Feature Article - Geodiveristy and biodiversity
Feature article published in the Tasmanian Year Book, 2000 (cat. no. 1301.6)
Contributed by Nature Conservation Branch, Department of Primary Industries, Water and Environment
Tasmania, including all its islands, supports a wide variety of landforms, plants and animals. There are approximately 1900 native plant species, 37 native mammals, 159 resident terrestrial species of birds, 21 land reptiles, 11 amphibians and 44 freshwater fish. Isolated from the Australian mainland for at least 10,000 years, Tasmania has both supported the continent’s biodiversity by providing a refuge for species that have died out on the mainland, and it has been protected from most of the introduced animal species that have so affected the flora and fauna of mainland Australia. The dingo is absent; the fox has never become established; and feral goats and pigs have restricted Tasmanian distributions. Among introduced species, the feral cat and rabbit are the greatest threats to native populations and their habitat.
Geodiversity or the diversity of our non-living environment is a prerequisite for biodiversity. For example, Pedra Branca Rock is the only place in the world where the Pedra Branca skink lives. If we allow our waterways, landforms and soils to become degraded then this will adversely impact on natural diversity. Geoconservation is an essential part of bioconservation, as geodiversity provides the variety of environments and environmental pressures which directly influence biodiversity.
For its size, Tasmania has a large variety of rock types. These are representative of the different geological periods stretching back as far as one billion years. The break-up of the Gondwanic supercontinent has had a major influence on the geological evolution of the State, particularly the final break-up, which resulted in Tasmania taking up its roughly triangular shape by about 20 million years ago.
Other significant events have included a number of glaciations. These played a critical role in moulding the States mountains over the last two million years, but also influenced land-forming processes down to and below sea level, which was about 120 m below the current level, 20,000 years ago.
Perhaps one of Tasmania’s most famous geological sites is Macquarie Island. It is a very rare example of a geological feature which occurs far below the ocean surface and the earth's crust. Rocks from the upper mantle are well-exposed on the island, as are basalt lavas which extruded on the sea floor about 10 million years ago. It is for these reasons that the island was nominated and listed as a World Heritage Area in 1997.
The loss of native vegetation is widely regarded to be the single most significant threat to biodiversity. Since European settlement in 1803 Tasmania has lost 30% of its original native vegetation. The greatest losses are an estimated 66% of swamp forests, 47% of coastal heathland, 46% of dry forest and woodland, and 40% of grasslands. The percentage decrease is lowest in the vegetation types on the most infertile soils. In the period 1988-94 an average of 10,500 ha were cleared annually (Kirkpatrick and Jenkin 1995).
Reservation is used to protect significant species, communities and habitats. A conservation reserve of world class standard requires 15% of pre-European extent or 60% and 100% of 1996 distribution of specific vegetation communities in the reserve system. For forest communities, these targets are being pursued through the Regional Forest Agreement implementation of the Comprehensive, Adequate and Representative (CAR) reserve system. Targets for non-forest native vegetation will be more accurately determined after the completion of detailed vegetation mapping. In many instances private land is required. The majority of the reservation targets are being met through special covenant agreements with landowners through the Regional Forests Agreement Private Land Reserve Program.
Most of the plant communities with the highest priority for conservation occur in the drier areas of the State, the Midlands and northern slopes and south eastern Tasmania bioregions, where extensive agricultural clearance or settlement has occurred. Over 95% of this land is in private ownership. Of the 50 forest communities mapped throughout Tasmania approximately 25 require some private land to be included into the reserve system. The dry sclerophyll forest/grassy woodlands are especially rare.
Wetlands are areas permanently or periodically inundated by water where sediments and nutrients accumulate. They form a critical link in many terrestrial and aquatic food webs, and the accumulation of nutrients means that they are highly productive areas. Wetlands support flora and fauna that have evolved to survive in a diverse and changeable environment. Because of their specialised physical adaptations and the isolation of many wetlands, there is a high rate of endemism (species that live only in Tasmania), among these organisms. Wetlands have an important role in ensuring water quality and controlling flooding.
Agricultural development conducted without prior ecological assessment is the major cause of the decline and degradation of wetlands in Tasmania. Urbanisation is another pressure responsible for the decline in the diversity and complexity of wetlands in Tasmania.
Around 800 wetlands covering 41,546 ha have been studied in Tasmania. Of these, 92 are considered of national importance, with a further 61 considered to be of State significance. Ten are considered to be of international significance and are listed on the RAMSAR Convention on Wetlands.
Since European settlement began in 1803, many changes to the Tasmanian landscape have occurred. Although these changes have benefited some species, some are now threatened with extinction. The demise of the Tasmanian Tiger or Thylacine in 1936, and the lack of concrete evidence for its existence since, should serve as a lesson. The last 100 years has seen the recognition that fauna protection includes not only the rich and diverse vertebrate fauna, but the equally fascinating invertebrate fauna. That recognition has resulted in the listing of 118 threatened invertebrates.
Up to 32 priority vertebrate and invertebrate species are being considered for protection under the private land Comprehensive, Adequate and Representative reserve system. Protection is achieved through a combination of dedicated and informal reserves, and off-reserve management prescriptions. Species include 3 mammals, 4 birds, 5 fish, 4 freshwater crayfish, 6 beetles, 5 snails and 4 other invertebrates, which include a butterfly and 3 velvet worms. Currently, minimum reservation requirements for 20 of these species are being met on public land.
Land clearing and soil degradation have been relatively less in Tasmania than on mainland Australia. Nevertheless, the habitats of much of Tasmania have been altered on a large scale by activities such as agriculture and forestry. Subdivision of land into small to medium-sized blocks for residential development poses a threat to conservation of native fauna in many municipalities. Large areas of native habitat are being divided, and reduced to isolated fragments, too small to sustain viable populations of native animals. Sensitive and careful land planning is needed, coupled with joint co-operative action involving the whole community, to ensure that the needs of native wildlife are met.