Stephens Island (Cook Strait, New Zealand) is home to large populations of fairy prions (Pachyptila turtur) and tuatara (Sphenodon punctatus, an ancient reptile), which share burrows in the ground. It has been assumed that guano deposition by seabirds increases nutrient availability to plants, and that large populations of the carnivorous tuatara are the result of flow-through effects from plants to invertebrate herbivores. We examined the within-island scale effects of seabirds and tuatara on forest soils and vegetation along a gradient of burrow densities. High burrow densities were correlated with extremely low soil pH (down to 3.4), very high soil P (up to 3.9 mg P g-1), and high litter deposition but low ground litter. Seedling numbers declined marginally as burrow numbers increased. Seedling emergence rates from field soils, and germination and growth of a phytometer species also decreased in soils from areas with high burrow densities. In contrast, soil available N (NO3- and NH4+) was high everywhere and did not change with burrow density. Furthermore, of three common shrub species examined (Macropiper excelsum, Coprosma repens and Melicytus ramiflorus), only one (Macropiper excelsum) showed an increase in leaf N concentration with increased burrow density, and only one (C. repens) showed increased herbivory (number of holes per leaf and area per leaf removed) with increased burrow density. Birds descending through trees may increase litter loss. There is little evidence for changes in herbivory or nutrient availability to herbivores along burrow density gradients. We suggest that, although overall nutrient availability may be increased by the presence of these animals, very high densities have negative effects on seedling populations.
Keywords: Burrowing seabirds; Guano deposition; Herbivory; Island restoration.