The battle for dominance is underway on the West Coast as the creeping Acaena (foreground and middle ground) struggles to compete with the increasingly taller grasses while the megaherb Stilbocarpa gains a foothold.
In April 2015 the short grasslands of Macquarie Island are at the end of their summer growing season. The plants are the tallest they have been for years, probably for many decades. There is virtually no bare ground visible as the lush growth covers dirt previously laid bare by rabbits. These changes were predictable consequences of the rabbit eradication nearly four years earlier. It is too early to tell for sure, but it does look like other predicted changes are starting to occur, such as the ascendancy of the megaherbs.
While historical information about Macquarie Island’s vegetation is quite limited, better data are available from the 1980s. One of several long-term ecological monitoring programs on the island is a set of six vegetation monitoring sites established by Geof Copson in 1980. These include locations on the west coast, east coast and plateau, all with some form of short grassland vegetation.
Location of Tasmanian Parks and Wildlife Service vegetation monitoring sites on Macquarie Island. Source: Whinam et al. (2014).
Each site comprises five randomly located 20 x 20 metre quadrats. These 30 quadrats are typically surveyed every 2 to 3 years by a pair of botanists who record the vascular plant species present and estimate the foliage cover of each species.
Rabbits kept these short grassland ecosystems very short. There was little point in measuring the height of the plants in what resembled a mown lawn. Now the short grassland is getting taller. And that is something worth measuring. Measurements of the maximum foliage height of each major species have been made during the past two surveys. Not surprisingly, there is a consistent increase in height across all species and sites.
Height of native grass species Festuca contracta at four study sites has increased as much as three-fold between summer 2013 and summer 2015.
Since the peak in the rabbit population around 2006 some changes in the extent of plant species are evident in the monitoring sites. The two largest plants on the island, Stilbocarpa polaris and Poa foliosa, have declined. Both these megaherbs are known to be highly palatable to rabbits so it is likely they suffered during the rabbits’ final rampage and have yet to recover. They are still present in very small numbers at all sites so it seems that the rabbit browsing did not wipe them out locally and, therefore, there is capacity for recovery. Given what we know about these species it is quite possible they will increases from less than 1% foliage cover on some sites to nearly 100% over the next few years.
Average foliage cover class of the megaherb Stilbocarpa polaris and tussock grass Poa foliosa at six sites since 1981.
Plants which have benefited from disturbance by rabbits, such as Poa annua and Acaena magellanica, display a very different trend. These disturbance-lovers have expanded since the mid-2000s. It is reasonable to assume that without rabbits these plants will become less abundant as they compete with other plants. Unless there has been a state-shift in the ecosystem or there are other factors at play which make such simplistic predictions wrong – as is often the case in ecology!
Average foliage cover class of the introduced grass Poa annua and the native disturbance-colonizing herb Acaena magellanica at six sites since 1981.