Beyond the 'extinction debt'

The best examples of temperate grasslands of SE Australia, like tallgrass prairies in the USA, now largely exist in small, isolated pockets, and are of variable quality due to a century of more of land use. They're the last bastions of these once great ecosystems.

Basic conservation biology theory tells us that long-term conservation of the ecosystem will be challenging - populations are small and isolated, habitats have been degraded by weed invasion to varying degrees, and interactions (such as plant : pollinator mutualisms) may have already been disrupted.

High quality native grassland on three-chain wide roadside at
Derrinallum. Mt Elephant looms large in the background.
(Photo: John Morgan)

Hence, conservation of the biota will be a challenge.

To illustrate the scale of the problem, thanks to generous funding from the The Myer Foundation, our research team has just recently completed a floristic survey of Melbourne's Urban Grasslands. Our fabulous botanist Ben Zeeman (and team) recently surveyed the floristic composition, structure and population size of selected plant species. In total, we surveyed 79 public grasslands for composition (totalling 1393 ha), did 904 transects defining structure, and made 45,200 species observations. Wow! What an effort. Of the 250 native species we observed, 80% were confined to 20% or fewer sites. Hence, while there is still lots of native plant biodiversity in urban Melbourne, much of it is perhaps on the brink of extinction.

Extinction debt is a concept in ecology that may have great relevance to Melbourne's grasslands.

Extinction debt occurs because of time delays between impacts on a species, such as destruction of habitat, and the species' ultimate disappearance. For instance, long-lived trees may survive for many years even after reproduction of new trees has become impossible, and thus they may be committed to extinction. Extinction debt is most likely to be found in long-lived species and species with very specific habitat requirements (specialists).

Extinction debt has important implications for conservation, as it implies that species may go extinct due to past habitat destruction, even if continued impacts cease, and that current reserves may not be sufficient to maintain the species that occupy them.

So what can be done to reverse the extinction debt in Melbourne grasslands? This is a conversation we need to have. Biologists are telling us there will be a continuing crisis (and they have been very good at documenting declines) but is there a way to slow / reverse the extinction debt?

There are probably several answers here. Using a basic understanding of the threats to remnant vegetation that apply anywhere where land use has transformed the native landscape, I can think of four key things we should be doing.

1) Supplement / enhance small populations: we know one of the main reasons that populations go extinct is that they become small and hence, vulnerable to loss of genetic diversity, environmental stochasicity, and regeneration failure due to the fact so few propagules being produced. Hence, having identified which species are in this category in Melbourne's grasslands, perhaps we should look at bulking up population numbers. This might be done with transplanting plants and sowing seeds. Importantly, such activities should not be restricted to just rare species. Our data tells us lots of species might be in trouble.

2) Promote population turnover: one of the reasons populations become small is because recruitment ceases to replace mature plants. If deaths > births, then it is inevitable that populations decline to local extinction. Hence, rather than just maintaining adults (the thing that we see in grasslands), it is imperative that the mechanisms that allow seedling recruitment are well-understood and, more importantly, the conditions for regeneration are maximised. A focus on seedling recruitment needs to occur. My own work suggests that seedlings of many species require lots of light to survive, and that recruitment is episodic and probably related to years of above-average rainfall. Maybe we can enhance recruitment success by removing canopy (by fire or slashing) and watering plots to see if seedling recruitment improves.

3) Maintain important ecosystem processes: one of the critical processes in grasslands is the effect that dominant grasses have on smaller intertussock species. Strong, asymetrical competition means that Kangaroo Grass tends to win the battle when left undisturbed. Hence, frequent implementation of fire (or some other mechanism such as slashing) needs to occur if you want to maintain intertussock species of the majority of the flora. This is well known, but needs to be implemented.

4) Empower people to care: if we are to avoid a massive loss of Melbourne's grassland biodiversity, we have to make people care about this. Many people already do. There are dozens of terrific Friends Groups, individuals and agency staff who are working tirelessly to improve grassland condition and recognition. This energy needs to be harnessed, and a broader audience cultivated. Those people who live near grasslands have to care about them, otherwise conservation of local diversity will always be a challenge.

Recently I saw some excellent signs that advertise the values of grasslands and their habitats. This was on one of the best rural roadsides for native grasslands there is, so the values are already well-known. But I applaud the local Shire for emphasizing the values of remnant grasslands, and why we should care. This needs to happen in Melbourne, least we lose lots of our grassland flora and no-one notices.

The fabulous signs on the Cressy-Shelford Rd promoting the values of the native grassland
as habitat for endangered species. The signs measures approx. 7 m x 2.5 m and are produced in high quality materials.
(Photo: John Morgan)