Butterfly and moth habitat specialisation changes along an elevational gradient of tropical forests on Mount Cameroon

Niche breadth, reflecting the range of environmental conditions or resources a species can exploit, influences its distribution, persistence, vulnerability to environmental change, and interspecific interactions. The elevational niche‐breadth hypothesis predicts broader ecological niches at higher elevations due to increased environmental stress and heterogeneity, yet empirical tests have focused on food specialisation, leaving habitat specialisation largely overlooked. Here, we provide the first test of this hypothesis using habitat‐related niches quantified by 27 characteristics of forest structure and canopy openness, analysing fruit‐feeding butterflies and moths along a full tropical forest gradient (lowland to timberline) on Mount Cameroon. Habitat‐related niche breadth was estimated using the outlying mean index multivariate ordination across 96 forest plots spanning 350–2200 m a.s.l., employing two complementary metrics: absolute niche breadth (ANB) based on the total range of habitat conditions occupied across the gradient, and within‐elevation niche breadth (WENB) reflecting the proportion of local heterogeneity utilized at each elevation. Forest structural heterogeneity increased steeply with elevation. Butterflies showed a substantial upslope broadening of ANB, particularly in two (Charaxinae and Satyrinae) of the three abundant subfamilies. All three abundant moth families (Erebidae, Noctuidae and Geometridae) displayed a similar, though weaker, pattern to butterflies, although full moth communities showed no clear elevational trend in ANB. These results support the elevational niche‐breadth hypothesis for habitat specialisation in some tropical butterflies and moths, likely reflecting increased habitat opportunity space and reduced competition at higher elevations. In contrast, WENB showed no consistent elevational patterns, indicating that broader availability of environmental conditions does not necessarily translate into more generalised local habitat use and may be constrained by behavioural preferences, resource distributions, or biotic interactions. By disentangling overall and local habitat niche breadths, our study reveals unrecognised complexity in elevational specialisation and provides new insights into how tropical insects respond to environmental gradients.