Boundaries

This ecoregion largely follows the boundaries of the Murray-Darling Basin, but also includes a small piece of land on the Gulf of St. Vincent immediately to the west of the basin (the South Australian Gulf region). The northern boundary in the Gulf of St. Vincent is the Wakefield River.

Topography

This ecoregion is flanked by the Great Dividing Range in the south, east, and north with maximum elevations typically greater than 500 m, but up to 2,228 m (Mt Kosciuszko), while the western boundary primarily consists of low hills mostly below 200 m. The majority of the basin is of low elevation (< 200 m asl) and consists of extensive plains and low undulating areas.

Freshwater habitats

The majority of this ecoregion consists of one massive interconnected river system covering over 1,132,000 square kilometers, or 14% of Australia (Lintermans 2007). The western section contains a series of independent small coastal streams. There is a large variety of habitat types throughout the ecoregion, including saline lakes, alpine to intermittent streams, sub-alpine bogs, vast floodplains, lowland rivers, swamps, and lakes. The Murray-Darling Basin Commission estimates that there are greater than 30,000 wetlands found within the basin (Murray-Darling Basin Commission 2005). Floods and droughts are crucial components relative to the health of lower portion of the province, especially floodplain habitats. Most rivers are now dammed and river flow is highly regulated.

Terrestrial habitats

Dry forest and eucalypt woodlands dominate the vegetation in this ecoregion, although alpine and sub-alpine communities occur on the flanks of the Great Dividing Range (World Wildlife Fund 2001).

Description of endemic fishes

No genera are endemic, but a number of species are: Galaxias fuscus, G. rostratus, Craterocephalus amniculus, C. fluviatilis, Melanotaenia fluviatilis, Gadopsis bispinosus, Maccullochella macquariensis, M. peelii (Murray cod), Macquaria australasica, Bidyanus bidyanus, and Hypseleotris sp. (Murray-Darling carp gudgeon).

Other noteworthy fishes

Maccullochella peelii is the largest Australian freshwater fish, reaching 113 kg and 1.8 m. Today, fish over 45 kg are rare.

Ecological phenomena

Several species in this ecoregion are known to be highly migratory, with one of these, Macquaria ambigua (golden perch) having been recorded moving over 2,300 km completely within freshwater environments (Reynolds 1983). Macquaria ambigua and Bidyanus bidyanus (silver perch) are both unusual in that they have pelagic eggs, which is rare in freshwater fishes. This is thought to be an adaptation to floodwater dispersal away from spawning areas and into areas with more abundant food resources (Merrick & Schmida 1984). Craterocephalus fluviatilis frequently inhabits quite saline lakes, some of which have salinities significantly greater than sea water.

Justification for delineation

The Murray-Darling ecoregion has a moderate level of endemism and complex relationships with surrounding regions. It has high similarity to parts of Eastern Coastal Australia [807], the Bass Strait Drainages [809], and the Great Artesian Basin [806]. Overall, the ecoregion appears to have experienced mixing of faunas from surrounding regions with distinctive faunas, while maintaining a high degree of endemism. With the possible exception of southwestern Victoria drainages in the adjacent Eastern Coastal Australia ecoregion, all faunal connections must have occurred across drainage divides (Unmack 2001).

Level of taxonomic exploration

Overall, the taxonomic exploration of the fauna is good, with aspects of the fauna being relatively well known. A number of taxonomic problems remain, especially within Galaxias olidus (which probably represents 4-5 taxa) and morphological and molecular examinations of other components of the fauna will likely result in a number of species being separated into multiple taxa (e.g., Retropinna (Hammer et al. 2007), Gadopsis (Miller et al. 2004), and Hypseleotris (Thacker et al. 2007)).