Nutrient input by seabirds to the forest on a coral island of the Great Barrier Reef

The rate of deposition of droppings from WhiteCapped Noddies Anous minutus Boie, the principal seabird nesting and roosting in the trees of Pisonia grandis R. Br. on Heron Island, Great Barrier Reef, was measured 3 times during 1983. Deposition at ground level was extremely variable, partly because much of the material was intercepted by foliage. An average rate of deposition was about 2 g m-2 d-' (dry matter), equivalent to a total input of about 45 tonnes yr-' to the soil under the P. grandis canopy of the island. This represents the transfer of about 1.4 tonnes yr-' of phosphorus from the surrounding waters to the island's soil where P may become stabilised in insoluble form. Coral islands are largely dependent on the surrounding regions of the sea for influx of plant nutrients, mostly deposited by roosting and nesting seabirds. Such nutrients are crucial for the plants on coral cays, which often maintain a stable vegetation, in turn providing suitable habitats and shelter for many bird and other species. Nutrient contents of guano deposits are known for a large number of localities (e.g. Hutchinson, 1950) including some Great Barrier Reef (GBR) islands where phosphate has been mined (cf. Cribb, 1979). However, little is known about the rate of input of plant nutrients by birds to islands of the GBR (cf. Heatwole et al., 1981). This report presents measurements of the input of nitrogen, phosphorus and other elements under the forest of Pisonia grandis R. Br. (Nyctaginaceae), the major plant community on Heron Island (23'27' S, 151'55' E), a coral cay about 16 ha in area. This species is closely associated with colonies of seabirds, having hooked sticky anthocarps leading to dispersal of seeds by birds, and providing nest-sites and nesting material. On Heron Island the most abundant bird nesting and roosting in the P. grandis trees is the White-Capped Noddy Anous rninutus Boie: there were about 70,000 adults in January 1983, and 10,000 young were raised to fledging (K. Hulsman, pers. comm.). On this island Noddies nest principally in P. grandis (cf. Shipway, 1969). Droppings-catchers, polyethylene bags 30 X 45 cm, were pegged out for 2 to 3 d on the ground under closed Pisonia grandis canopy. In January and SepQ Inter-Research/Printed in F. R. Germany tember, 10 bags were set out in a transect at 3 m intervals in the central P. grandis closed-forest (cf. Specht, 1981) to give an unbiased selection of sampling sites. Because this may not have sampled the full range of variation, 10 additional catchers were placed in pairs so that one was in a place heavily contaminated with droppings and the other was in a clean place under the same tree; the 5 trees chosen were widely separated over the island from the edge to the centre and from closed-scrub to closed-forest. The paired catchers were set out in January, May and September. Catchers were visited night and morning to remove leaf litter and debris cast over them by foraging Banded Land-Rails Rallus philippensis L. and burrowing Wedge-Tailed Sheanvaters Puffinus pacificus (Gmelin). No rain fell during any of the periods, and A. minutus was always the principal species in the trees above all catchers. The catch was dried at 50°C, scraped from the bags, and samples were analysed for carbon, hydrogen and nitrogen with a Heraeus CHN-Rapid analyser. Eight of the largest samples were analysed for phosphorus by the molybdenum blue method (Allen, 1974) and for cations by atomic-absorption spectrophotometry. The area covered by P. grandis was measured by tracing round the canopy of every tree on a map from January 1983 air-photographs. The catch (expressed as dry-mass of material d-' m-' of ground area) was abundant at all 3 times of year (Table 1). The material consisted principally of bird droppings, but included some feathers, small pieces of plant material, coral sand and other matter. There was extreme variation between individual catchers. Undoubtedly much material is intercepted by leaves and stems in the canopy of Pisonia grandis, and the density of the foliage as well as number and activity of birds vary from time to time and place to place. Material caught on foliage may b e liberated as flakes during windy conditions or washed to the ground during rain. No attempt was made in this study to estimate 298 Mar. Ecol. Prog. Ser. 19: 297-298, 1984 31 Jan to 30 May to 3 to 5 Sep 3 Feb 1983 2 Jun 1983 1983 Transect (n = 10) 2.2f 1.2 n. d . 1.7k0. 8 (Range) (0.002-1 1.6) (0.001-7.0)