Top down or bottom up? Comparing the impacts of introduced arboreal possums and ‘terrestrial’ ruminants on native forests in New Zealand
- G Nugent, W Fraser, PJ Sweetapple
Ungulates have been widely introduced to new locations, often increasing to high densities that impact on native plant communities. Himalayan tahr (Hemitragus jemlahicus) were introduced to New Zealand in 1904 and now occupy about 9600 km2 of the Southern Alps. Managers aim to control tahr to reduce impacts to native montane grasslands. We used a network of 111 permanent plots in eight catchments to estimate the long-term impacts of tahr on total vegetation cover and snow tussock (Chionochloa spp.) height. The proportion of sub-plots containing faecal pellets was used as a measure of tahr activity. Total vegetation cover increased during the study period but declined non-linearly with increasing tahr activity, with the most rapid decline occurring as tahr activity increased from low levels (i.e. a highly-vulnerable relationship). Tussock height declined weakly as a function of time during the study period, but declined strongly with increasing tahr activity (a proportionate relationship). A proportional effect of tahr activity on adult tussock height and a non-linear logarithmic effect of tahr activity on vegetation cover indicate that species other than tussocks were highly sensitive to tahr activity, even at very low levels. We conclude that tahr had significantly impacted total vegetation cover and tussock height during 1990–2013. Although vegetation cover appears to still be recovering from the high tahr densities that occurred prior to the 1970s, managers need to control tahr to lower levels, to further reduce their impacts on montane grasslands.