 2: 279–305.

Appendix 1. Assessment

RED LIST ASSESSMENT (IUCN 2001): Aerva congesta

Kingdom: Plantae

Phylum: Tracheophyta

Class: Magnoliopsida

Order: Caryophyllales

Family: Amaranthaceae

Subfamily: Amaranthoideae

Genus: Aerva

Species: congesta

Authority: Isaac Bayley Balfour, 1877.

Common name: Mascarene Amaranth

Taxonomic notes: The species was described first by Isaac Bayley Balfour (1877) with distribution in Rodrigues, where its presence was observed as frequent on the island. Type specimen is from Rodrigues: Balf.f. s.n., 1874 (holo. K, 1 sheet; isotyp. M, 1). The species is considered valid and Aerva congesta is an accepted name as per version 1.1 of The Plant List (2013).

Assessment Information

Red List Category and Criteria (Version 3.1): Critically Endangered CR B1ab(iii,v) & B2ab(iii,v); C2a(i); D.

Justification: The whole population of the species is restricted to two severely fragmented locations comprising two sub-populations. The area of occupancy is estimated as 8km² and the number of mature individuals is 29. Major threats such as erosion, fire, and alien species keep this species as Critically Endangered due to the continuing decline observed in the area/quality of habitat and number of mature individuals.

Geographic Range / Distribution Information

Range description: The species is endemic to Mauritius and is found at an elevation range of 3–90 m.

Countries of occurrence: Endemic to Mauritius and presumed extinct in Rodrigues

Extent of Occurrence (EOO): Not estimated, but is very small.

Area of Occupancy (AOO): AOO is estimated to be 8km².

Number of locations: The species is currently found in two severely fragmented locations

Range map: See Image 2.

Population Information

Population: The species is estimated to have about 29 mature individuals, of which 10 are estimated to be found at Gris Gris and the remaining 19 on Round Island.

Population trend: The population is declining. Over the last 10 years the population has declined by 50–60 %, mainly on Round Island due to various threats primarily due to competition with fast growing native and alien grasses and soil erosion.

Habitat And Ecological Information

Habitat and ecology: This halophyte species is a small prostrate herb, 0.8–3.2 cm tall, grows in open areas, in cracks and bare rocks, within the salt-sprayed region on border of the cliffs and in rock crevices of exposed cliffs at an elevation between 3-90 m. The species is mostly restricted to cliffs and predominantly brown soils. In association with the following native species of grasses and sedges (Zoysia matrella (L.) Menill, Stenotaphrum dimidiatum (L.) Brongn. and Fimbristylis cymosa R.Br.) as well as other native species (Dichondra repens J.R. Forst. & G. Forst., Ctenitis maritima (Cordem.) Tardieu and Selaginella obtusa Spring). In Gris Gris, the fire prone alien invasive grass Heteropogon contortus (L.) P.Beauv. ex Roem. & Schult. is also present. Key threats to the species observed in the study area are competition with fast growing native and alien grasses; sometimes competing for resources like light, water and nutrients and leading to poor plant growth.

System: Small prostrate perennial herb.

Information On Threats

Major Threats: The main threats to the two species population are:

Direct competition for resources such as water, light, nutrients with exotic grasses and other herbaceous weeds.

Increasing erosion, mainly during the dry season where there is less ground cover due to die off of exotic grasses like Heteropogon contortus and where the ground is more vulnerable to erosion.

Additional threats: Drought; fire; cyclones; diseases; pest attack; littering (at Gris Gris) burrowing by Shearwater birds (on Round Island) and thus leading to damage of plants are other possible threats.

Use And Trade Information

Use: Not known.

Livelihoods and sustenance: Not reported.

Trend in off take from the wild: Not reported.

Trend in off take from cultivation: Not reported.

Information On Conservation Actions

Conservation actions:

The Mauritian Wildlife Foundation is propagating the species through seeds and cuttings for augmentation on Round Island and introduction on Ile aux Aigrettes. Forms part of the Round Island Plant Restoration Plan

Weeding and planting in localized patches

Research in place:

Optimising choice of sites for the augmentation, re-introduction and introduction programmes to ensure that the correct conditions for the species, including microclimatic requirements are fulfilled.

Investigating the translocation of Aerva congesta to other offshore islets and Rodrigues.

Investigating the fate of native herbaceous community on Round Island and carrying out weed management in localized patches.

Research needed:

Setting up systematic research to better understand the ecological requirements of the species.

Determining the success of different planting techniques regimes on plant survivorship.

Monitoring in place:

Protecting planted areas against Shearwater burrowing

Sowing seeds into restoration areas

Treating restoration planting and seed sowing as field trials in order to gain valuable information to aid future restoration work on Round Island and elsewhere.

Education in place: Not known

Education needed: Awareness and sensitization campaign