Cliff nesting birds provide nutrient inputs to cliff ecosystems

Abstract

Allochthonous inputs often link nutrient cycles between different ecosystems as nitrogen (N) that originates in one ecosystem is transferred to another. Mobile animals such as birds and fish can translocate nutrients between locations. Consequently, animal-mediated nutrient subsidies can shape plant communities, especially in nutrient-poor ecosystems. Terrestrial cliffs are typically nutrient-poor, receiving N from atmospheric deposition and cliff-top runoff. Cliff nesting birds deposit N-rich excrement onto the cliff during nesting, yet this potential nutrient input has rarely been demonstrated. Here we document an allochthonous input of N from cliff-nesting birds to terrestrial cliff vegetation. We sampled mineral N and vegetation on eight cliff systems in western North Carolina (USA) with nests from peregrine falcon (F. peregrinus; two sites) and common raven (Corvus corax; five sites). Two sites had nests but species identity was not confirmed. Cliff faces below bird nests had higher levels of ammonium (NH₄⁺) than did faces without nests. Both ammonium and nitrate (NO₃⁻) explained variability in cliff vegetation community composition, but site-level characteristics explained more of the variation than did N inputs alone. Lichens, the dominant taxonomic group, contributed the most to this variation and had higher species richness and diversity below nests. Indicator species analysis identified two nitrophilic lichen species below nests: Flavoplaca citrina and Physcia caesia. A third nitrophilic lichen, Polycauliona candelaria (syn. Xanthoria candelaria) was also found under nests. Bird excrement is likely a significant subsidy to nutrient-poor terrestrial cliff ecosystems, thus linking cliffs to more productive ecosystems and highlighting the need to include birds in conservation and management of cliffs.