Forest edge encroachment by rural orchards shifts bird communities in favor of understory birds: Forest birds as indicators of landscape changes in agroecosystems

Land use change is a critical factor affecting the structure and functioning of biological communities, particularly forest birds. The Hyrcanian forests in northern Iran are undergoing significant transformations due to increasing agricultural expansion and the growth of orchards encroaching upon forest edges. While research often suggests that the edge effect can enhance species diversity in heterogeneous agricultural-forest landscapes, the effects of different agricultural practices have received less attention. In this study, we integrated remote sensing, landscape ecology, and statistical analysis, and served bird communities as ecological indicators aiming to identify the impact of various land use modification on biological communities of an agricultural-forest landscape. We used Sentinel-2 images to generate a land use map of the region based on the random forest classification method. Explanatory variables extracted from the derived land use were measured in 75 cell grids (1200 x 1200 m) and bird information in 400 x 400-meter units. We calculated 15 landscape metrics for each 1200-meter landscape unit cell and combined them with residential area and road length data for statistical analysis. The Redundancy Analysis (RDA) method was employed to examine the effects of environmental variables on bird community composition. We identified a total of 38 breeding bird species belonging to 21 families in the region. The RDA variance partitioning accounted for 0.554 as the constrained fraction, indicating that 54 % of the bird community variance was explained by the explanatory variables. Generalist and urban-dwelling birds were mainly dispersed across negative values of canonical axis 1 (RDA1). Conversely, forest specialist birds were scattered along positive values of RDA1. Less specialized understory birds, depending on more open woods with higher species diversity, were mostly illustrated with shorter projections across RDA2 extremes, correlated with orchards. Our findings demonstrate that the marginal habitats of Hyrcanian forests, predominantly consisting of orchard plantations, exhibit greater bird community diversity compared to dense forests or croplands. Although species diversity has been observed to increase initially, the ongoing trend of orchard encroachment on forest patches may result in heightened edge effects, an increased presence of generalist species, and a decline in specialized forest species. These results can help guide land use planning to achieve biodiversity goals in heterogeneous agricultural-forest landscapes.