Boundaries

The Madagascar Eastern Highlands ecoregion encompasses a relatively narrow strip of land extending from north to south, essentially the entire length of Madagascar, and covers the upper and middle reaches of the eastern coastal rivers. The ecoregion covers about 20% of the island, or about 80% of the eastern slope region (Aldegheri 1972). 

Topography

Terrain in the region is steep, with the summits of many eastern mountains located as little as 50 km from the coast (Aldegheri 1972). 

Freshwater habitats

Rivers in this ecoregion are generally small to moderate in size, rocky in substrate, and swift in current. Drainages are steep in their upper reaches, with numerous sections of rapids and cascades interspersed with flatter stretches of more moderate flows. These coastal rivers are short and terminate on a narrow coastal plain over a contracted continental shelf. In undisturbed areas, the water is clear and well oxygenated. Compared to rivers in western Madagascar, eastern rivers have lower pH (slightly acidic vs. neutral), higher conductivity, and much lower dissolved carbonate levels (Riseng 1997). Rainfall is high in the Eastern Highlands, 2,500-3,000 mm annually according to Aldegheri (1972), and water levels vary considerably with the occurrence of seasonal storms. In contrast to many river systems in western Madagascar, rivers in the east flow year round.

Terrestrial habitats

Dense evergreen forest is the principal vegetation type throughout this ecoregion, except at elevations above 1,800 m where a shift to thicket and shrubland occurs (Lowry et al. 1997). 

Description of endemic fishes

Two fish families, Bedotiidae (rainbowfish) and Anchariidae (catfish), are endemic to Madagascar and are represented in this ecoregion (Sparks & Stiassny 2003). Endemic fish faunas are associated especially with Pandanus and palm swamps, such as are found in the upper Mangoro and Mananara drainages. Due to land conversion, these habitats are now quite rare in Madagascar.

Both the Mangoro-Nosivolo and Mananara Rivers harbor endemic cichlids, bedotiids, and anchariids, as well as a diversity of other native species (Reinthal & Stiassny 1991; Sparks & Reinthal 2001). The upper sections of these two rivers are regions of noteworthy diversity within the ecoregion. A small stretch of rapids in the Nosivolo River near Marolambo is the only known habitat of the endemic cichlid Oxylapia polli (Reinthal & Stiassny 1991), and an area of only a few square kilometers in the Mananara drainage encompasses the distribution of the cichlid Ptychochromoides vondrozo (Sparks & Reinthal 2001). 

In contrast, the headwaters of many other eastern rivers support markedly depauperate fish assemblages, and native fish species are frequently absent from the uppermost reaches. An example is the upper reaches of the Namarona River, with only two species of goby having been collected there (Sparks, pers. obs.). If native fishes occur at all in many of these headwaters, communities are comprised of at most a few gobioids (gobies and eleotrids), such as rock-climbing gobies of the genus Sicyopterus. Provided there are no significant barriers to upstream dispersal, eels (Anguilla spp.) also frequently migrate into these areas. Fish communities in the middle reaches of these rivers are more diverse, and are characterized by communities including endemic cichlids, bedotiids, gobioids, and anchariid catfish. 

Rivers of the Madagascar Eastern Highlands (and Madagascar Eastern Lowlands [823]) are home to number of closely related species of endemic rainbowfish of the genus Bedotia. The results of preliminary studies of this group suggest that each major eastern basin may contain a unique species (Loiselle & Stiassny 2003; Sparks & Stiassny 2003; Sparks & Smith 2004). Bedotiids of the genus Rheocles are also relatively diverse in the middle and upper reaches of eastern rivers, although nearly all species (particularly R. sikorae, R. wrightae, and R. pellegrini) are now quite rare and restricted in distribution.

Justification for delineation

This ecoregion is defined by the upper reaches of the eastern coastal drainages above about 200 m elevation and distinguished by an endemic aquatic fauna. 

Level of taxonomic exploration

Fair. Of the five major aquatic ecoregions of Madagascar, ichthyologists have probably explored the Madagascar Eastern Highlands and Madagascar Eastern Lowlands [583] most thoroughly. This is largely due to the presence of a number of larger towns and a network of passable roads along the coast that permit access to the upper and middle reaches of many eastern rivers. Yet, there are still many east coast drainages that remain to be surveyed for fishes and other freshwater taxa. For example, a major gap exists in our knowledge of ichthyofaunal communities for the middle and upper reaches of rivers extending from the Masoala Peninsula south to Lake Alaotra (Sparks & Stiassny 2003). The middle and upper reaches of many of these rivers have yet to be inventoried for freshwater fishes and should be considered top research priorities.