The effects of vertebrate herbivore grazing on the alpine vegetation of the Eastern Central Plateau, Tasmania

This thesis examines the relative influences of domestic stock, rabbits and native vertebrate herbivores on alpine and treeless subalpine vegetation on the Eastern Central Plateau Tasmania, with special reference to tall alpine herbs. Results from 25 year old grazing exclosures on Liawenee Moor indicated that domestic stock had a much greater impact on the vegetation than rabbits and native vertebrate herbivores. Vegetation cover was greatest and structure was most complex in the ungrazed exclosure, whilst vegetation cover was least in the sheep, rabbit and marsupial grazed control area. Aerial photographic analysis showed that, over a 19 year period, regeneration in these exclosures was most successful in the ungrazed exclosure, whilst bare ground increased in the sheep grazed area. Most plant species that were present in the grazing exclosures were also present in sheep grazed areas. However, sheep grazing had a dramatic impact on flowering success of many tall alpine herbs, with significantly more herbs flowering in the rabbit and native vertebrate herbivore grazed plot and the ungrazed exclosure than the sheep grazed area. A few tall herbs were tolerant of grazing by rabbits and native herbivores as there was no significant difference in flowering success between the two exclosures (the ungrazed exclosure and the rabbit plus native herbivore grazed exclosure). Vegetation cover in 23 year old plots at Liawenee and Augusta tended to increase at approximately 1% per year, whether grazed by domestic stock (Liawenee) or not (Augusta). Regeneration at Liawenee was dominated by shrubs and grasses. Shrubs were the most important recolonisers of bare ground at Augusta. Regeneration occurred at a slightly faster pace at Augusta, despite its higher altitude. At other sites, where domestic stock grazing ceased 6 years before the study commenced, the average annual increment in vegetation cover was also approximately 1%. The greatest increase in vegetation cover was found in the ungrazed exclosures, followed by the rabbit grazed exclosures, whilst the rabbit plus native vertebrate herbivore grazed controls had the lowest increase in vegetation cover. Flowering success at these sites was greater for some tall herb species in the ungrazed exclosures, but differences were less marked than at Liawenee. There were very few differences in flowering success between rabbit grazed exclosures and those areas that were open to rabbits and native vertebrate herbivores, indicating that rabbits may have a larger impact on flowering success than native herbivores. Naturally ungrazed areas, small islands in a fast flowing stream, were dominated by tall alpine herbs and palatable grasses, which were non-existent or sparse on the native vertebrate herbivore grazed banks. Tall herbs were dominant on the upstream ends of these islands, which experienced physical disturbance from fluctuating stream levels. These islands had very little bare ground other than that created by stream erosion. A major implication of this study is that the continuation of domestic stock grazing in treeless subalpine environments will contribute significantly to the deterioration of the landscape through a decrease in vegetation cover in exposed sites, a reduction in the structural diversity of the vegetation, a loss or reduction of some palatable plant species (mainly tall alpine herbs), the reduction of flowering of some tall herbs, and the maintenance of bare ground patches. Sheep plus rabbits and marsupial grazers have a much greater impact than rabbits and natives alone. Rabbits in their present numbers, may be considered to be an additive effect to native vertebrate herbivore grazing as recovery is slower under combined grazing (rabbits and native vertebrate herbivores) than under rabbit grazing alone. Rabbit grazing substantially affects the flowering success of some tall herb species. It is estimated that most of the Eastern Central Plateau will regenerate naturally within 50-80 years. Tall herbs are more prevalent where bare ground is less than 20%, but are not the dominant lifeform in areas that are grazed. Tall herbs may dominate in naturally ungrazed environments (islands) but only where physical disturbance of the ground has occurred.