Location and General Description
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 mm or less per year. 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 last 9 to 11 months (Donque 1972). 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 prolonged periods of drought lasting several years do occur. The highly porous nature of the soil and bedrock provides little capacity for vegetation to absorb moisture. Many plants have adaptations to store what little water is available and means to minimize water loss (see below). Average annual temperatures for the ecoregion range between maximums of 30° to 33° C and minimums of 15° to 21° C. The topography of the ecoregion is relatively flat and running from sea-level to altitudes of typically between 55 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 (Du Puy and Moat 1996). 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 (Morat 1973, White 1983). 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 4,000 years or so, and anthropogenic use over the past 2,000 years, with the grasslands having relatively low numbers of endemic plants (Phillipson 1996). 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.
The spiny thicket or spiny forest is usually 3 to 6 m 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 (Rakotomalaza and Messmer 1999).
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 Didierea madagascariensis, dwarf baobabs Adansonia fony, the very large Pachypodium geayi, and Delonix spp.
Some notable, smaller centers 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 trees, Moringa drouhardii. The limestone cliffs are home to several endemic animal species including the mongoose (Galidictis grandidieri) and a blind cave-dwelling fish (Typhleotris madagascariensis) This area constitutes also a restricted distribution zone for the nocturnal gecko (Ebenavia maintimainty) as well as for the skink (Mabuya vezo). The Mahafaly Plateau area, as well as Cap Saint Marie on the southern tip of the island, are known for pronounced dwarf 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 largely different, and gallery forests are 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 3 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.
The ecoregion has the highest percentage of plant endemism in Madagascar (Jolly et al. 1984, Davis et al. 1994, Phillipson 1996). 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 11 species and 4 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 the 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.
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) (Mittermeier et al. 1994). The mongoose species is considered endangered on the current IUCN Red Data List, and Verreaux’s sifaka and the ring-tailed lemur are vulnerable (IUCN 2000). Some mammals have very restricted ranges within the ecoregion. Grandidier’s mongoose (Galidictis grandidieri), was described as new to science in 1986 and has a restricted range around Lake Tsimanampetsotsa. Subfossils have been identified from a cave near Itampolo, south of Lake Tsimanampetsotsa (Goodman 1996).
Species of reptiles endemic to the ecoregion include the chameleons Furcifer belalandaensis and F. antimena. Further, the 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 boa Acrantophis dumerilii is found in this region, although not exclusively so. 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.
There are 8 bird species endemic to the ecoregion and an additional 2 bird species that live only on the western drier side of the island (Nicoll and Langrand 1989, Stattersfield et al. 1998). 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 (Langrand 1990). Another, the recently described red-shouldered vanga, is known only from the Toliara region (Goodman et al. 1997, Hawkins et al. 1998). 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 (IUCN 2000).
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 km2 of spiny forest still remains in the ecoregion (Du Puy and Moat 1996). 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 (Sussman et al. 1994), 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 (Du Puy and Moat 1996). Much of the inland area has been replaced by secondary grassland and wooded grassland (Morat 1973, White 1983, Davis et al. 1994, Lowry et al. 1997). 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 3 percent of remaining habitat at best. The existing protected areas exclude important habitats 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 (Morris and Hawkins 1998).
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 habitats 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 (Ganzhorn et al. 1997).
Types and Severity of 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 habitats. 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 habitats, 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 ecoregion extends inland approximately 80 km, 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.
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Prepared by: Helen Crowley
Reviewed by: In progress