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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, highaltitude 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 …