Community characteristics of forest understory birds along an elevational gradient in the Horn of Africa: A multi-year baseline

Abstract

Tropical mountains are global hotspots for birdlife. However, there is a dearth of baseline avifaunal data along elevational gradients, particularly in Africa, limiting our ability to observe and assess changes over time in tropical montane avian communities. In this study, we undertook a multi-year assessment of understory birds along a 1,750 m elevational gradient (1,430–3,186 m) in an Afrotropical moist evergreen montane forest within Ethiopia’s Bale Mountains. Analyzing 6 years of systematic bird-banding data from 5 sites, we describe the patterns of species richness, abundance, community composition, and demographic rates over space and time. We found bimodal patterns in observed and estimated species richness across the elevational gradient (peaking at 1,430 and 2,388 m), although no sites reached asymptotic species richness throughout the study. Species turnover was high across the gradient, though forested sites at mid-elevations resembled each other in species composition. We found significant variation across sites in bird abundance in some of the dietary and habitat guilds. However, we did not find any significant trends in species richness or guild abundances over time. For the majority of analyzed species, capture rates did not change over time and there were no changes in species’ mean elevations. Population growth rates, recruitment rates, and apparent survival rates averaged 1.02, 0.52, and 0.51 respectively, and there were no elevational patterns in demographic rates. This study establishes a multi-year baseline for Afrotropical birds along an elevational gradient in an under-studied international biodiversity hotspot. These data will be critical in assessing the long-term responses of tropical montane birdlife to climate change and habitat degradation.

Building a mechanistic understanding of climate-driven elevational shifts in birds

“…Since this landmark work, the number of studies documenting upslope elevational shifts in tropical birds has remained relatively low. This paucity of research likely reflects a lack of carefully standardised and available baseline or time-series data with which to calculate trends over time [59,60]. Many attempts to quantify tropical elevational shifts have had to make use of less traditional data sources.…”

“…At medium scales, the maturation of monitoring schemes will provide longer-term perspectives on elevational ranges including both trends and interannual variation [67,69] while accounting for imperfect detectability [98,221], especially when paired with data on other taxa. Mist-netting studies could be used to monitor the demographic rates of populations as a function of elevation [59]. Perhaps we would find that upslope expansions are driven by higher recruitment rates at higher elevations or that lower survival rates at lower limits are causing range contractions.…”

Elevational patterns of species richness and community structure of understorey birds in an East African montane forest

“…Additionally, due to high rates of forest degradation in tropical regions (Laurance et al., 2011), biodiversity is severely threatened (Eglington & Pearce‐Higgins, 2012; Vieira et al., 2008). Besides temperature, a variety of other factors including precipitation (Abebe et al., 2019; Santillán et al., 2018), vegetation cover (Forero‐Medina et al., 2011; Jankowski et al., 2013; Kittelberger et al., 2021; Santillán et al., 2018), habitat quality (Forero‐Medina et al., 2011; Freeman et al., 2018), resource availability (Forero‐Medina et al., 2011; Santillán et al., 2018), interspecific competition (Jankowski et al., 2013; Lenoir et al., 2010) and evolutionary history (Santillán et al., 2018) may also exert strong influence on the distributional limits and diversity of birds along elevational gradients. Montane ecosystems are important because they are viewed as a potential future refugia for species displaced by a changing climate, while at the same time supporting habitat types considered at highest risk of decline due to global warming (Dillon & Conway, 2021).…”