Wetlands Australia 32: Biodiversity across wetlands in the Derwent estuary, Hobart
Authors: Inger Visby (Derwent Estuary Program) and Dr Vishnu Prahalad (University of Tasmania)
The Derwent Estuary faces pressures due to population growth in Hobart as well as climate change and sea level rise, yet there has been a lack of knowledge about the plants, birds and ongoing human impacts on these urban wetlands.
One of the earliest settlements of colonial Australia was on the shores of the Derwent estuary, directly impacting the Sullivans Cove’s now extinct wetlands. Hobart grew in this location, to become the largest city and capital of the state of Tasmania, with surrounding coastal settlements affecting almost all of Derwent’s saltmarsh wetlands.
There has been a lack of knowledge about the plants, birds and ongoing human impacts on of the urban wetlands of the Derwent Estuary. In response to this information gap, an estuary-wide, cross-tenure, saltmarsh survey project was started in 2018. From a management perspective, we began to examine human impacts, historic saltmarsh loss and likely future extent. The aim was to provide landholders with information about a threatened vegetation community on their property, what state their saltmarsh was in, what does its future look like, and what actions should they be taking now, and going forward, in order to preserve and improve biodiversity and other values.
Derwent Estuary. Photo: Inger Visby
We also conducted plant and bird surveys for the first time covering the whole estuary. The flora survey across our 16 sites documented 102 species, of which beaded glasswort (Sarcocornia quinqueflora subsp. quinqueflora), sea rush (Juncus kraussii subsp. australiensis), and coast speargrass (Austrostipa stipoides) were the most recurrent and dominant native species. Among the 37 introduced species (36 % of total), sweet briar (Rosa rubiginosa), blackberry (Rubus fruticosus agg.) and African boxthorn (Lycium ferocissimum) were identified for active control, while others were considered to be more benign.
Bird surveys (two 20 min counts in one summer) recorded 37 species, of which the welcome swallow (Hirundo neoxena) and Tasmanian native-hen (Tribonyx mortierii) were the most abundant and common. Planting of native buffer vegetation was identified as a priority in some sites to attract terrestrial birds using saltmarsh for feeding, as was signage and education of dog walkers.
The availability of the Saltmarsh App allows further contributions to be made to this database to continue to track the biodiversity in our urban wetlands. We expect this monitoring and outreach to tie in with on-ground works including buffer plantings and infill removal (e.g. at Windermere Bay), combined with interpretation walks and signs, to highlight wetland biodiversity. This data will be used to increase public awareness of the important habitat these remnant wetlands provide and to leverage on-ground management works with key land stakeholders.
Derwent Estuary. Photo: Inger Visby