Loss of big mammals breaks alliance between ants and trees

Blogging on Peer-Reviewed ResearchThe natural world is full of alliances forged between different species, cooperating for mutual rewards. The relationship between ants and acacia trees was one of the first of these to be thoroughly studied. But new research suggests that this lasting partnership may be sundered by the unlikeliest of reasons – the decline of Africa’s large mammals.

Giraffe next to whistling-thorn acaciaAcacias are under constant attack from hungry animals, from tiny caterpillars to towering giraffes. In response, many species like the whistling-thorn tree (Acacia drepanolobium) recruit colonies of ants as bodyguards. Any hungry herbivores eager to chomp on the acacia’s leaves quickly get a mouthful of biting, stinging ants. The tree is a fair employer. In return for their services, its ant staff receive a sugary and nutritious nectar as food and hollow swollen thorns called ‘domatia’ as board.

But this pact is a fragile one. Todd Palmer from the University of Florida and colleagues from the USA, Canada and Kenya have found that it rapidly breaks down if the large animals that graze on the acacia disappear. Without the threat of chomping mouths, the trees reduce their investments in bodyguards to the detriment of both partners.

Palmer demonstrated this with plots of land in Kenya’s Laikipia Plateau, where fences have kept out large plant-eaters for over a decade. Since 1995, no herbivore larger than a small antelope has entered the four-hectare “exclosures” in an attempt to study the effect of these animals on the local ecology.

Within these 10 years, Palmer found that the majority of trees produced fewer domatia and less nectar and unexpectedly, the strongest alliances were hit the hardest. What were once happy partners quickly became selfish rivals.

Tale of three ants

Several species of ant compete for a place on the acacia’s branches and the most abundant of these – Crematogaster mimosaeis also the most cooperative partner. It relies heavily on the domatia for shelter and aggressively protects the tree in return. But with no herbivores around, its services were not required and the trees started evicting it. The proportion of acacias colonised by C.mimosae fell by about a third.

C.mimosae defends the whistling-thorn acaciaThe partnership soured from both ends. The ants, with less food and smaller homes, became twice as likely to farm sap-sucking scale insects. Their waste fluid is a sugary liquid called honeydew that the ants drink, but to make it, they need to suck the juices of the tree. The ants also slacked off and were half as likely to marshal a defence against marauding plant-eaters.

A second species Crematogaster sjostedti actually seemed to benefit from the trees’ reduced investment. It is much less common than C.mimosae and takes a more relaxed attitude to the partnership. It could even be viewed as a parasite, for it defends the tree less aggressively and ignores the domatia, nesting instead in boreholes excavated by beetle larvae. As such, when the trees reduced their provisions, C.sjostedti wasn’t fussed and with its competitor suffering, it more than doubled in abundance.

A third species, Crematogaster nigriceps is even rarer and even more parasite than partner. Although it relies on domatia like C.mimosae, it also actively prunes any growing shoots that the tree sends out. This isolates it from the rest of the canopy, which prevents more competitive species like C.mimosae and C.sjostedti from invading. But it also sterilises the tree.

A zebra and an acaciaDespite this, trees harbouring C.nigriceps still produced the same sizes of nectaries and domatia even in the herbivore ‘exclosures’. Palmer thinks that the trees can’t tell the difference between the munching of large herbivores and the ants’ own pruning. Both send the same physiological signals that tell the tree it is being attacked, and that it should house defenders.

Everybody loses

Palmer is keen to point out that the changes seen in the acacias are not evolutionary ones. The experiment’s 10-year span is much shorter than the life of an acacia and with no passing generations, there is no clay for natural selection to sculpt with. The changes in domatia and nectar are part of a flexible program of behaviour enacted by the tree in response to changing pressures from plant-eaters.

But in this case, it works to the acacia’s disadvantage. You might expect that taking away large animals that feed on the trees should benefit them in the long run. In fact, the opposite is true. The key change is that the parasitic C.sjostedti replaces the cooperative C.mimosae as the dominant species in the ant-acacia community. And trees that bear this ant grow more slowly and are twice as likely to die as trees that harbour the others.

Long-horned beetleLong-horned beetles are the problem. They are a destructive species that bore into the acacias’ trunks. C.mimosae and C.nigriceps kill these pests, but C.sjostedti actually encourages them because it lives in the holes they leave behind. As the herbivores left and C.sjostedti rose to power, the beetles thrived and the trees paid the price.

Palmer’s study elegantly shows that the fall of one species can ripple across an entire habitat in unforeseen ways. It’s an increasingly common theme in modern ecology and I have blogged about it in a few other posts before. Here, it sundered the collaboration between two cooperating species to the detriment of both, and left the door open for an invading parasite.

While the situation in the study was created by scientists, it is being mirrored all over Africa. Several large plant-eating species are endangered or vulnerable, including the African elephant and both species of rhino. If they decline further, the entire ecosystem could feel the effects, from the tallest tree to the smallest ant.

Images: Ants, giraffes and sunset taken by Todd Palmer; beetle taken by Ivan Stankovic

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