Mutualism and Antagonism in a Post-Extinction Hawaiian Bird-Lobelioid Pollination System.

Extinction of specialized pollinators can alter the balance between mutualism and antagonism in plant-animal interactions, yet the mechanistic consequences of partner loss remain poorly resolved. We examined plant-bird interactions among three endangered Hawaiian lobelioids (Cyanea shipmanii, Clermontia pyrularia, and Clermontia lindseyana) and extant nectarivorous birds in montane forests on Hawai'i Island. These plants were likely pollinated historically by now-extinct long-billed Hawaiian honeycreepers, creating a system in which contemporary bird-plant interactions may differ from historical pollination dynamics. Using motion-triggered cameras, we quantified floral visitation by 'I'iwi (Drepanis coccinea) and Hawai'i 'Amakihi (Chlorodrepanis virens), evaluated whether nectar acquisition was coupled with contact with fertile floral structures, and measured the frequency and form of nectar robbing. To test the consequences of nectar robbing, we experimentally simulated corolla piercing and flower splitting using a 3D-printed 'Amakihi bill model, quantified subsequent nectar replenishment, and evaluated fruit development and seed viability. Contact with fertile floral structures (anthers or stigma) occurred in 43% of floral visits overall. Nectar robbing occurred in 39% of all visits and was more frequently performed by 'Amakihi. Experimental damage significantly altered subsequent nectar replenishment, demonstrating that nectar robbing modifies floral resource dynamics beyond immediate removal. Despite frequent antagonistic behaviors and altered nectar dynamics, fruit production and seed viability remained high across treatments. These findings indicate that co-occurring nectarivores perform different functional roles that alter floral resource dynamics and plant reproductive outcomes. In this post-extinction system, mutualistic and antagonistic processes operate simultaneously, underscoring the importance of mechanistic evaluation when assessing the persistence of species interactions under biodiversity loss.