Not long ago, I wrote about how the structural complexity of tall mountain ash forests allows a diverse array of animals to co-occur at the same place. Grasslands, with the dominant plants rarely standing more than a metre tall, offer little in the way of opportunities for animal communities to assort themselves vertically. But just as they say you shouldn’t judge a book by its cover, so too should you never judge a landscape by its vegetation cover. I was recently able to fully appreciate this while doing fieldwork in the Western Grassland Reserves right on the fringe of Melbourne’s western suburbs.
Structural diversity also plays a role in shaping grassland biodiversity; the process occurs at a smaller scale, though. For instance, small depressions have higher water availability and rushes, such as gold rush (Juncus flavidus), grow best in these locations. Similarly, breaks in the grass cover that result in patches of bare ground are where the small rosette form of spur velleia (Velleia paradoxa) is most likely to be found. Just like giants of the forest that emerge above the canopy of a mountain ash forest, so too does artichoke thistle (Cynara cardunculus) tower above the surrounding grasses and herbs. In the same way that small differences in habitat influence the plants that occur at a site, animals respond to variation in vegetation structure. Nestled tightly amongst dense grass tussocks, millipedes can be found; this may be due to the microclimate created as the many tightly clustered grass stems trap a pocket of moist air. Sitting atop the taller artichoke thistles, Eurasian skylarks (Alauda arvensis) or Horsfield’s bush larks (Mirafra javanica) may often be sighted singing their rich and melodious songs. These species are also dependent on the dense cover provided by thick grass because it is amid these grasses that they most often situate their nests.
As you can see from the species I’ve mentioned, there is more than just grass in a grassland. So why then, if many different plant species are able to grow in a grassland, do trees not occur there and turn the grassland into a woodland or forest? Grassland biomes predominantly occur where there is low to moderate rainfall; any less and the landscape becomes desert, any more and forests or woodlands predominate. Disturbance is also a key factor. Grassland plants are able to reach reproductive maturity rapidly after disturbances such as fire and grazing. Therefore, in environments that would otherwise be amenable to the growth of woody vegetation, repeated disturbances at frequent intervals favour the presence of grasslands.
Succession post-disturbance can be key to determining what grassland plants are present at a site. In the immediate aftermath of a grass fire, light is able to reach the ground layer. This prompts many sun-loving species to germinate and, for a short period of time, flourish. As time passes, other plants re-colonise the landscape and compete for light and nutrients. This process may see some species disappear from a landscape until the next fire passes through. Re-colonisation may rely on seed stored in the soil seedbank, or plants may have seeds with a high dispersal capacity and recolonise via seed arriving from other locations. Many grassland plant species have underground stems and structures that are protected from the passage of fire by the soil. These enable them to re-grow quickly in the post-fire landscape. The successional changes following fire also influence animal assemblages. Once plant species have recovered, ground-dwelling stubble quail (Coturnix pectoralis) may re-appear at a site. However, if the grass cover becomes too thick with the passing of time, they too may disappear in search of more favourable habitat elsewhere.
Whilst fire is a major method by which biomass is removed from grasslands in our state, it is not the only process by which this occurs. Across the Indian Ocean in Africa, large herbivores such as zebras, wildebeest, and many species of antelope extensively graze the grasslands. In Australia, grassland herbivores are typically smaller. Of course there are eastern grey kangaroos and the introduced European rabbit, but the biomass of invertebrates is much greater than these groups. Across Australia’s Top End, the mounds of grass-eating termites interrupt the horizon; closer to Melbourne, orthopterans (grasshoppers, locusts, and crickets) as well as the larvae of many lepidopterans (butterflies and moths) are major consumers of above-ground plant material. The effects of invertebrate-grazing in Victoria’s grasslands remains little studied. However, although they are only small, individuals from these groups can occur in high abundance and the cumulative effect of their feeding is likely to exceed that of grazing vertebrates (except of course where native grasslands are subject to grazing by sheep and cattle!).
We Melburnians are lucky. Our metropolitan area is fringed to the west by the Victorian Volcanic Plains. The importance of the grasslands that occur on these fertile plains cannot be understated. They are one of the most heavily impacted ecosystems in our state and suffer at the hands of urban expansion, weed invasion, loss of characteristic fire regimes, and past land use. Many of the best examples occur on roadside and railway reserves because these have escaped intensive agricultural use, and may have received regular burning. These grasslands support a wealth of biodiversity including the Critically Endangered golden sun moth (Synemon plana) and spiny rice-flower (Pimelea spinescens subsp. spinescens). At a cursory glance, the flat landscape and uniform grass cover may seem uninspiring. Yet, if you are intrepid enough to take a closer look, you will see that they are every bit as deserving of your attention as the tall mountain ash forests to our east that gain so many visitors.
Rowan is a PhD student studying seabird ecology. When he's not thinking about the ocean, he likes to think about woodland birds.
You can find him on Twitter at @roamingmoth
Banner image courtesy of Rowan Mott.