Madagascar spiny thickets
Berenty, Madagascar. Source: David Olson
The Madagascar spiny thickets or spiny desert of southern Madagascar, also referred to as deciduous thicket, is a globally distinctive ecoregion. This ecoregion is part of the Deserts and Xeric Shrublands biome, within the Afrotropics Realm. While the island of Madagascar is notable for exceptional levels of endemic plants and animals, the spiny thicket is particularly distinctive with 95 percent of the plant species endemic to the ecoregion. Members of the endemic Didiereaceae family present dominate the thicket, which have similar xeric adaptations to New World cacti, such as small leaves and spines, but with the Madagascar spiny thickets displaying more woody rather than succulent characteristics.
There are a total of 288 recorded vertebrate taxa in the Madagascar spiny thickets, including several endemic Madagascar families, including the charismatic Ring-tailed lemur or Maki (Lemur catta), Verreaux’s sifaka (Propithecus verreauxi verreauxi), and the regionally endemic Microcebus griseorufus. Several endemic animals are restricted to the dry southern part of Madagascar, including the recently described Grandidier’s mongoose (Galidictis grandidieri) and the Madagascar radiated tortoise or Sokake (Geochelone radiata).
Very little of the ecoregion is formally protected and information on the biota of the region is somewhat limited. The forests are rapidly being destroyed by indigenous people and becoming fragmented by charcoal production, agricultural expansion (for maize and cattle grazing), and man-made fires associated with generation of new cattle pastureland.
Source: World Wildlife Fund
Location and general description
Near Tulear, Madagascar. Source: David Whitacre One of seven ecoregions covering Madagascar, the spiny thicket ecoregion extends across southern and southwestern Madagascar with its northern border at the Mangoky River on the west coast and the western slopes of the Anosyennes Mountain chain in the southeast.
It falls in the extreme rain-shadow of Madagascar behind the eastern chain of mountains and far from the prevailing northeastern rains. Consequently, the average annual rainfall for the ecoregion is 500 millimetres (mm) or less per annum. The driest areas are in the southwestern coastal region where the annual rainfall may be less than 350 mm per year, and the dry season may endure nine to eleven months. The climate is dominated by a wet and a dry season with most of the rain falling between October and April. Rainfall can be erratic from year to year, with prolonged periods of drought lasting several years sometimes occurring. The highly poroussoils and bedrock provide little opportunity for vegetation to absorb moisture. Many plants have adaptations to store what little water is available and means to minimize water loss. Average annual temperatures for the ecoregion range between maxima of 30° to 33°C and minima of 15° to 21°C. The topography of the ecoregion terrain is relatively level and extends from mean sea-level to altitudes of typically between 55 metres (m) and 200 m above sea level, with elevation increasing gradually from the coast inland towards the Central Highlands.
There are two major rock types in the ecoregion; the Tertiary limestone of the Mahafaly Plateau and the unconsolidated red sands of the central south and southeast. This geology corresponds to a major division in the habitat. The taller, dense dry forest on the sandy soils is dominated by Didieria madagascariensis, and the more xeric adapted vegetation on the calcareous plateau around Lake Tsimanampetsotsa is characterized by dwarf species.
The general vegetation distributed across the ecoregion is spiny bush in the south and west and a mosaic of spiny bush and secondary grassland in more inland areas. Plants represented include a mixture of deciduous woody plants, and deciduous and evergreen succulents. This mosaic of vegetation is believed to be a result of degradation of the thicket through climatic desiccation over the past 4000 years, and anthropogenic use over the past 2000 years, with the grasslands having relatively low numbers of endemic plants. Many of the spiny bush plants possess extreme adaptations to the aridity of the ecoregion, such as extended root systems with massive tubers, enlarged, succulent trunks and branches, succulent and reduced leaves, thorns, and waxy and hairy coatings. Through convergent evolution, the plants of the Agave-Boojum desert in the Vizcaino region of Mexico have evolved similar adaptations, so that the two regions are very similar in structural appearance.
Berenty, Madagascar. Source: David Olson The spiny thicket or spiny forest is usually three to six metres in height, but sometimes includes emerging trees of the Didiereaceae family which reach more than 10 m in height, such as Alluaudia ascendens and A. procera. Other emergents in the forest include Commiphora spp. (Burseraceae family), Tetrapterocarpon geayi (Leguminosae family), and Gyrocarpus americanus (Hernandiaceae family), as well as other species from the Euphorbiaceae, Leguminosae, and the baobabs of the Bombacaceae. The scrub layer includes Leguminosae and many lianas from the Asclepiadaceae. A recent survey in the eastern spiny forest reported that the following families were the most dominant and diverse: Burseraceae, Didiereaceae, Euphorbiaceae, Anacardiaceae, and Fabaceae.
Vegetation changes throughout this region, from east to west, resulting in vegetation communities with varied appearance throughout the spiny thicket. Parts of the spiny thicket are completely lacking in species of Didiereaceae. Alluaudia procera and A. ascendens dominate the eastern portions of the ecoregion. To the west, in the north, there are no emergent Alluaudia spp., but instead this area is dominated by the Octopus tree (Didierea madagascariensis), Perrier's baobab (Adansonia rubrostipa), the massive Pachypodium geayi, and Delonix spp.
Some notable, smaller centres of endemism occur within the ecoregion. The limestone cliffs to the east of Lake Tsimanampetsotsa, on the Mahafaly Plateau, have unusual vegetation comprising four main families: Euphorbiaceae, Didiereaceae, Bombacaceae and Fabaceae. The vegetation here includes the distinctive succulent, Euphorbia stenoclada, and the Bottle tree (Moringa drouhardii). The limestone cliffs are home to several endemic animal species including the Endangered Grandidier's Mongoose (Galidictis grandidieri) and a blind cave-dwelling fish (Typhleotris madagascariensis). This locale also constitutes a restricted distribution zone for the nocturnal gecko (Ebenavia maintimainty) as well as for the Onilahy mabuya skink (Mabuya vezo). The Mahafaly Plateau area, as well as Cap Saint Marie at the southern tip of the island, are known for pronounced stunted vegetation.
Other significant plant communities and habitat within the ecoregion include the low bushy scrub of the coastal dunes, and the gallery forests on the alluvial soils bordering major rivers (the Mandrare, Onilahy, Linta and Fiherenana). Although gallery forests occur along rivers within spiny forest, their floristic composition is quite different, with gallery forests being similar to the western tall deciduous forest. There are also important areas of transition forest. One such area is on the western side of the Anosyennes Mountains in the southeast where the humid forest grades into spiny forest. This habitat is represented in parcel three of the National Park of Andohahela and is the only protected habitat of the locally endemic Triangular palm, Dypsis decaryi. However, most of the zone with the important transitional habitat is not protected.
Biodiversity features
Ring-tailed lemur (Lemur catta), Madagascar. Source: David Olson Flora
The ecoregion has the highest percentage of plant endemism in Madagascar. Forty-eight percent of the genera and 95 percent of the species occurring in the ecoregion are endemic to the island. Some of the dominant forest species belong to the endemic family Didiereaceae. There are eleven species and four genera (Didierea, Alluaudia, Alluaudiopsis and Decaryia) in this family. Some of the endemic plants are extremely rare due to restricted ranges, such as Aloe suzannae (Liliaceae) and the palm, Dypsis decaryi, as well as tiny Euphorbia herbs, Pachypodium spp., and Hibiscus shrubs exploited for the ornamental and nursery trades.
The harsh, drought-prone environment has produced extreme adaptations among the plants found here. Woody species have long tap roots, swollen storage organs, and waxy caducus leaves. Pachycaulous stems are common amongst the succulents, seen in Adansonia, Pachypodium, and Moringa spp. Spines are abundant among many plants, including the genera Allaudia, Pachypodium, Mimosa, and Didierea, and serve several functions. They protect plants from moisture seeking animals and combine with large terminal leaves to reduce surface area and moisture loss. In fact, several Euphorbia spp. have no leaves and conduct photosynthesis and respiration from the trunk and stems alone. Other techniques used to deal with unpredictable rainfall include drying and reviving, toxic sap and precocial flowering.
Verraux's Sifaka (Propithecus verrauxi), Madagascar. Source: David Olson Mammals
The fauna of the ecoregion is also distinctive and includes three strictly endemic mammals, the White-footed sportive lemur (Lepilemur leucopus), Grandidier’s mongoose (Galidictis grandidieri) and Microcebus griseorufus. Near-endemic mammals include the Large-eared tenrec (Geogale aurita), and the Lesser hedgehog tenrec (Echinops telfairi). Six other lemurs are found only in spiny thicket and the adjacent Succulent Woodlands ecoregion, Red-tailed sportive lemur (Lepilemur ruficaudatus), Verreaux's sifaka (Propithecus verreauxi), the Ring-tailed lemur (Lemur catta), Forked-marked lemur (Phaner furcifer), Fat-tailed dwarf lemur (Cheirogaleus medius), and Gray mouse lemur (Microcebus murinus). The mongoose species is considered endangered on the current IUCN Red Data List, and Verreaux’s sifaka and the Ring-tailed lemur are classified as Vulnerable. Some mammals have highly restricted ranges within the ecoregion. Grandidier’s mongoose (Galidictis grandidieri) was described as recently as 1986 and has a restricted range around Lake Tsimanampetsotsa. Subfossils have been identified from a cave near Itampolo, south of Lake Tsimanampetsotsa.
Reptiles
Species of reptiles endemic to the ecoregion include the chameleons Furcifer belalandaensis and F. antimena. Further, the Madagascan spider tortoise (Pyxis arachnoides), and the Radiated tortoise (Geochelone radiata) are found in this ecoregion and the zone to the north, the succulent woodlands. The Madagascar ground boa (Acrantophis dumerilii) is found in this ecoregion, although not exclusively. Many more species are endemic to the ecoregion including the rock dwelling iguanids Oplurus saxicola and O. fihereniensis, the Day gecko (Phelsuma breviceps), nocturnal geckos Ebenavia maintimainty and Matoatoa brevipes, and the snake Liophidium chabaudi.
Avifauna
There are eight bird species endemic to the ecoregion and an additional two bird taxa that live only on the western drier side of the island. Endemic species include Verreaux's coua (Coua verreauxi), running coua (Coua cursor), Lafresnaye’s vanga (Xenopirostris xenopirostris), red-shouldered vanga (Calicalicus rufocarpalis), Archibold’s newtonia (Newtonia archiboldi), and littoral rock-thrush (Monticola imerinus). Some of these endemics are quite restricted in their geographical range. For example, two endemic species are known only from a narrow coastal strip on the northwest edge of the ecoregion. They are subdesert mesite (Monias benschi) and long-tailed ground roller (Uratelornis chimaera). Each of these species belong to monospecific genera and are representatives of two of the five families endemic to Madagascar. Another, the recently described red-shouldered vanga, is known only from the Toliara region. The Madagascar plover (Charadrius thoracicus) is near-endemic to this ecoregion, but is also found along the west coast into the Succulent Woodlands and the Dry Deciduous Forest ecoregions, while the Thamnornis warbler (Thamnornis chloropetoides) extends only slightly outside this ecoregion into the Succulent Woodlands ecoregion.
The Red-shouldered vanga and Long-tailed ground roller are recorded as Vulnerable species on the recent IUCN Red List of Threatened species.
Amphibians
Goudot's bright-eyed frog. Source: John Sullivan There are a number of amphibian taxa present within the ecoregion, the totality of which are: Ansouhy tomato frog (Dyscophus insularis); Betsileo Madagascar frog (Mantidactylus betsileanus); Brown rainfrog (Scaphiophryne brevis); Dumeril's bright-eyed frog (Boophis tephraeomystax); Madagascar bullfrog (Laliostoma labrosum); Mascarene grassland frog (Ptychadena mascareniensis); the Endangered Blue-legged mantella (Mantella expectata); and the Goudot's bright-eyed frog (Boophis goudotii).
Ecological status
The ecoregion still has a range of intact habitats, some of which encompass large areas. From satellite images, it has been estimated that between 14,000 and 17,000 square kilometers (km2) of spiny forest still remains in the ecoregion. The rate of habitat loss and degradation is lower relative to other habitats around Madagascar, in part because of the low human population density throughout much of this ecoregion. However, recent developments, such as an irrigation pipeline and several other factors, have increased the movement of people into the ecoregion. Traditionally, hunting "fady," or taboos, of two local tribes (Antandroy and Mahafaly) protected many animal species in this region. However, with the increased movement of people across the region, the local fady on certain animals is becoming less effective as a means of protection.
There are some blocks of relatively intact forest, particularly in the northwestern portion of the ecoregion and the extreme southeast of the range. Much of the inland area has been replaced by secondary grassland and wooded grassland. There are several relatively small reserves in the ecoregion, including Tsimanampetsotsa National Park and Ramsar site, Beza-Mahafaly Special Reserve, Cap St. Marie Special Reserve, and Berenty Private Reserve. However, these protect about three percent of remaining habitat at best. The existing protected areas exclude important [[habitat]s] for endemic species of birds and reptiles, such as the coastal area around the Onilahy River, the strip of forest between Mangoky and Fiherenana rivers and Lake Ihotry.
As is the case in many reserves in Madagascar, the reserves themselves are not well managed or protected. For example, cattle and goat grazing continues in the fragile coastal[[habitat]s] of Cap St. Marie, an area of heavy exploitation by succulent plant collectors. A priority-setting workshop held in 1995 identified the following sites as in need of immediate conservation: the region around Lake Tsimanampetsotsa, Fiherenana region, Cap St. Marie, and the Mahafaly Plateau.
Ecological threats
The principle threats to the spiny thicket are the small-scale, but widespread, exploitation for firewood and charcoal production. Selective logging of forests for construction wood is also a significant threat, particularly as the spiny thicket forest type has a naturally slow rate of growth and regeneration. The increasing cultivation of corn and grazing of domestic species (primarily cattle and goats) also pose very serious threats to the ecoregion’s [[habitat]s]. The degradation of forests for agriculture has been exacerbated in recent years by the extreme periods of drought. Growing of maize is also expanding in the area, forming an additional threat to the habitats.
Invasive plant species, such as prickly pear (Opuntia spp.) and the rubber vine (Cissus spp.), which is only a threat in gallery forest, have increased the degradation of the [[habitat]s], especially in disturbed forest areas. As in other regions of Madagascar, the collection of endemic species of plants and animals for international trade poses a threat to the integrity of the habitats. Illegal collection is a particularly significant threat for the populations of the two endemic species of tortoises and various species of succulent endemic plants.
Justification of ecoregion delineation
Extending from Morombe to Tolagnaro along the southwest coast of Madagascar, this ecoregion is based primarily on Humbert’s (1955) southern vegetation domain, within Cornet’s (1974) larger ‘subarid’ bioclimate. It also falls within Udvardy’s (1975) ‘Malagasy thorn forest’ biogeographic province. The dominant vegetation is deciduous thicket, representing a center of endemism for Didiereaceae. The Madagascar spiny thickets is accorded the ecocode AT1311 by the World Wildlife Fund.
The ecoregion extends inland approximately 80 kilometres, with the eastern limits stretching further inland along the Mandrare River where it meets the southern edge of the Central Highlands. There has been some debate on whether the boundary should be delineated using Cornet’s bioclimatic zone or Humbert’s vegetation zone, each differing in extent and ecological parameters. It was finally decided to use Humbert’s linework for the spiny thicket ecoregion, but form an additional ecoregion defined by the remaining extent of Cornet’s subarid bioclimate, as it shares floral affinities with both the spiny thicket and western dry-deciduous forest, yet contains distinct assemblages.
Neighbouring ecoregions
The following ecoregions have some degree of tangency with the Madagascar spiny thickets:
- Madagascar succulent woodlands, bordering most of the north
- Madagascar subhumid forests, to the north and northeast, but lying east of the Madagascar succulent woodlands
- Madagascar lowland forests, at the southeast
References
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