Burrowing seabirds and reptiles: impacts on seeds, seedlings and soils in an island forest in New Zealand

Stephens Island (Cook Strait, New Zealand) is home to large populations of fairy prions (Pachyptila turtur) and tuatara (Sphenodon punctatus, an ancie nt reptile), which share burrows in the ground. It has been assumed that gu ano deposition by seabirds increases nutrient availability to plants, and t hat large populations of the carnivorous tuatara are the result of flow-thr ough effects from plants to invertebrate herbivores. We examined the within -island scale effects of seabirds and tuatara on for est soils and vegetati on along a gradient of burrow densities. High burrow densities were correla ted with extremely low soil pH (down to 3.4), very high soil P (up to 3.9 m g P g(-1)), and high litter deposition but low ground litter. Seedling numb ers declined marginally as burrow numbers increased. Seedling emergence rat es from field soils, and germination and growth of a phytometer species als o decreased in soils from areas with high burrow densities. In contrast, so il available N (NO3- and NH4+) was high everywhere and did not change with burrow density. Furthermore, of three common shrub species examined (Macrop iper excelsum, Coprosma repens and Melicytus ramiflorus), only one (Macropi per excelsum) showed an increase in leaf N concentration with increased bur row density, and only one (C. repens) showed increased herbivory (number of holes per leaf and area per leaf removed) with increased burrow density. B irds descending through trees may increase litter loss. There is little evi dence for changes in herbivory or nutrient availability to herbivores along burrow density gradients. We suggest that, although overall nutrient avail ability may be increased by the presence of these animals, very high densit ies have negative effects on seedling populations.