Banksia Woodlands of the Swan Coastal Plain – an Assessment Challenge

Threatened Ecological Communities (TECs) are naturally occurring biological assemblages that are under threat of destruction or significant modification (DEC 2001).

The Banksia Woodlands of the Swan Coastal Plain Ecological Community (Banksia Woodlands TEC) was approved for inclusion as an Endangered TEC under the Commonwealth EPBC Act on 16 September 2016. This ecological community is a woodland associated with certain soils of the Swan Coastal Plain with a prominent tree layer of Banksia, and scattered Eucalypts and other tree species, with a species-rich understorey (Threatened Species Scientific Committee 2016).

The Banksia Woodlands TEC is largely restricted to the Swan Coastal Plain, within the Perth and Dandaragan subregions. It extends into the adjacent Jarrah Forrest and areas of the Whicher and Darling escarpments (Threatened Species Scientific Committee 2016).

Twenty-two Floristic Community Types (FCTs) described by Gibson et. al (1994), in Bush Forever (Government of Western Australia 2000), by Keighery et. al (2008) and by the Urban Bushland Council (2011) best correspond to the Banksia Woodlands TEC (Threatened Species Scientific Committee 2016).

The Conservation Advice (Threatened Species Scientific Committee 2016) lists the key characteristics of the TEC and therefore criteria for use in diagnostic tools.

The Focused Vision Consulting team has developed a specialised diagnostic tool, which is being refined with each new Banksia Woodland TEC assessment we undertake.

The front-end of the tool relies on publicly available spatial data including IBRA regions and soil systems, with the bulk of the criteria relying on data collected in the field, predominantly structure and species composition. So far so good. However, the challenge in accurate diagnosis arises when attempting to allocate FCTs to Banksia woodlands within study sites.

FCTs are defined as major regional (vegetation) community types (Gibson et. al 1994), and for the southern Swan Coastal Plain, are based largely on the cited study from 1994. Floristic analysis of plot-based data collected from study sites is compared with the 1994 dataset, and FCTs are allocated based on cohesion in a cluster analysis dendrogram or simple affinities based on highest scores for species in common. As most WA botanists familiar with this analysis and this dataset will attest, the results are rarely conclusive; with several possible FCTs usually matching to each vegetation type or plot.

The constraints are due to a number of factors, including the relatively limited sample size of the reference data. For example, the Gibson et. al 1994 study was limited to just 509 plots across the entire southern Swan Coastal Plain, for 30 defined FCTs. Additionally, this dataset is now significantly out-of-date, and with taxonomic revisions, requires constant updating. Furthermore, not all of the reference data incorporates cover or identifies dominant taxa within each type. Whilst “typical” species are listed for each FCT, a lack of dominant flora identified for each prevents the option to apply weightings to the these when carrying out comparative analysis. Therefore, we are limited to simple presence-absence comparisons.

The importance of FCT allocation enters when proposing offsets for unavoidable impacts to the Banksia Woodland TEC.

The relevant regulatory authorities and advisory departments require that offset sites match impact sites in terms of FCTs represented, and rightly so.

But this becomes significantly challenging when allocation of these FCTs is limited by the reference material.

In an attempt to overcome this challenge, Focused Vision Consulting is working alongside some key clients, consulting with fellow botanical specialists, including academics and key personnel from the Department of Parks and Wildlife, as well as gauging regulatory and advisory attitudes towards both FCT allocation and the need for FCT-specific sites for offsets.

As with all new conservation listings and guidelines for environmental assessment and management, there will be teething problems and some matters will be identified for adjustment in an inevitable revision of the Conservation Advice. For now, we are committed to working together with the best minds for the job, to achieve outcomes that are focused on the main objective; appropriate conservation of these important woodlands.