Spatial structuring and floral avoidance behavior prevent ant-pollinator conflict in a Mexican ant-acacia

Abstract

Ant-acacias are a classic example of insect-plant mutualism, the ants defending the plant from attack by herbivores. A potential drawback of such mutualism is that ants may drive away useful insects, particularly pollinators. We assess the potential for ant-pollinator conflict in a Mexican ant-acacia, Acacia hindsii Benth. (Fabaceae: Mimosoideae). Healthy trees maintain large populations of a highly aggressive ant, Pseudomyrmex veneficus (Formicidae: Pseudomyrmecinae), housed in swollen-thorns. Ants are provided with trophic rewards in the form of extrafloral nectar and protein-rich Beltian bodies, and forage entirely within the acacia canopy. A. hindsii flowers are presented in spicate inflorescences of ca. 200 densely-packed florets, all of which last a single day and open before dawn, dehiscing together from 0700h to 0830h. Pollinator activity began shortly after dehiscence and continued until 1300h. Daily activity patterns of ants and pollinators overlapped in time, but were largely separated in space. The resources exploited by the ants during the period of overlap - particularly extrafloral nectar - are predominantly concentrated within new growth, while inflorescences are present only on shoots from the previous year. Ants do collect nectar from nectaries on flower-bearing shoots, but very rarely move onto inflorescences. Bioassays of ant behavior suggest that A. hindsii inflorescences are highly repellent to P. veneficus. This repellence is long-lasting and presumably reinforces the spatial segregation between ants and pollinators. Further experiments showed inflorescences of two non-myrmecophilous Acacia species also to be ant-repellent, though to a lesser extent than in A. hindsii. This suggests a general role for ant-repellents in Acacia with selective enhancement in myrmecophilous species.