Cliff nesting birds provide nutrient inputs to cliff ecosystems

Authors: LANGEVIN, ANGELA - Appalachian State University; BOGGESS, LAURA - North Carolina Agricultural And Technical State University; Harrison, Georgia; MADRITCH, MICHAEL - Appalachian State University

Submitted to: Basic and Applied Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/9/2024
Publication Date: 6/25/2024
Citation: Langevin, A.E., Boggess, L.M., Harrison, G.R., Madritch, M.D. 2024. Cliff nesting birds provide nutrient inputs to cliff ecosystems. Basic and Applied Ecology. 79:74-83. https://doi.org/10.1016/j.baae.2024.06.001.
DOI: https://doi.org/10.1016/j.baae.2024.06.001

Interpretive Summary: In this research, scientists explored how birds nesting on cliffs, like peregrine falcons and common ravens, contribute nitrogen to the plants growing on those cliffs. Cliffs are usually low in nutrients and mainly get nitrogen from the air and runoff. The study found that the bird nests add nitrogen-rich guano to the cliffs, significantly affecting the nutrient levels on the cliff face. Cliffs below bird nests had higher levels of ammonium, a form of nitrogen, compared to cliffs without nests. The study revealed that both ammonium and nitrate influenced the types of vegetation growing under nests. Two lichen species were identified as indicators of nitrogen below nests. This shows that bird droppings provide an important nutrient boost to cliffs, connecting them to more fertile ecosystems. The study highlights the importance of considering birds in the conservation and management of cliffs.

Technical 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.