Four years without bunnies

Walking is getting harder on Macquarie Island. Previously short-cropped grass only centimetres tall is now a knee-deep meadow of grass matted with mosses and herbs. This is one of the initial observations from last week’s field trip to the Subantarctic island.

Short grassland at Green Gorge study site has more than doubled in height since rabbits were removed.
Short grassland at Green Gorge study site has more than doubled in height since rabbits were removed. The previous day this was under snow but after overnight rain it became possible to do a vegetation survey.

Another obvious change is a shift in the dominant species. Under the rampant rabbit regime, one grass species in particular flourished – Agrostis magellanica. Now other native grasses which were present in lower numbers are now prominent, even dominating sites where rabbits had previously kept them in check. The mat-forming herb Acaena which thrived in heavily grazed grasslands is struggling to compete with the vigorous growth of grasses and sedges.

Many of the smaller herbs which favour bare ground and occupy openings amongst the grasses appear to be less common. In the absence of rabbit diggings and overgrazing they are now abundant only on creek banks and landslides, which is perhaps their original niche in a pre-grazing era.

Boggy short grassland at Bauer Bay on the west coast. The Macquarie Island cabbage plants which have appeared here since rabbits were removed have the potential to reach over 1 metre in height.
Boggy short grassland and herbfield at Bauer Bay on the west coast. The Macquarie Island cabbage plants which have appeared here since rabbits were removed have the potential to reach over 1 metre in height. Note the tussocks on the slopes in the background, most of which have appeared in the past four years.

Tussock grass (Poa foliosa) is clearly increasing in abundance and in height, signalling a future shift in vegetation type from short grassland to tall tussock grassland. And the Macquarie Island cabbage, which was virtually eliminated from much of the island by rabbits, is popping up as young plants throughout many grasslands. In a few years time it is likely to outgrow the short grasses and take over. The tall tussock grass and Stilbocarpa vegetation described in Nineteenth Century accounts might be making a comeback. The return of the megaherbs seems imminent.

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