Ecology of the Pembrokeshire Islands: V. Manuring by the Colonial Seabirds and Mammals, with a Note on Seed Distribution by Gulls

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Abstract
Nutrients of seabird guano had marked effect on vegetation of nesting colonies. Where excessive, all macroscopic vegetation was eliminated, where present in moderate amounts, rank growth was induced in some spp. but this was usually depressed by grazing, treading and burrowing. Halophytes were more tolerant of high concentrations of guano than were mesophytes. Shearwaters defaecated largely within burrows, puffins at burrow entrance, in a localized area within or on a communal "standing ground" outside. Soil nitrate was about 30 times more than normal in rabbit colonies, 100 times in bird colonies, half of this being leached out during 6 weeks following birds'' departure. P2O5 was increased from 0.02% to 0.7% by guano, exchangeable calcium and carbonates were more closely related to soil organic matter. Both guano and sea salt increased soil alkalinity when acting alone, guano on spray-washed cliffs had no effect on pH, guano on salt marshes increased soil acidity. Organic matter of guano had low water-retaining capacity. Flora of bird colonies (angiosperms, algae, bryophytes, lichens and fungi) is descr. Arable weed seeds and cereal grain transported to islands by gulls germinated on cliffs.