Andrew MacDougall and co-authors have just published a paper in Nature (494, 86-89) that potentially has implications for understanding disturbance ecology here in Australian temperate grasslands. It's about fire impacts in degraded savanna and ecosystem stability. I've admired Andrew's work in oak savanna in British Columbia for some time. He's written about whether exotic species are 'drivers' or 'passengers' of change in degraded systems, and how to define conservation strategies using historical perspectives. It's worth checking out his Lab Homepage.
In the original oak savanna of British Columbia, a high diversity of grasses and forbs was historically maintained by frequent fire. But with fire suppression, invasions and pastoralism, present day oak savanna are species-poor, and dominated by exotic grasses. These dynamics are captured in Supplemenatry Fig 1 of the paper:
|Fig S1 from MacDougall et al (2013)|
|Present day oak savanna. Species-poor, and dominated by exotic grasses.|
Because of fire-prevention measures since the mid-nineteenth century, oak savanna have lost many of their plant species - including the fire-tolerant species that contribute most of the diversity. Ironically, present day savanna produces a relatively stable annual biomass (from the remaining exotic grasses) and remains resilient to climate fluctuation.
MacDougall et al. conducted a 10 yr study in which the low diversity savanna plots were periodically burned after a very long period without fire. Savanna were able to recover from burning only in areas that had a relatively high diversity of native plants. (i.e. the system had maintained native species from the original flora that were capable of regenerating after fire). While these native species were rare and mostly functionally redundant, they proliferated after burning and rapidly recover the structure and function of the savanna, as well as preventing invasion by woody species. The exotic grasses, by contrast, were not well-adapted to fire and the system subsequently crashed after fire. They concluded that the study demonstrates how persistent human activity can homogenize both structure and function of an ecological system and this can weaken the diversity-related mechanisms needed to compensate for sudden disturbance.
So, why did I get excited?
Well, in a similar vein here in southern Australia, many native grasslands exist as species-poor systems due to fire exclusion and grazing. Exotic grasses are common in many grazed grasslands, many of the daisies and lilies that characterise the grasslands have been lost, and in many cases, native C4 grasses have been replaced over vast areas by C3 species.
And it is exactly these types of grasslands that are being acquired to improved the conservation status of grasslands near Melbourne and offset losses to urban expansion. What has me most interested is the idea that managers will need to manage new grassland reserves for biodiversity and one key way to do this is to remove biomass by burning. This is exactly what scientists (like me) have been saying is necessary to manage for grassland diversity.
But, if fire is introduced into a grassland that perhaps has had a century of fire exclusion, and occurs in a system where many of the fire-tolerant native herbs have been lost because of grazing, what might we expect as the outcome?
From work that Ian Lunt, Andrew Scott and others have conducted, we know there is likely to be little soil seed bank of native forbs in grasslands. Hence, fire won't promote a flush of native species to bring about a miracle cure for the grasslands. And the most fire-tolerant grasses (like Kangaroo Grass) are now absent. So, might fire actually de-stabilise grassland structure and function as a result of one hundred years of land use and the loss of native species? Might ecosystem stability be compromised by the re-introduction of fire because there is a hidden vulnerability to sudden environmental change in ecosystems that have had the buffering effects of high species diversity eliminated?
These are crucial questions for which we do not yet have answers. But it is work like that of MacDougall et al. that point to the real, perhaps even urgent, need to examine these effects, least we assume that species-poor systems function as if they were still species-rich.
MacDougall, McCann, Gellner & Turkington (2013) Diversity loss with persistent human disturbance increases vulnerability to ecosystem collapse. Nature 494: 86-89.