Seedling ratios: a simple method for assessing ungulate impacts on forest understories

Abstract

Introduced ungulates threaten the indigenous biota of remote oceanic islands such as Hawaii and New Zealand. The effectiveness of sustained animal-control programs to protect these forests is reduced by the lack of affordable and robust tools to monitor their outcomes. We developed a simple method (the seedling ratio index) to monitor forest understory condition. This index compared species richness of tall seedlings (30–200 cm) with that of short seedlings (<30 cm) for groups of species in different ruminant-feeding-preference classes (high, moderate, low). We assessed the method's utility by measuring seedling ratio indices at sites covering a wide range of forest types, ruminant species, and ruminant densities in Hawaii and New Zealand. The relationship between seedling ratio indices and pig (Sus scrofa) abundance in Hawaii also was investigated. Seedling ratio indices for high-preference plants were negatively correlated with ruminant abundance (r=−0.93). The regression equation fitted to these data successfully predicted changes in seedling ratio indices for high-preference species following managed reductions in ruminant densities. For low-preference species, seedling ratio indices remained near or above zero over the range of ungulate abundance examined. The method appears to provide a robust indicator of probable impact of ungulate browsing on forest understories, although other measures are required to assess the level of understory disturbance resulting from rooting by pigs.