Feral rabbits and the baseline problem

Rabbits were already locally abundant on Macquarie Island by the time the first descriptions of the island’s vegetation were made by Scott (1880) and A. Hamilton (1884). Consequently there is no information to give us a baseline of the vegetation prior to the impact of rabbits (there are no native grazing animals on the island). Another problem is that these early vegetation records of Scott, A. Hamilton and, later, his son H. Hamilton (1911-14) provide only basic information with no precise details. Hamilton noted in 1884 that:

The tussocks and the Stilbocarpa become smaller as you ascend and, at about 300 feet, you gain a plateau so swept by the Antarctic gales that vegetation is reduced to compact, closely growing mosses, small Uncinias and the conspicuous cushion-like masses of Azorella selago.

An exclosure plot showing the difference between ungrazed (inside the fence) and grazed vegetation (everything else) around 18 months after rabbits were removed.
An exclosure plot showing the difference between ungrazed (inside the fence) and grazed vegetation (everything else) around 18 months after rabbits were removed.

The first detailed scientific treatment of the Macquarie Island vegetation was by Taylor, who spent some months on the island in 1950. He noted that rabbits were in “plague proportions” in some areas. Rabbit population estimates are problematic since rabbit populations are highly variable over space and time. Rabbits would reach great numbers in certain areas, until the food resource was reduced to a point where the rabbits would either starve or move elsewhere. Data collected from the 1970s onward provides a good indication of trends in rabbit populations.

Population trends of rabbits on Macquarie Island. Red line is smoothed population estimate based on models, with 95% confidence intervals (dotted lines).
Population trends of rabbits on Macquarie Island, 1974-2012. Red line is smoothed population estimate based on models, with 95% confidence intervals (dotted lines). Source: Terauds et al. (2014) Polar Biology.

Following the successful introduction of the biocontrol agent myxoma in 1978 rabbit populations declined by around 90% and remained relatively low until an increase in the early 2000s likely resulting from removal of cats and cessation of the myxoma program. Vegetation recovery was observed during the 1980s and 1990s following the damage caused by high rabbit populations in the 1970s on both steep coastal slopes and on other grassland sites (coastal terraces, plateau and inland valleys). Vegetation change consistent with heavy grazing pressure was subsequently observed in the recent period of high rabbit numbers which peaked around 2006.

A rabbit skull in 2013 is a reminder of the now-extinct feral species which thrived on Macquarie Island.
A rabbit skull in 2013 is a reminder of the now-extinct feral species which thrived on Macquarie Island.

After more than 130 years of running rampant in the subantarctic ecosystem, the story of rabbits on Macquarie Island ends in 2011 with the successful efforts of the Macquarie Island Pest Eradication Project. So we would expect the vegetation to now respond to the absence of grazing…