A review of the
conservation status of the threatened western Indian Ocean island tree Pisonia sechellarum (Nyctaginaceae)
Justin Gerlach1, Bruno Senterre 2 & Fabien Barthelat 3
1 Nature Protection Trust of Seychelles,
PO Box 207, Victoria, Mahé, Seychelles
2 Evolutionary Biology &
Ecology - CP 160/12, Université Librede Bruxelles, 50 Av. F. Roosevelt; 1050 Bruxelles, Belgique
3 IUCN, ORMA-Caribe, c/o Parc National de
Guadeloupe Habitation Beausoleil, Montéran 97120
Saint-Claude, Guadeloupe
1 gerlachs@btinternet.com (corresponding author), 2 bsenterre@gmail.com,3 fabien.barthelat@iucn.org
Abstract: The status of the tree Pisonia sechellarum F. Friedmann(Nyctaginaceae) endemic to the western Indian Ocean
is reviewed. Five populations have been located: four in the Seychelles Islands
(three on Silhouette Island and one on Mahé) and one
on Mayotte. The species is
associated with montane forest and ravine habitats
and the genetic identity of different populations remains to be
investigated. This species is considered to be endangered.
Keywords: Conservation status, Mayotte, Nyctaginaceae,Pisonia, Seychelles, tree.
doi: http://dx.doi.org/10.11609/JoTT.o3310.4621-9
Editor: Anonymity
requested. Date
of publication: 26 August 2013 (online & print)
Manuscript
details: Ms# o3310 | Received 18 August 2012 | Final received 25 April 2013 | Finally
accepted 29 June 2013
Citation: Gerlach, J., B. Senterre& F. Barthelat (2013). A review of the
conservation status of the threatened western Indian Ocean island tree Pisonia sechellarum(Nyctaginaceae)a. Journal of Threatened Taxa 5(12): 4621–4629;http://dx.doi.org/10.11609/JoTT.o3310.4621-9
Copyright:© Gerlach et al. 2013. Creative
Commons Attribution 3.0 Unported 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:No specific funding was received for the research described here.
Competing
Interest: Authors declare no competing interest.
Acknowledgements: We
are grateful to everyone who has assisted with the field research.
Author Details and Contribution: Justin Gerlach- Scientific Cooridnator of Nature Protection Trust
of Seychelles, has carried out research on all aspects of the biodiversity of
the Seychelles islands since 1986, including all the ecological surveys of the Pisonia sechellarumforest. Lead author of the manuscript. Bruno Senterre,
PhD in agronomy science, at the Université Libre de Bruxelles, is
specialized on the study of vegetation types in tropical environments. He also
worked on phytogeography of central Africa, functional ecology, island
ecosystems and taxonomy. He is currently consultant mostly for the government
of Seychelles and United Nations Development Programme(UNDP). Fabien Barthelat is a botanist and
a public agent that has worked for the French ministry of agriculture from 2000
to 2007 in Mayotte where he was inventorying the flora in collaboration with
the “Muséum National d’Histoire Naturelle” of Paris. Then he was working for the
national forest department in Guadeloupe (French West Indies) and he was
seconded to IUCN in 2010 to implement projects in the Caribbean.
For figures, images, tables -- click here
Introduction
Pisonia sechellarum F. Friedmannis a moderately sized tropical tree confined to the islands of the western
Indian Ocean. According to Friedmann (1994) it is thought to be
related to the Indo-Pacific P. umbellifera(J.R. Forster & G. Forster) Seemann but there
have been no detailed studies of its affinities to date. As with many of the plants of the region
its survival is considered to be threatened by
invasive species. In the context of
the western Indian Ocean islands its 20m height is relatively large, and it is
a locally significant component of the forest canopy. The species was discovered in 1986 from
one small area of Silhouette Island (Friedmann1986). Subsequently (in 2009 and
2012) two more populations were discovered on the island, on the slopes of MontDauban (Senterre &
Nolin pers. obs. 03 November 2009) and Mont Pot à Eau (Senterrepers. obs. 27 November 2012). A
sterile specimen from Mahé island (Morne Blanc) from 1874appeared to be this species but this could not be confirmed by Friedmann (1994). Nevertheless, this record was confirmed in 2010 when a group of local
botanists explored a steep ravine close to MorneBlanc and observed a single plant, still sterile but definitely belonging to
the species P. sechellarum (Chong-Seng & Senterre pers. obs. 24
October 2010). It is now known not
to be a Seychelles endemic, being also found on the main island of Mayotte (Barthelat & Boullet2005).
In 1990 the main population
(known as the ‘Pisonia forest’) was the focus
of a three-month ecological study which identified plant and animal diversity
of the site, estimated the number of trees and investigated the reproductive
ecology of P. sechellarum (OUSE 1990). During this study only one potential
pollinator species was identified, an undescribedspecies of heliodinid moth, Epicroesa sp. This
moth was observed visiting flowers and ovipositing in
them. Larvae of the moth were found
to feed on developing fruit resulting in 100% fruit loss in 1990 (Floater 1995). The lack of recruitment was considered
to be a threat to the long-term survival of the species, with most trees
apparently originating as suckers from a small number of parent trees. As with other Pisoniaspecies, this tree produces sticky fruits which are presumed to be adapted to
bird dispersal. The fruits are
larger than those of most Pisonia (around 4cm)
and no dispersal agent has been identified.
Prior to the discovery of theMahé, Mayotte and smaller Silhouette populations this
tree was considered to be Endangered on the basis of
its restricted range and small population, and the presence of invasive species
(Ismail et al. 2011). The new
discoveries require a re-evaluation of its status. The available data are collated here,
combined with monitoring data for the main population gathered over 20 years.
Recorded populations
The populations (marked on
Fig. 1) are defined as:
1. Pisoniaforest (4028’54S & 55014’01E) - This was the first
population to be discovered (in 1986). It is the main Silhouette population in the Anse Mondon Valley, found at 450–550 m. P. sechellarum is the dominant tree at this site.
2. South of
Mont Pot à Eau(4029’02S & 55014’13E) - This population was discovered
in November 2012 in a ravine near the “Pisoniaforest”. This is an extensive P.sechellarum dominated forest similar in size and
composition to the “Pisonia forest”. It extends from 496–557 m.
3. AboveGrande Barbe (4029’03S & 55013’55E)
- The population above Grand Barbe was located in
2009. It is found towards the
southern slopes of Mont Dauban, in the upper part of
the Rivière Quatre Cent
(410m elevation). In this area,
many small ravines are running southwards down from the slopes of Mont Dauban, with much smaller granite boulders than in the “Pisonia forest”, which may explain why it does not form a
distinct forest type like in the Pisoniaforest. No detailed inventory has
been done, but the two botanists who discovered it estimated that fewer than
ca. 10 trees were present. All were sterile and rather small, i.e. less than
10m high. Other species observed in
this new locality included several rare species found in the “Pisonia forest”, e.g. Tectariaspecies.
4. MorneBlanc (4039’19S & 55025’56E) - The population of Morne Blanc is limited to only one individual tree observed
in 2010 in a very similar habitat to the “Pisoniaforest”, i.e. a very steep ravine with large granite boulders at 475m elevation. Nevertheless, the local botanists who found it could not explore the
difficult terrain further, and no other expedition has been organisedto return to this site.
5. Mayotte, Réserve Forestière de Majimbini (12046’10S & 45010’42E)
- on Mayotte, this species was first collected in 2003. The first identified locations was in
the Réserve Forestière de Majimbini where few specimens of P. sechellarum are found at 400–500 m on a river
side in a humid forest with Grisollea myrianthea Baillon, Labramia mayottensis Labat, Pignal& O. Pascal,Cynometra floretii Labat & O. Pascal, Marattia fraxinea Sm. ex J.F.Gmel and also the invasive Syzygium jambos (L.) Alston.
6. Mayotte, Réserve Forestière des Crêtes du Sud (12051’29S
& 45009’01E) - The second known location is a single tree and
situated in the Réserve Forestièredes Crêtes du Sud (foothill
of the Tchourembo) at 350–400 m. This tree is found in a disturbed place
alongside indigenous and exotic plants including Mangifera indica L. and Litsea glutinosa (Lour.) C.B. Robinson. However, the slopes of this mountain are
little studied; it is possible that more trees occur in the “rocky” humid
forest of the slopes. Basaltic
boulders are present in both known locations. The identification of
the Mayotte trees was confirmed by F. Friedmannand they have been referred to in literature (Barthelat & Boullet 2005) but not discussed in
detail. The Conservatoire Botanique National de Mascarin is
carrying out an inventory of the Réserve Forestière de Majimbini.
Methods
The only site to have been
studied in detail is the Pisonia forest on
Silhouette Island. This site was
studied in 1990, 2000 and 2010. In
1990 the forest was mapped, with forest boundaries being defined as the
outermost P. sechellarum trees known at the
time. Subsequently, some isolated
trees have been located outside this boundary but these are sparse and the
boundary remains a reasonable definition of the area. A transect of 180m length was taken down
the centre of the valley and used as a reference line
for all studies. This line was not
straight due to the extremely rocky nature of the terrain, but provides a
reference line rather than a true line of orientation. Climatic factors (temperature and
humidity) were recorded at a fixed point at the top of the line throughout
July–September 1990.
The number and size of trees
was studied in 10x10 m quadrats at 10 points along the transect line. In each quadrat all the trees over 1.5m
tall were counted. This was
repeated in the top section in 2000 and 2010. Tree heights were measured by
trigonometry with a clinometer and four trees were measured in each of three
areas along the transect line. This was carried out once, in 1990.
Vegetation was determined
using 2x2 m quadrats along the line, spaced 2m apart. Every 2m along the line two quadrats
were surveyed either side of the line, giving a total of 130 quadrats and totalling 520m2. In each quadrat the percentage of ground
covered by each species was recorded. This was repeated in 2010 using the upper 10 original points. These could be accurately relocated as
the position of the transect line in the upper section was well defined by the
trees and rocks used to secure the line in 1990.
Canopy cover was investigated
by taking 10 photographs from the upper vegetation quadrat points. The camera was positioned vertically at
head height and a 35mm lens was used to photograph the canopy. The resultant digital photograph was
converted to a black and white image and the number of black and white pixels
counted (analysed in Gimp), giving a percentage of
cover. This was calculated for 1993
and 2010.
Analysis
Changes in the cover of each
plant species in the two surveys of the Pisoniaforest were evaluated using a paired sample t-test of the arcsine transformed
percentage cover, following tests for normality and homoscedasticity, the
transformed data were confirmed to show a normal distribution. Numbers of Pisoniatrees were not normally distributed and were compared using a Wilcoxson matched pair test.
Surveys
Surveys of potentially
suitable sites were carried out on all islands in attempts to locate further
populations of P. sechellarum. The species is associated with steep
boulder filled ravines above 500m and most of these areas have been
visited. On Silhouette all the
larger ravines have been examined. On Mahé more ravines exist and surveys are not
fully complete. The areas surveyed
comprise the ravines around Morne Blanc, Congo Rouge
and Morne Seychellois, Mont Cotonand Pérard (two ravines explored, others remain
unexplored) and Montagne Planneau (extensively
explored, but many ravines are difficult to access and remain unexplored).
Results
Pisonia forest (Image 1)
The area defined as the Pisonia forest on Silhouette covers 4820m2. The additional isolated trees expand this
area to about 5500m2. P.sechellarum were
recorded from 446–560 m. In
this site in July–September 1990 temperature remained in the range
18–26 0C with a mid-day mean of 230C and humidity
ranged between 84–100 % with a mid-day mean 93%. The 1990 study estimated the density of Pisonia trees at 410±50 per hectare, giving an
estimated 190 trees within the Pisoniaforest. Height decreased down the
line with means and standard deviations varying from 23±0.6 m (0–40 m),
to 21±1.5 m (80–120 m) and 13±2.25 m (120–180 m). The tallest tree measured was 24m
tall. The full list of plant
species recorded at the site is given in Appendix I and the distribution of the
main species is shown in Fig. 2.
No significant change was
found in the number of P. sechellarum trees,
or most other species, between 1990 and 2010. The only significant changes in plant
cover throughout the forest were a decrease in Cinnamomum verum Presl (0.758–0.001,
t=2.460, P(2)59<0.05) and Dracaenareflexa Lam. (1.531–0.001, t=2.808, P(2)59<0.01). In the top section of the forest four
species disappeared from quadrats: Clidemia hirta (L.) D. Don (from 4.313; t=2.952, P<0.01), Ci.verum (from 0.688;
t=2.460, (2)9<0.05), D. reflexa Lam.
(from 2.313, t = 2.808, P(2)9<0.05) and Begonia seychellensis Hemsl. (from 2.188; t = 1.898, P(2)9>0.05). The decrease in cover
of Cl. hirta and Ci. verum seedlings in the
top sections of the transect (Fig. 2) may be related to shading due to
increased canopy cover in this area (Fig. 3).
Seedlings of P. sechellarum were not located in 1990 but were observed
subsequently. A single seedling was
found in 2003 at the top edge of the forest and five found scattered throughout
the forest in 2011.
South Mont Pot à Eau
The number of trees in this
site is not known, it is estimated to be similar to
the Pisonia forest (at least 100 trees). Several saplings under 2m tall were
observed. The flora is similar to
that of the Pisonia forest but apparently
lacks Piper seychellarum Gerlachand Psychotria silhouettae F. Friedmann.
Above Grande Barbe (Image 2)
About 5–10 individuals
ranging in size from 5–8 m high. Surrounding vegetation consists of closed forest, mostly submontane but with elements of montaneforests, typically associated with small ravines. Two Tectariawere observed plus the endemic Impatiens gordonii Horne ex Baker, among other rare species,
but no detailed species inventory was made.
Morne Blanc
One tree
4–6 m high. Surrounding
vegetation open, with sparse tree cover, in a very steep ravine with big granite
boulders, just at the foot of a cliff; the ravine
extended westwards but could not be explored. No detailed inventory has been made for
associated species. Nevertheless,
one of the Tectaria species found in the two
sites of Silhouette was also observed here.
Mayotte (Image 3)
No quantitative data are
currently available for Mayotte.
Discussion
The
population of P. sechellarum is stable in the
“Pisonia forest” on Silhouette island. Some
change in vegetation composition in the site has been recorded since 1990 but
this is attributable to a local effect of increased canopy cover. The number of mature trees has remained
unchanged and some successful recruitment has been observed (contrary to
earlier suggestions that seed predation by the endemic Epicroesasp. moth was preventing any regeneration (Floater 1995)).
The
other Seychelles populations are very poorly known and may be larger than
recorded here. However, searches for P. sechellarumhave been undertaken in many other montane ravines onMahé without success. It therefore seems to be extremely rare
on that island. The second largest
Silhouette population (south of Mont Pot à Eau) is
similar in importance to the “Pisoniaforest”. The third population, on
the southern slopes of Mont Dauban, contains two
distinct sites in two adjacent ravines. This may be a significant population due to the abundance of many such
ravines running down the slopes of Mont Dauban. Nevertheless, these ravines are all much
smaller than in the “Pisonia forest” and the
Mont Dauban population may be less healthy than the
upper Anse Mondon one. Small populations may also exist in the
numerous and poorly explored ravines of the northern side of Mont Dauban.
Pisonia sechallarum can
probably be regarded as a “montane” species, with the
Mont Dauban stand being at its lower elevation
limit. It is known that a number ofmontane species can occur at lower elevation in
ravines (Senterre et al. 2009). The genetic identity of the
geographically widely separated and morphologically varied Seychelles and
Mayotte populations need to be investigated.
Red List
status
Pisonia sechellarum is
currently listed on the IUCN Red List as Endangered on the basis of criterion D
(population size estimated to number fewer than 250 mature individuals) and
threatened by invasive plant species. At the time of the Red List assessment only the main population was
known. Although the new data result
in dramatic increases in the known range (Area of Occupancy from 0.05–5
km2 and Extent of Occurrence from 0.05–20,000 km2),
they do not significantly change the threat status of the species, with a
Seychelles population of around 200–300 individuals and an unknown number
on Mayotte. The population
structure is precarious with only six populations (with at least one probably
not being viable) and there is a decline in habitat quality in most sites due
to the impacts of invasive species. This would qualify the species for Endangered status on the basis of
restricted range B2 (small Area of Occupancy) subcriteriaa (only five viable subpopulations) and b(iii)
(deteriorating habitat): Accordingly P. sechellarumshould be regarded as Endangered (B2 a, b(iii)).
Conservation
needs
Conservation
of P. sechellarum requires the continued
protection of the known populations (all are within protected areas and are
currently safe from any development or logging threats) and control of invasive
species. Invasive species are not causing significant problems to the main
population at present but do need control in the other four sites. Wider invasive species management and
habitat restoration is also needed as the populations are isolated by a
combination of natural restriction due to their association with boulder fields
and by areas of heavily degraded habitat.
The limited
recruitment of the species should be investigated; predation by the Seychelles
endemic Epicroesa moth is a natural process
and does not completely prevent set of viable seed in the Pisoniaforest, but it is not known whether this contributes to the lack of significant
recruitment elsewhere. As many of
the mature trees appear to have grown from suckers or from fallen branches, and
this growth strategy is known in other Pisoniatree species it should be relatively easy to propagate for ex situ conservation
and augmentation of the more restricted populations. There is an urgent need to create a
viable population on Mahé near the single Morne Blanc tree. This could be supplemented by cuttings from Silhouette but should be
preceded by thorough searches of the remaining unexplored sites on Mahé in order to maintain any genetic distinction between
the island populations. Genetic
research into the distinctiveness of the Mahé,
Silhouette and Mayotte populations should also be undertaken, including an
analysis of genetic diversity within the populations.
References
Barthelat, F. & V. Boullet (2005). Index de la flore vasculaire de Mayotte - Version 2005-1,
pp. 103–197. In: Rolland, R. & V. Boullet (coord.). Mayotte, Biodiversité et évaluation
patrimoniale, Contribution à la mise en œuvre de l’inventaire ZNIEFF.
Document DAF - CDM, Mamoudzou et CBNM, Saint-Leu.
Barthelat F., M. M’changama& B.A. Sifari (2007). Atlas de
la flore protégée. DAF de
Mayotte, 51pp.
Floater, G.J. (1995). Seed predation and flower visiting by Epicroesa sp. (Lepidoptera: Heliodinidae)
on a rare Seychelles tree. Phelsuma 3: 31–40.
Friedmann, F. (1986). Flowers and Trees of Seychelles. Editions Delroisse forest Department of Finance, Seychelles
Friedmann, F. (1994). Flore des Seychelles: Dicotylédones. Editions de l’Orstom, Paris
Ismail, S., M.J. Huber & J. Mougal(2011). Pisonia sechellarum. In: IUCN 2011. IUCN Red
List of Threatened Species. Version 2011.2. <www.iucnredlist.org>.
Downloaded on 30 April 2012.
OUSE (1990). Oxford
University Silhouette Expedition 1990. Final Report, Unpublished.
Senterre, B., J.Gerlach, J. Mougal & D.Matatiken (2009). Old growth mature forest
types and their floristic composition along the altitudinal gradient on Silhouette island (Seychelles) - the telescoping effect on a
continental mid-oceanic island. Phytocoenologia39(2): 157–174; http://dx.doi.org/10.1127/0340-269X/2009/0039-0157