Boundaries

This ecoregion begins at the Berkeley River Basin in the west and extends east across northern Australian drainages to the Herbert River, incorporating all rivers draining to the northern coastline. This region includes Arnhem Land, the Gulf of Carpentaria, Cape York Peninsula, and the wet tropics.

Topography

Most northern coastal rivers descend from an interior plateau at about 200 m elevation to sea level. The Durack Range and Eastern Highlands with maximum elevations of about 600 m occur in the western and eastern parts of the ecoregion, respectively. There is also a large sandstone massif with elevations up to about 350 m asl in Arnhem Land in the northern portion of the ecoregion. Most rivers flow across flat region with low gradients, except those draining to the eastern coastline. River gradients in the east tend to be higher as the main Eastern Highlands are close to the coastline.

Freshwater habitats

This is a large ecoregion, covering 1,077,000 square kilometers or 13% of Australia. Massive flooding may be caused by large tropical depressions and cyclones. Most of the large rivers of this ecoregion have vast floodplains. Rivers in the southern Gulf of Carpentaria portion of this ecoregion typically rise in relatively low hills, and many of these rivers are somewhat intermittent throughout much of their length. In contrast, many rivers in the “Top End” of the Northern Territory rise on a large plateau and have large waterfalls that restrict the upstream movement of most fishes. These rivers tend to have more permanent flow for much of their length. Rivers draining to the eastern coast tend to be much shorter but originate in much higher mountains with high rainfall. These are some of the few high gradient rivers in northern Australia. There are almost no major dams in this ecoregion, although one, the Ord River Dam, is the largest in Australia at 741 sq. km.

Terrestrial habitats

Tropical savannas dominated by open eucalypt forests interspersed with small patches of rainforest occur along the coast. Eucalypt woodland with a grassy understory occurs further inland (World Wildlife Fund 2001b).

Description of endemic fishes

Over twenty species are endemic to this ecoregion, including two monotypic genera: Guyu and Cairnsichthys. Each has a small distribution in the wet tropics of northeastern Queensland. Endemics from the genus Melanotaenia and the family Terapontidae are most numerous, although there are also endemic Gobiidae, Soleidae, Percichthyidae, Ambassidae, Eleotridae, and Carcharhinidae.

Other noteworthy fishes

The bizarre Kurtus gulliveri, nurseryfish, is present in many coastal drainages. Females somehow attach the eggs to a hook on the head of males (Berra & Humphrey 2002). Several other fish species are unusual in that they are mouth brooders. These include the two Scleropages spp. (females hold eggs), all ariid catfish (males hold eggs), and Glossamia aprion, mouth almighty (males hold eggs) (Allen et al. 2002). All of these groups are found in surrounding ecoregions and/or New Guinea.

Justification for delineation

This vast region is likely typified by high connectivity between rivers during low sea levels. Most of the area between northern Australia and southern New Guinea becomes terrestrial, and many rivers coalesce together thus allowing their faunas to mix. The separation between Kimberley [803] is likely due more to decreasing connectivity between rivers and the isolated, localized occurrence of endemic species there. The break between Northern and Eastern Coastal Australia [807] is probably due mostly to differences in climate as rainfall drops very rapidly from the wet tropics south (Unmack 2001).

Level of taxonomic exploration

Taxonomic exploration varies between good and fair. Few species have been studied in any detail and several areas remain poorly sampled. The high faunal similarity within this ecoregion implies a lack of isolating mechanisms. But some widespread groups have diversified, with multiple taxa likely being recognized in the genus Mogurnda (M. Adams, unpub data), Craterocephalus, and within some species of Melanotaenia (Unmack 2005) and Pseudomugil signifer (southern blue-eye) (Wong et al. 2004).