Ants are not always heroes. Invasive ants are a significant threat to global biodiversity. Alba Costa and colleagues conducted a study on the island of Mahé, Seychelles, into how two common invasive ant species affect the pollination and reproduction of native plants in restoration projects. They found that the Yellow Crazy Ant (Anoplolepis gracilipes) and White-Footed Ant (Technomyrmex albipes) deter flying pollinators from landing on plants, disrupting pollination networks.
While ants can be pollinators, they can also harm plants by preventing pollination. They can eat reproductive organs without contributing to pollination, and they can also deter pollinators from landing on a plant or else chase them off.
Costa and colleagues conducted a combination of ant exclusion experiments and pollinator observations to see what was going on. They staked out plants on eight granitic inselbergs. ‘Inselberg’ comes from the German for island mountain. On four of the inselbergs, ecologists were restoring sites, and the other four remained as invaded control sites.
Inselbergs can be quite short, but the tops can still be distinct from the terrain they rise up from. That means native plants on Seychelles’ inselbergs rely primarily on flying visitors for pollination. The scientists found that the invasive ants don’t reduce pollinators’ visits but change what they do once they arrive. Costa and colleagues write:
Our visitation data showed that the presence of ants is not deterring pollinators from approaching plants and visiting one flower, which means that the number of visits is unaffected by ants. In the absence of ants, flying pollinators will continue their foraging bout and probe neighbouring flowers. Ants appear to disrupt such foraging bouts as the numbers of probed flowers declines in the presence of invasive ants. This change in foraging behaviour may be triggered by aggressive behaviour of the ants directed at flying pollinators (Junker et al., 2007) or possibly through indirect interactions such as resource competition (Lach, 2005)Costa et al. 2023
As a result of the curtailed visits, the plants suffer reduced fruit set. That, in turn, leaves fewer seeds for the next generation.
Costa and colleagues emphasise the importance of their study in understanding plant-animal interactions in restoration projects. They write:
Our results showed that the frequency of flower visits and fruit set in the presence of invasive ants did not vary with restoration status, which indicates that the impact of invasive ants on pollinator foraging behaviour and plant reproductive performance was not affected by the removal of non-native plants in inselberg plant communities. Previously, it was shown that pollination resilience and function are reduced in unrestored inselberg communities dominated by non-native plants (Kaiser-Bunbury et al., 2017).Costa et al. 2023
The research shows invasive ants can have long-term impacts, even when restoration efforts begin in an ecosystem.
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Costa, A., Heleno, R., Freide, E.F., Dufrene, Y., Huckle, E. and Kaiser-Bunbury, C.N. (2023) “Impacts of invasive ants on pollination of native plants are similar in invaded and restored plant communities,” Global Ecology and Conservation, 42(e02413), p. e02413. Available at: https://doi.org/10.1016/j.gecco.2023.e02413.