‘Invasive ants dig too greedily and too deep into the roots of whistling acacia saplings’

Pat Milligan discusses his recent articleA soil-nesting invasive ant disrupts carbon dynamics in saplings of a foundational ant–plant‘. Find out more about how big-headed ants disrupt photosynthesis and plant growth.

Human trade and travel across the planet often introduce species to new regions where they may have few or no natural competitors or threats. These “invasive” species often trigger declines in native biodiversity in their new ranges, and mounting evidence suggests that biological invasions also disrupt species interactions and ecosystem functions. So, what are the common ways in which invasive species can have profound impacts on ecosystem services and processes? Investigating this question is a key step in predicting how biological invasions will affect habitat conservation, the benefits received by humans from the environment, and the ongoing climate and biodiversity crises.

Many areas in the tropics and subtropics have been invaded by one or more invasive ant species, and ant invasions can contribute to large habitat changes. Past research shows that many invasive ants tend to outcompete or prey upon native insect species, and they also often form novel relationships with phloem-feeding (feeding on plant stem nutrients) insects. Killing off native insects tends to disrupt important services for native plants that were once provided by those insects, such as pollination and protection from herbivores. Formation of new partnerships with phloem-feeders usually causes those phloem-feeders to increase in abundance and damage native plants. Thus, invasive ants often reduce plant productivity via interactions that can be quite specific to a given ecosystem, but which are perhaps not common or predictable.

A mature whistling acacia in a ‘black cotton’ savanna at Ol Pejeta Conservancy. A young acacia sapling, about 50 cm tall, is nearly hidden in the grass at the left edge of the image. (Photo courtesy: P. Milligan)

In ‘black cotton’ savannas of East Africa, the invasive ‘big-headed ant’ (Pheidole megacephala) has steadily expanded into forests made up almost entirely of ‘whistling acacia’ trees (Acacia drepanolobium). Usually, these trees are protected against elephants and other large herbivores by highly aggressive native ants, which live inside of the tree crown (its leafy aboveground sections) and eat tree nectar; however, big-headed ants kill many insects as they invade, including native ants, and decrease survival and growth of the tree community.

Big-headed ants swarm a mature whistling acacia tree, searching for native ants to kill and eat. (Courtesy: P. Milligan and B. Cherry)

When we first learned of this invasion, our attention was consumed by events in the tree crown: lively battles between native and big-headed ants, trees completely empty of native defenders after these ant battles, and then catastrophe for the undefended trees once elephants found them.

An elephant breaks a branch from a whistling acacia that was recently invaded by big-headed ants, and is now defenseless. (Courtesy: P. Milligan)

But then we noticed another phenomenon: big-headed ants digging large nest entrances around the stem bases of whistling acacia saplings and other savanna plants. We suspected that a trait common to invasive ants, the digging of extensive tunnel networks, may also strongly impede plant productivity if done in and around the roots.

We constructed a series of greenhouse and field experiments to isolate the effects of phloem-feeding insect partnerships and underground tunnel excavation by big-headed ant on the growth and carbon dynamics of whistling acacias. While this project focused on acacia saplings, it has implications for the whole savanna: whistling acacias are what we call a “foundation” species, affecting the cycling of nutrients and even visibility of predators and prey in its savanna environment.

First, we wanted to test if big-headed ants were affecting whistling acacia saplings by themselves or through a novel partnership with a native phloem-feeding hemipteran (called ‘wax scales’). In a 6-month long greenhouse experiment at Mpala Research Centre in Kenya, we grew groups of whistling acacia seedlings with or without big-headed ants present on the plants. Within each of those groups, we also allowed wax scales to colonize the seedlings, or removed them from the seedling branches and stem. Our experiment showed that big-headed ant presence negatively impacted sapling growth and photosynthesis, but the removal of wax scales did not.

L) “Wax scale” phloem-feeding insects on a whistling acacia branch. R) Big-headed ant workings traveling along a whistling acacia lateral root that that has been excavated as part of a nest network. (Courtesy: L) P. Milligan, R) P. Milligan, E. Pringle).

Having confirmed that big-headed ants were not impacting saplings indirectly through a wax scale partnership, we then conducted a second greenhouse experiment where we grew saplings from seeds under three levels of big-headed ant presence. From months 2 to 12, we grew seedlings in 1) a control group with no big-headed ants present, 2) a ‘root exposure’ treatment in which big-headed ants could access the sapling root system but not the stem, branches, or leaves, and 3) a ‘full exposure’ in which big-headed ants could freely traverse the entire plant. Root exposure significantly reduced whole-plant photosynthesis, leaf water status, and availability of sucrose, a key carbohydrate for plant respiration and maintenance. Full exposure saplings tended to have less leaf and root biomass and more big-headed ants present in their planting pots, and so we suspect that bigger ant colonies may dig larger and more disruptive tunnel networks that disrupt the production of new leaves and roots.

Illustration of second greenhouse experiment (Fig. 1 from manuscript): A) Control group, B) Root exposure group, and C) Full exposure group. (Courtesy: P. Milligan, B. Kolbus)

Finally, we wanted to know if these findings from the greenhouse matched with real savannas that were experiencing big-headed ant invasion. At Ol Pejeta Conservancy, we counted the numbers of leaves (an approximation of ‘crown size’) of saplings growing in big-headed-ant-invaded and uninvaded savannas. Half of trees in both invaded and uninvaded areas had also been protected under herbivore-excluding large metal cages for 9 months preceding the survey, which allowed us to separate the impacts of large herbivores and invasive ants on crown size. Our surveys showed that presence of big-headed ant but not of herbivores strongly reduced sapling crown size, suggesting that big-headed ants indeed directly impact native plants.

Our research helps to explain why this ant invasion is causing whistling acacia population declines in East African black cotton savannas, where whistling acacias play large roles in nutrient cycles, grass productivity, and other important functions of the ecosystem. More broadly, our research raises an alarm: if invasive ants dig large nest networks around tree roots and if this tunnelling often reduces plant carbon fixation and storage, then we can expect that ant invasions will reduce forests’ abilities to absorb atmospheric carbon across the tropics and subtropics. For this reason, we emphasize a need for more studies that monitor carbon fixation and storage for forests that have experienced invasions by ground-nesting ant species.

Pat Milligan University of Nevada, Reno, USA

Read the full research article online: A soil-nesting invasive ant disrupts carbon dynamics in saplings of a foundational ant–plant

You can also read the BES press release here

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