Landscape Effects on Ecosystems: Birds as Active Vectors of Nutrient Transport to Fragmented Urban Forests Versus Forest-Dominated Landscapes

Abstract

In urban landscapes, nearby birds contribute to allochthonous nutrient flow from residential areas to fragmented forests by consuming food in residential areas and depositing feces in forests. To estimate allochthonous nutrient flow qualitatively, the stable nitrogen (δ15N) and carbon (δ13C) isotope approach is useful. However, the quantitative allochthonous flow rate cannot be estimated by the stable-isotope approach. To quantify allochthonous input, we compare two different landscapes. We assume that the input rate in deep forests in the forest-dominated landscape is the basic autochthonous flow that is common to various forests, and estimate the allochthonous nutrient input by subtracting the autochthonous flow from the total input in urban forests. The observed nutrient-input rate in the forest-dominated landscape as the autochthonous flow is 0.0298 kg P ha−1 y−1 and 0.319 kg N ha−1 y−1. Using these values, the allochthonous P input (kg P ha−1 y−1) is estimated at 0.0307 in urban fragmented forests and 2.31 in forests with crow roosts, whereas N input (kg N ha−1 y−1) is 0.397 in urban fragmented forests and 23.2 in forests with crow roosts. Our estimation shows that, in urban forests with roosts, birds contribute 2.7 times the amount of allochthonous P contributed via other pathways, and in urban forests without roosts they contribute 0.035 times the amount; and in addition, birds contribute 0.66 times the amount of allochthonous N input via other pathways in urban forests with crow roosts and 0.011 times the amount in urban forests without roosts. We also measure stable nitrogen and carbon isotope ratios, %N, %P, and %C to estimate the diet of birds. High δ15N and δ13C values in crow roosts indicate that they eat foods such as livestock meat, C4 maize, or fish. High avian biomass is the major reason for the large nutrient input in urban landscapes, especially in urban forests with crow roosts.