Forest structure determines spatial changes in avian communities along an elevational gradient in tropical Africa

Abstract

Aim To test if tree species richness and forest structure drive spatial variation in avian communities along a tropical elevation gradient and to present information about the role of detailed forest parameters. Location A 2,000‐m long elevational gradient of tropical forest on Mt. Cameroon, west‐central Africa. Taxon Birds and trees. Methods We performed bird censuses and vegetation mapping at the same plots across six forested sites at elevations of 350, 650, 1,100, 1,500, 1,850, and 2,200 m a.s.l., with 16 plots per elevation. We tested the effects of elevation, forest structure and tree diversity on the species richness, functional diversity and β‐diversity of birds (Bray–Curtis dissimilarity). We used conditional inference trees based on random forests (RF) to investigate these relationships across all elevation sites as well as within elevations. Results Both tree and bird species richness declined monotonically with elevation. Vegetation structure correlated with elevation, and all vegetation attributes significantly differed among elevations. The RF explained 70% of the variance in avian species richness, with the most important predictors being elevation, proportion of dead trees, tree species richness and herb layer coverage. We found that elevation (and shrub layer) was a particularly important predictor of avian functional diversity. We identified no important predictor of bird species richness after standardization within elevations, and the proportion of dead trees was the sole important predictor of functional diversity. Within‐elevation β‐diversity in avian community composition was determined by the dissimilarity of the tree community and differences in leaf area index, solar radiation and spatial distance. The functional dissimilarity was best explained by leaf area index. Main conclusions Apart from elevation itself, spatial distance even within elevations correlated with compositional and functional variation among avian assemblages. Forest structural traits can have a significant influence on distribution of birds. Thus, gaps in the spatial distribution of species such as along elevations might be caused by fine‐scale recognition of suitable habitats.

Climate as a major driver of avian diversity in riparian Amazonian habitats along an environmental gradient

“…Importantly, these variables likely affect biological communities in different ways and have a key function determining vegetation structure (ter Steege et al, 2003), which provides specific habitats for the biota, including birds (Hořák et al, 2019).…”

“…Average species frequency (a and b) and median biomass (c and d) decrease along the Rio Branco and its major tributaries, with mean annual precipitation, lower average annual temperatures and with lower evapotranspiration. Points represent our surveyed localities (n = 16); grey areas refer to confidence intervals, and black lines denote significant relationships [Colour figure can be viewed at wileyonlinelibrary.com] has been shown in other tropical forests (Hořák et al, 2019). Also, among the most important factors defining structural complexity and productivity of alluvial vegetation, are the physicochemical parameters of river water (e.g.…”

Effects of disturbances by forest elephants on diversity of trees and insects in tropical rainforests on Mount Cameroon

“…Moreover, the communities of all insect groups were highly homogeneous in both forest types in this high elevation. The montane forests on Mount Cameroon are already relatively open and with limited tree diversity 40 that additional disturbances by elephants could hardly increase habitat heterogeneity even for butterflies. Moreover, some tree dominants in the montane forests, such as Schefflera abyssinica and S. mannii , are (semi)deciduous during the dry season which generally open the higher canopy even in the undisturbed forests.…”

“…At each of four sampling sites, eight circular plots (20 m radius, ~ 150 m from each other) were established in high canopy forests (although sparse in the undisturbed sites), any larger clearings were avoided. In the disturbed forest sites, the plots were previously used for a study of elevational diversity patterns 40 , 42 . In the undisturbed forest sites, plots were established specifically for this study.…”

“…To assess the tree diversity in both disturbed and undisturbed forest plots, all living and dead trees with diameter at breast height (DBH, 1.3 m) ≥ 10 cm were identified to (morpho)species (see 40 for details). To study impact of elephant disturbances on forest structure, each plot was characterized by twelve descriptors.…”