Assessing habitat condition – an old approach to an old problem

The drawing above, depicting a one yard by one yard quadrat, was published in 1936 (R.T. Patton Ecological studies in Victoria; Part IV – Basalt Plains Association. Proc. Roy. Soc. Vict., 48, 172–190). It is one of the first descriptions of the diversity in native grasslands near Melbourne. Historically, botanists tended not to focus on the area west of Melbourne. Von Meuller (the famed 19th century botanist) seldom visited the area. It was not until 1916 (Sutton) that the flora was reviewed and then only the Keilor Plains. By the time of John Stuwe’s review of native grasslands in western Victoria in 1986, he was only dealing with 0.15% of the original area of Themeda grasslands.

Such historical information is of immense importance when trying to determine how the composition and structure of native grasslands has changed in the almost complete absence of floristic information, and to identify remnants that most approximate the historic condition.

Despite the small samples size (n=1), we can gain many importance pieces of information from this 90 x 90 cm quadrat:

Native species richness = 12 (3 grasses – all perennial, 9 forbs – 1 annual, 8 perennial)

Exotic species richness = 1 (the annual grass Aira)

Themeda tussock density = 13 (comprising many small tussocks)

So how does a current day native grassland dominated by Themeda compares to one historic example we have?



Quadrat depicting current species density at Mt Cottrell
(Drawing by Hannah Forrester)

 

Recently, some students of mine re-drew grassland quadrats in a conservation reserve that is less than 3 km from Patton's original site. The grassland is dominated by native species . A typical example of the current day diversity is shown above.

Native species richness = 3 (the dominant perennial grass + 2 forbs)

Exotic species richness = 4 (1 annual grass, 1 annual legume, 1 Iridaceae, 1 perennial forb)

Themeda tussock density = 31 (comprising many small tussocks)

The grassland at Mt Cottrell looks like a high quality remnant
- but comparison with historical data suggests otherwise
(Photo: John Morgan)

It's clear that, despite the grassland looking like a native grassland from a structural perspective, profound changes have occurred. Invasions of non-native species are one obvious change - but they are largely sub-ordinate species and do not necessarily strongly influence the function of the grassland. Rather, it is the almost complete loss of interstitial native species that has been most profound. Where there might have been 11 (largely perennial) species in the inter-tussock spaces, our survey showed just two. And these occurred at incredibly low density.

It's likely that the loss of native species has had two principle drivers - the loss of palatable species during the period of stock grazing that preceeded the declaration of the reserve probably accounts for the absence of many typical grassland forbs unaccustomed to on-going and heavy herbivory. And some loss might have also occurred due to the lack of frequent burning. Themeda assumes monodominance at the site, and appears to strongly compete for light and space with interstitial species. Many temperate grasslands are burnt irregularly after conservation, leading to rapid declines in diversity, so similar processes might be at play here.

While the comparison we have made is very simple, and perhaps old fashioned, the historical data gives us something to aim for when considering the restoration objectives for the site. It tells us what sort of native species might be returned and at what densities. This is an incredible challenge, and not for the faint hearted. But at least our objectives are evidence-based and this is likely to lead to restoration that has a clearly defined end-point. It tells us that the re-establishemnet of interstitial forbs, not weed control, is our priority, and that to do this, reducing the dominance of Themeda is paramount.