Long-term vegetation changes in Otari-Wilton's bush following possum control

Abstract

Since being introduced into New Zealand in the 1830s, the brush-tailed possum (Trichosurus vulpecula) has caused widespread damage to the native flora. Here, I investigate long-term trends in Otari-Wilton's bush. Otari-Wilton's bush has been browsed by possums for approximately a century. I specifically investigate how this heavily browsed plant community has responded since the introduction of possum control in 1993. Measurements were made on six permanent plots set up around the forest, which were censused three times, twice before possum control (1934 & 1992), and once after possum control (2004). Plant densities were calculated for common species, and the individuals were assigned into three height classes. Of the 18 species analysed, 15 decreased in stand density before possum control. This trend was reversed with 14 species showing an increase in stand density after the possums were controlled. The total number of plants recorded in the plots after possum control has more than doubled, compared with the plants recorded before possum control. Most species followed the trend of decreasing in density before possums were controlled, and increased in density after possum control. The trend was reflected in all three height classes, with seedlings usually showing the biggest increase after possum control. This decline and rebound corresponded with the feeding preference shown by the possum in its diet, with highly preferred species like Alectryon excelsus, Melicytus ramiflorus and Dysoxylum spectabile showing the greatest increase in density after possum control. I conclude that possum browsing significantly reduced the densities of several plant species, and their control has resulted in substantial improvements in plant numbers.