Lippia ('Phyla canescens', Verbenaceae) is an invasive perennial forb from South America that is now widespread throughout the Murray Darling Basin in Australia, threatening the biological values of internationally significant wetlands (Ramsar sites) and the productivity of the grazing industry (Earl 2003). In the Gwydir Wetlands in north western NSW, the spread of Lippia has been associated with large and widespread flooding that occurred in the late 1990s (McCosker 1998). Lippia dominance is believed to be favoured by the altered flood regimes associated with river regulation, namely a reduction in the frequency and duration of flooding. Studies have found Lippia to occur in infrequently flooded sites (Blanch et al. 1999; Mawhinney 2003) and at higher elevations in the floodplain (Blanch et al. 2000). Glasshouse studies have found that Lippia growth is stunted at inundation depths of 20 cm, although no mortality was reported (McCosker 1994; Hobson 1999). This has lead to a widespread belief that restoration of a more 'natural' flood regime may assist in Lippia control. However, few data are available to support this notion.

The Gwydir Wetlands and Macquarie Marshes are both examples of variable and dynamic floodplain wetland ecosystems. Both systems have experienced a major alteration to their hydrology as a result of river regulation, and the two regions have a long history of floodplain grazing. As efforts are made to address the hydrological issues in these wetlands, there is also a need to better understand how other disturbances such as grazing may be impacting on the ecological functioning of these ecosystems. This study investigated five areas of wetland condition, namely vegetation composition, soil seed bank, soil chemistry, water quality and aquatic invertebrates, to determine the nature and the magnitude of any impacts that grazing by domestic stock may be having on these two wetland ecosystems.

1. Characterization of the seed bank is one of the most important demographic assessments that can be undertaken for a plant community. Overlapping generations, evidence of past above-ground vegetation and histories of invasion and disturbance are recorded in the seed bank. 2. Two broad approaches have been used to elucidate seed bank components - sifting-sorting techniques and germinability assays. The utility of these approaches varies with community type and habitat although a common theme among studies has been the quest for an efficacious method. Here, we compare the two approaches for semi-arid ephemeral wetlands: seed extraction through flotation and seedling emergence. 3. Species composition of the soil seed bank differed dramatically depending on the technique, with only 19 species common to both methods and a total of 66 species detected using both procedures. 4. Both techniques provided similar estimates of seed density and species richness of the seed bank in the top 5 cm of soil. However, samples collected from 5 cm to 20 cm had lower seed densities using the flotation technique than with the seedling emergence technique. 5. Differences in seed detectability between the two approaches may be related to seed size, seed dormancy and specific germination requirements. 6. The community composition of soil seed banks for ephemeral wetlands depends on the choice of technique.

The Murray-Darling Basin is a Social-Ecological System (SES) of major importance to Australia and includes extensive wetland areas in the north-western parts of New South Wales. The Gwydir Wetlands and the Macquarie Marshes are the particular focus of this paper. These two wetland SES have undergone five successive adaptive cycles (phases) since they were first visited by Europeans in the early 19th century and the ecological, economic and social drivers initiating each transformation to a new cycle are described and analysed. The arrival of the European settlers with their domestic livestock rapidly displaced the Indigenous SES and the wetlands were extensively grazed; during wet periods the livestock were moved out of the wetlands and moved back in as the water receded. More recent land-use changes resulted from the building of major dams to enable storage of water for use in irrigated agriculture. A consequence of dam construction and water use has been a reduction in the frequency and extent of flooding, which has allowed many parts of the wetlands to be continually grazed. Furthermore, as machinery capable of cultivating the very heavy textured soils became available, dryland cropping became a major enterprise in areas of the floodplain where the likelihood of flooding was reduced. With the reduction in flooding, these wetland sites have been seriously degraded. The final phase has seen the invasion by an exotic weed, lippia ['Phyla canescens' (Kunth) Greene], which is a perennial that grows mat-like between other species of plants and spreads to produce a virtually mono-specific stand. The domestic livestock carrying capacity of the land becomes more or less zero and the conservation value of the wetlands is also dramatically decreased. Therefore, we suggest that lippia should be classed as an ecosystem engineer that has caused the latest transformation of these wetland SES and suggest research directions to investigate how they can be managed to revert to a state in which lippia is no longer dominant.