Species richness can decrease with altitude but not with habitat diversity

Abstract

In the paper by Allouche et al. (1), the authors suggested that species richness decreases at high levels of habitat diversity because the area available per habitat decreases [area–heterogeneity tradeoff hypothesis (AHTO)]. They showed a hump-shaped relationship between Catalonian bird richness and altitudinal range in grid cells, the authors’ surrogate for environmental heterogeneity. However, birds select habitats mainly based on vegetation structure and floristic composition (2). Catalonian high altitudes are dominated by uniform coniferous forests or simple habitats with low vegetation cover (outcrops, grasslands, and scrublands) that are known to be poor in bird richness. Furthermore, high-altitude grid cells have the largest altitudinal ranges but much fewer habitats than lower altitudes (using 48 habitat categories obtained from Inventario Nacional Forestal III, 2007–2008, Spanish Ministerio de Medio Ambiente). Elevation range in Catalonia is tightly correlated with maximum altitude (r = 0.951; P < 0.0001) and mean elevation (r = 0.858; P < 0.0001), but poorly correlated with environmental heterogeneity (number of habitats per cell: r = 0.049, P = 0.349; Shannon index across all habitat categories: r = 0.119, P = 0.024). When analyzed together (Generalized Additive Model; Fig. 1), bird-species richness shows a hump-shaped relationship with mean elevation [nonlinear P (n-Lp) = 0.003], a negative linear relationship with altitudinal range (P = 0.001; n-Lp = 0.426), and a positive, monotonic relationship with habitat diversity (P < 0.001; n-Lp = 0.336), as predicted from ecological theory (3). Thus, the unimodal relationship between altitudinal range and richness (1) merely reflects the well-known hump-shaped relationship between species richness and altitude (4), not a tradeoff between richness and environmental heterogeneity.