Marsupial grazing lawns in Tasmania : maintenance, biota and the effects of climate change

Native vertebrate grazers have been shown to be a critical element in the structuring of natural grasslands and lawns. Tasmania has a high density of native grazers and lawns have formed sporadically within woody landscapes. They are locally known as ‘marsupial lawns’ and can be found at altitudes from alpine to coastal and are often associated with wetland areas. While grazing lawns have been studied elsewhere, (particularly in southeast Africa), little is known in Tasmania on the causes of persistence of lawns. This study set out to investigate what maintains lawns associated with wetlands in Tasmania through researching the following questions: 1) What is the role of native vertebrate grazers and environmental factors, either singly or interactively, in the maintenance of grazing lawns? 2) Are there distinctive biotic communities associated with grazing lawns? 3) Is a changing climate affecting the extent of grazing lawns? Two marsupial lawn sites (Bangor at sea level, and Central Plateau in the highlands) were established to investigate the first and second questions. Scat counts confirmed the presence of marsupials in high numbers and also the presence of rabbits (but to a lesser degree), and exclosures proved that by excluding grazers, woody seedlings can establish and flourish on the lawn. Soil moisture was significantly higher in the soils of lawns than in the adjacent taller vegetation, suggesting that high moisture levels create productive lawns that in turn attract a level of native grazing (by wallaby, pademelon and wombat) that prevents invasion by woody species. Other environmental factors such as waterlogging, extreme temperature fluctuations and soil fertility do not appear to exclude woody species from invading the lawn at either site. The Bangor and Central Plateau lawns were biotically distinct from the adjoining bush vegetation. Lawns had a dense cover of herbaceous species while ground cover in bush consisted of litter and bare ground with little herbaceous cover. Dicotyledonous herbaceous species provided the dominant cover in the lawns. The composition of herbivorous invertebrates was also distinct between lawn and bush at both sites. One third of the taxa found in lawn pitfall traps and sticky traps were not found in bush traps. The Bobilla cricket (Gryllidae) dominated the lawn taxa at Bangor and the weevil Desiantha (Curculionidae) dominated lawns at the Central Plateau. Since the late 1970s dry conditions have been experienced in southeastern, eastern and northern parts of Tasmania with significant rainfall reductions in eastern, central and Flinders Island compared to the thirty years prior to 1978. Without adequate moisture, herbaceous lawns have failed to thrive. A consequent reduced grazing pressure may have enabled woody species to establish. A comparison of old (pre 1978) and current (post 1978) aerial photographs and aging (using ring counts) of invading shrubs, confirmed that woody species invasion of lawns post 1978 is widespread in eastern and northern parts of Tasmania, reducing the area occupied by lawns. Not affected by woody invasion were high rainfall areas, lawns around saline lagoons, coastal areas subjected to salt spray and low lying lawns occasionally inundated by sea water in extreme storm events. The invasion of grasslands by woody species is a global phenomenon of concern, as shifts from herbaceous to woody vegetation affect the flora and fauna that are dependent on a non-woody ecosystem. The causes have variously been attributed to changed domestic stock grazing interacting with environmental variables, fire regimes, increased rainfall, increasing CO2 levels, nitrogen deposition and warming due to climate change, though few studies have attributed woody growth to lack of native grazing pressure because of drought. The maintenance of ephemerally wet marsupial lawns in Tasmania is the result of the interaction of ‘top down’ and ‘bottom up’ controls. If either moisture or grazing levels are changed substantially, as appears to be happening with changing weather patterns in Tasmania, then this habitat and the associated biotic communities appear vulnerable to decline