Seabirds Enhance Primary Producer and Consumer Isotope Signals on a Sub-Tropical Island

Abstract

Seabirds are long-range transporters of nutrients, linking their marine feeding grounds with their terrestrial breeding and roosting sites. Seabirds can influence the terrestrial environment in which they reside by depositing nutrient-rich guano, which acts as a natural fertiliser. Here, we determined the nutrient content of Sable Shearwater (Ardenna carneipes) guano and used stable isotope analysis to determine changes in isotope signals and nutrient concentrations through the terrestrial environment across three sites on Lord Howe Island, Australia: active shearwater colony, abandoned shearwater colony and un-colonised area. The concentration of nitrogen in Sable Shearwater guano was like other Procellariiformes, and this was reflected in the palm leaves and invertebrates (slugs) sampled from seabird colonies, which were enriched in δ15N relative to the control site. In contrast, nitrogen stable isotope values in soils were not significantly different among sites, indicating the presence of historic seabird colonies. Guano was rich in phosphorous and potassium, which fertilised soils in the shearwater colony compared to sites without seabirds present. It is expected that the island will experience a reduction in nutrient inputs from guano because the shearwater population is in decline, and this may result in changes to vegetation assemblages in the colonies over time.

FIGURE 1

Map of the six Sable Shearwater (Ardenna carneipes) colonies on Lord Howe Island (Hunter Bay, Ned's Beach, Steven's Point, Middle Beach, Clear Place, Little Muttonbird Ground). Colony extents, as of 2018 (Lavers et al. 2019), are outlined in black. The three sampling sites (an active shearwater colony, ‘current colony’, an abandoned colony, ‘abandoned colony’ and an area that has never hosted a shearwater colony, ‘never colonised’) are indicated by orange circles. The location of Lord Howe Island is indicated by a black ‘X’ in the inset map.

FIGURE 2

The mean (±SD) stable carbon (δ ¹³C ‰) and nitrogen (δ ¹⁵N ‰) isotope values from Sable Shearwater adult and chick guano, kentia palm leaves, leopard slugs and soil samples collected from three areas on Lord Howe Island, Australia: a current shearwater colony (CC), an abandoned shearwater colony (AC) and an area that has never been colonised by shearwaters (NC).

FIGURE 3

Concentrations (mean ± SD) of N, P and K (all %) in soil, kentia leaf and leopard slug samples from three sites on Lord Howe Island: an active Sable Shearwater colony (CC), an abandoned colony (AC; abandoned within the last 10–15 years) and an area that has never hosted a shearwater colony (NC). Bars are grouped by sample type and colored by site. See Table S4 for exact mean ± SD and test statistics.

FIGURE 4

A principal component analysis (PCA) of nutrients and stable isotopes analysed in soil samples from a Sable Shearwater colony (CC; represented by triangles), an abandoned shearwater colony (AC; circles) and an area that has never had an active colony of shearwaters (NC; squares) on Lord Howe Island. Total N, P and K were reported as percentage (%), and δ ¹⁵N and δ ¹³C in permil (‰). Var., variance.