Close Encounters

Are ants a benefit or a detriment when they occupy a plant? New research says they may be both.

Research in two new papers shows how ants can interfere with pollination, but this interference is complicated.

Ants are known to benefit plants by providing nutrients to plants, pollination services and seed dispersal, as well as defending plants against herbivory. But do ants just drive away herbivores, or do they deter everything? And if they do, how does that affect plants that rely on pollinators other than ants? Two recent studies have been looking at the same problem from two different directions.

Nora Villamil and colleagues have been looking at the interaction between Turnera velutina and ants in coastal sand scrub at Troncones, Guerrero, Mexico. They had noted that biologists sometimes argue that aggressive ants could increase pollination by driving off pollinators after short visits, and so promote out-crossing of pollen. On the other hand they might deter pollinators from spending time at a plant, and so decrease pollination instances.

The same authors have already published research on T. velutina using ant corpses. They found that seeing the ants in the flowers deterred pollinator visits, but they also point out that this isn’t a ‘real world’ experiment. It’s likely that, out in the field, life is more complicated. So the team staked out an area and excluded ants from some flowers, but not others, to see what effect the ants had on pollinators.

They found that honey bees spent longer in the flowers without ants. As a consequence, they collected more pollen, meaning that they were having an effect on male fitness for plants. Ant exclusion also changed where the pollen went, with much of it ending up in the same plant. So the team also see an effect on female fitness in the plants. Ant-patrolled flowers may benefit from increased female fitness as, while there is less in total, the pollen is more likely to arrive from other plants rather than the same plant.

So the team concluded that while ants are deterring pollinators, the presence of ants helps the plants outcross their DNA, instead of relying on self-pollination. This, however, is a situation where plants have co-evolved with their ant occupants. What happens when unexpected ants arrive?

Francisco Fuster and colleagues have a paper published in the American Journal of Botany, looking at the effect of invasive alien ants in the Seychelles. They examined pollination of three plants, Thespesia populnea, Polyscias crassa, and Syzygium wrightii.

T. populnea and S. wrightii, can both be pollinated by sunbirds, but when the ant Anoplolepis gracilipes is present in the plants, things change. A. gracilipes, the yellow crazy ant, is an aggressive ant that protects scale insects that feed on the host plant, and excrete honeydew. When they’re not eating honeydew, the ants eat pretty much anything they can get away with, including vertebrate corpses. For visitors to the plants, the ants’ aggression means they don’t wait for vertebrates to be dead before taking a bite. This discourages the sunbirds from visiting the flowers.

Fuster and colleagues found that, for T. populnea this is not an insurmountable problem. Flying insects take on the role of pollinator. However, for S. wrightii, the situation is worse. Specialist pollinators do a much better job of pollinating the plant than generalist pollinators, and if the sunbirds aren’t visiting then reduced pollination means there’s likely to be a reduction in seeds. P. crassa has a slightly different problem, the authors found. The most effective pollinators were Phelsuma geckos, but these are nectar opportunists rather than specialists.

The team say that because each plant has one highly effective taxon pollinating it, ants disrupting visits by pollinators could have significant effects on population survival. However, it’s not as simple as saying a specific species of ant is a problem. They also note that an invasive ant causing disruption in one place can be an effective pollinator in another.

This would be consistent with Villamil and colleagues work in Mexico, which shows that ants can even have multiple effects on fitness, within the same plant. It appears that often ants may be more difficult to categorise than simply as antagonists or mutualists.