Abstract: Elevational gradients provide a unique opportunity to study the plasticity of plant-pollinator interactions, which is crucial for understanding ecological and evolutionary processes in plant pollination systems. Species-specific dispersal across elevation gradients of tropical mountains is constrained by the different tolerance of individual species to abiotic factors. Consequently, the composition of plant and pollinator communities, such as their interactions, changes continuously. For example, previous studies have shown a bee-to-fly transition as elevation increases, or that at high elevations, bird-pollinated plants may be more effectively pollinated than closely related bee-pollinated species, highlighting an altitude-driven bee-to-bird transition. We used Hypericum revolutum (Hypericaceae) as model plant, to explore how the identity and activity of floral visitors change along an elevational gradient in the montane grasslands of Mount Cameroon. We observed flower visitors across four elevations during two seasons. Our study confirmed the predicted bee-to-fly transition with increasing elevation. Bird activity followed a hump-shaped pattern, peaking around 2,800 meters above sea level. Male Cinnyris reichenowi individuals, the main bird floral visitor, exhibited higher activity than females throughout the entire elevational gradient and across both study periods. The observed patterns suggest that plants may face evolutionary pressures to adapt to these shifting pollinator communities, potentially driving local adaptations and diversification within populations.

Key words: Afrotropics, elevational gradients, floral visitor shifts, montane grasslands, Nectariniidae, nectarivory, pollination.