BIOGEOGRAPHICAL DIVISION
OF THE SOKOTRA ISLANDS
(draft)
Dr. Martin Culek, Ph.D.
Geographical Institute
Masaryk University in Brno, Czech Republic
Draceana cinnabari, Tertiary relict, endemit of Socotra Isl. It has water reservoir in its stem.
Woodland with no young trees – all being eaten by goats. These are about 400 years old. Max.
age being about 800 years. Grat threat to the future. Firmihin, ca. 550 m a.s.l., southern slopes.
The World context
• M. Udvardy (1975): Africotropical Realm, part of Somalian
province – no detailed division
• A. L. Tachtadžjan (Takhtajan) (1986): Paleotropical Kingdom,
Sudano-Zambezian Region, province No. 75 Socotra
• Many relicts
• ± 700 endemic species (flora + fauna) (Burdick, Alan, 2007) +
5 hard corals (from 230).
• 307 out of the 825 (37%) plant species on Socotra are
endemic (Miller, Morris, 2004) → archipelago the world's
tenth richest island group for endemic plant species
• No native amphibiants, no mammals except bats x
continental island ! Why:
• 7 endemic species of birds !
• Reptiles ≥ 90 % endemic
• insects - very rich: 600 species of insects, 90% endemic
Relations of biota on Socotra islands
http://www.socotraislandadventure.com/Socotra_%20fauna&flora.htm
• Flora: Relations to Somalia and Arabia, but:
• Dracaena cinnabari - tertiary relict, related species in S. Arabia, NE
Africa, the Canary Islands
• Kalanchoe, Helichrysum - strong links with S. Africa
• The genus Thamnosma (with T. socotrana on Soqotra) - related
species in S. Arabia, SW Africa and SW North America.
• Fauna – close to Africa (part of Afrotropical (Ethiopian) Realm,
some insects to Paleoarctic
• The marine fauna life: Socotra - a spectacular mixture of sp. from
different biog. regions - the western Indian Ocean, the Red sea,
East Africa and the wider Indo-Pacific.
680 species of fishes - comparable to the Red Sea.
• Socotra: more closely linked with Africa than Arabia
• Interesting affinities with other island groups - the granitic
Seychelles, remote islands of the Atlantic Ocean
M. Udvardy
(1975)
UNESCO – MAB
Biogeographical
provinces of
the world
Socotra
Division of Tachtadjan
Methodology of biogeographical
division of Socotra
• Complex approach – abiotic conditions taken in
account.
• ± Evolutionary approach – supposed refugia and
barriers taken in account.
• Regionalisation → Individual unites.
• Recent time (± 19. century) – before devastation
in 20 - 21. century.
• Priority given to the flag-ship species, esp.
Draceana cinnabari.
Biogeographical division
• Biog. Province: Socotra Archipelago
• Biog. Subprovinces: 1. Socotra Isl.
• 2. 3 Isl. left (15 end. flora sp.)
• Biog. Districts of Socotra: 1.1 Haghier Mts.
– Biog. Subdistricts: 1.1a Northern walls (Skant)
• 1.1b Southern slopes
• 1.2 Western Highlands
• 1.3 Eastern Highlands
• 1.4 Northern Highlands
1.1 Haghier Mts.
• Hyperalkaline plutonic rocks.
• Higher parts – climatically subtropic, rel. wet, fogs
• Permanent water courses, fresh water !
• Refugium, esp. „mountain“ species
• Altitudinal vegetation tiers (zones): 2 – 5
• Highest biodiversity
• 2 forms of Draceana cinnabari
1.1 Haghier Mts.
• 1.1a Northern walls – wet in „winter“(NovemberJanuary),
in April-June, Sept –October - fogs.
Relatively extremely cold and wet. Lower parts in
summer monsoon (May-September) hot, windy.
• Extremely steep – low use.
• Forests in gullies, broadleaf bushes (Trichodesma
scotii) ± continuous lichens.
• ± absence of Adenium and other „allien looking“
trees and bushes (excluding Draceana).
• Typical real tree – Sterculia africana.
Lower part of Northern slopes, low trees, flowering on
left side – Boswelia – source of resin (= kadidlo !)
Upper part of Northern walls, end of April 2004, 15 h.
Trichodesma scotii,
afternoon fogs.
The only forest on Island – Homheir Pass, cca 1400 m. End of April 2004
1.1 Haghier Mts.
• 1.1b Southern slopes – wet and foggy in summer.
Strong sunshine, warm surface. Green in summer.
• Slopes – accessible → intens. pastures, f. fields.
• Ridges – „macchie“ with Draceana, slopes – dry low
woodlands with Euphorbium, medium parts –
typical presence of Commiphora, flowering sp., in
lower parts – succulent trees with Dorstenia gigas
in upper part of canyons.
• Typical region for end. freshwater crab
Vegetation similar to the makchie, 1400 m a.s.l., end of April 2004
Dry woodlands – bushlands, 3 m high. Ca 1300 m, southern slopes.
Southern slopes, ca 1100 m, low trees – Commiphora planifrons
(source of myrha !)
1.2 Western highlands
• Limestone, dust, gravel, coastal sands.
• Dry, windy, mostly wet in summer, northern slopes
– wet in winter.
• Western part – caves – ext. dry x springs
• Separated by passes and basins from Mts.
• Strongly devastated by grazing.
• Poor vegetation, low biodiversity, probably naturaly
without Draceana → distinguished ± negatively.
• Positively – vegetation of gypsum soils, mangroves.
Bushes of Croton, strongly affected by grazing
Green bushes – similar to submediterranean Buxus
(=zimostráz)
The last mangrove on Noged, souther coast (Avicennia marina)
1.3 Eastern Highlands
• Limestone plains x canyons in hyperalkaline plutonites.
• Great ± periodical watercourses, waterholes, caves,
carstic springs.
• Wet in summer, medium dry, dry winds in winter
• Intensively used but more wet and green (pastures,
plantations, Tamarindus …)
• Connected to Haghier Mts. → descend of „mts.“
species.
• Punica protopunica
• Presence of Draceana, common Boswelia, freshwater
crab.
• Most endagered district.
Southern slopes, cover of limestone, to the right - tops of Hagghier Mts.
on granitoide rocks. Vegetation destrued, but rests surviving.
Draceana woodland on souhtern slopes (limestone plateau Firmihin, ca
600 m a.s.l.)
Punica protopunica (= wild predecessor of pomegranate = granátové
jablko !)
1.4 Northern Highlands
• Limestone, hyperalkaline plutonites, gravel, floodplains,
sand dunes.
• Medium wet in winter, extr. dry and hot rest of the
year. Strong turbulented winds, föhns. +8 °C.
• Water from Mts.
• Strongly used floodplains (plantations) x poor pastures
• Separeted partly by baselines, passes, limestone
boundaries.
• Poor vegetation, poor biodiversity, probably naturaly
Draceana absent
• Positively – Flamingoes, rich in Adenium o. s.
Főhn clouds on mountains during summer monsoon (wind
from SW), view from NE. Flamingoes in brakish water in a bay.
Vegetation on granitoide rocks with Croton (low bush) and
Adenium (in flower). Ca 400 m a.s.l.
Sand dune Houlef, ca 350 m high, and low bushes on windy coast
Date palm plantations in
floodplains, and highest
specimen (6,7 m) of endemit
Adenium socotranum.