Seabird guano influences on desert islands: soil chemistry and herbaceous species richness and productivity

An understanding of the effects of guano deposition on arid soil chemistry and the consequences for plant communities is lacking. This study examined patterns of herbaceous species richness and productivity, soil chemistry, soil moisture and soil respiration on 11 islands in the Gulf of California, six of which receive seabird guano deposition. Species richness was significantly lower on islands with guano (“Bird” islands) than islands without guano (“Non-bird” islands), with very little overlap in species composition; however, productivity was significantly greater on Bird than on Non-bird islands. As expected, Bird island soils had higher concentrations of NO3−, NH4+ and total nitrogen (N) than Non-bird island soils; and, measurements of δ15N indicate that the higher concentrations of N were derived from guano. We also found that soil moisture and respiration were significantly higher, but pH was significantly lower, on Bird than Non-bird islands. These results suggest that guano deposition in deserts stimulates productivity-even in dry years-due to elevated N and, indirectly, soil moisture. Guano deposition also results in a decrease in species richness and a change in species composition probably due to elevated N, N toxicity, or low pH. However, we also found that pH varied more on Bird than on Non-bird islands; and that salinity-while not different between island types-was significantly patchier on Bird than on Non-Bird islands. These results suggest that guano deposition affects not only the general chemical composition of soils, but also results in greater spatial variation in soil chemical composition, which may ultimately affect species richness and composition. Therefore, understanding spatial patterning in soil chemistry as a result of guano deposition is critical for understanding guano effects on plant richness and productivity.