A Nature Guide's -
Guide to the Wet Tropics
Black Mountain Natural History
The apparently barren landscape supports a surprisingly large range of animals and plants which are specially adapted to this unusual environment. The green patches on the otherwise bare mountainside are large fig trees. Seedlings able to establish themselves in rock crevices extend long roots to draw water and dissolved nutrients from deep within the mountain.
Around the base of the mountain there are a number of plants normally found in rainforest. Self-mulching ferns, umbrella trees and stinging trees have adapted to these very different conditions. Monsoon forest, technically known as semi-deciduous mesophyll vine forest, grows around the edges of the rock masses. This vegetation is a haven for animals that venture from or to the rocky shelter of the mountain.
Endemic Vertebrate Species
The boulders shelter three species found nowhere else in the world – the Black Mountain Skink (Carlia scirtetis) photo to the right., Black Mountain Gecko (Nactus galgajuga) and the Black Mountain Microhylid Frog (Cophixalus saxatilis).
The worldwide distribution of these animals is, remarkably, less than six square kilometres (just over two square miles)!
The rare Black Mountain skink (Carlia scirtetis) is a small lizard with longish legs and a distinctive duckbill-like snout. This slender skink is black in the shade but glistens green in the sunlight. Its long limbs are flecked with yellow, and a golden stripe runs down the length of its back. The skink is often seen basking or hunting over the mountain’s rocks during the day, except for the hottest hours when it retreats within the boulder jumble.
Black Mountain Boulder Frog
The vulnerable Black Mountain boulderfrog (Cophixalus saxatilis) or rock haunting frog is the largest (about the size of a walnut) of Australia’s microhylidds – a group of frogs normally confined to the leaf litter of tropical rainforests.
This large-eyed frog lays its eggs on land rather than in water. Adults tend to the eggs and young which hatch as fully formed froglets; they have no tadpole stage. The bright yellow female frog and the smaller mottled brown male are more easily heard than seen. Their call is a sharp tapping noise.
They have acquired an almost crab-like ability to scuttle on the granite boulders, although they can still disappear in a series of leaps when alarmed. At night these frogs emerge to forage on the boulders of the mountain and in and about the scattered figs and fringing monsoon forest..
Geology
Black Mountain’s rocks are actually light grey granite but appear black because they are covered in three main blue-green algae which fix nitrogen from the air and therefore help other vegetation to grow such as the fig trees through a symbiotic relationship.
The boulders are rich in feldspar, mica and hornblende and, on very hot days, rain can cause the boulders to explode.
The hard granite that makes up the boulders of the black mountain range originally formed from magma from the earths crust. As the softer surfaces eroded away the fractured magma became the mountains of boulders that can be seen today.
If you look closely you will see that the boulders aren’t actually black – they are grey but they are covered by a film of microscopic blue-green algae growing on the surface. Geologists believe the boulders were once a molten mass which solidified deep below the earth's surface 260 million years ago. Erosion gradually exposed the granite plug and fractures began to form the boulders we see today.
Reference:
Carmody, J. (2010) Wet Tropics of Queensland World Heritage Area Tour Guide Handbook. Published by the Reef & Rainforest Research Centre Ltd., Cairns (220 pp.).