Infestation by pollination-disrupting alien ants varies temporally and spatially and is worsened by alien plant invasion

Abstract

Plant-pollinator mutualisms may be disrupted by alien ants. Such threats are likely to vary spatio-temporally and with other biological invasions, but these aspects are rarely studied. We examined a threatened island endemic scrambler (Roussea simplex, Rousseaceae) about a decade after its pollination by an endemic reptile was found to be disrupted by an invasive alien ant and we investigated whether progressive invasion by plants has an influence on alien ant distribution. We quantified ant-infestations of the scrambler’s flowers and the vertical distribution of alien ants in the habitats through baiting experiment over 2 years and characterised the height of plants within habitat patches that face contrasting levels of alien plant invasion. Two alien and one native ant species were found visiting the flowers, namely Technomyrmex albipes, Anoplolepis gracilipes and Plagiolepis madecassa respectively. Few flowers (6.1%) had alien ants and ant-infested flowers occurred mostly nearer the ground. Mean alien ant abundance varied annually for A. gracilipes but not for T. albipes which however is much rarer than a decade previously. Alien ant-occupied baits showed higher probability of occurring predominantly nearer the ground. Roussea plants were much shorter where alien plant invasion is high, compared to where habitats are well-preserved. The severity of alien ant-infestation that leads to mutualism-disruption varies spatially and at different time scales. As alien plant invasion progresses, the resulting higher native plants’ mortality causes Roussea to lose its supporting host plants and collapse closer to the ground where it faces greater infestation of the alien ants that are documented to disrupt its pollination. Incorporating spatio-temporal variation of threats and unravelling possible interactions between sympatric invasive alien species should be important considerations for conservation managers.