Species → Whistling-thorn ant
Whistling-thorn ant
Crematogaster mimosae
What if it disappeared?
Impact ModerateAs soon as it disappears from a whistling-thorn acacia, the protector ant immediately stops actively defending the tree: nothing left to bite or sting the herbivores browsing its young shoots, until another ant species — often a less effective one — moves into its now-vacant swollen thorns.
HypothesisWhen the protector ant loses its place on an acacia to a non-protective rival ant, that tree suffers twice the mortality and more borer beetle attacks over a decade than a tree where it remained in place — measured directly in the large-herbivore exclusion experiment run in Kenya.
DocumentedDetail
Palmer, T.M., Stanton, M.L., Young, T.P., Goheen, J.R., Pringle, R.M. & Karban, R. (2008). Breakdown of an ant-plant mutualism follows the loss of large herbivores from an African savanna. Science, 319(5860), 192–195.
The whistling-thorn acacia "pays" its protector ant in nectar and housing hollowed out in its swollen thorns, in exchange for active defense against herbivores. When an outside factor pushes the tree to cut back on this investment — in the experiment, excluding large herbivores for ten years — the mutualist ant, which depends on this reward to thrive, gets displaced by an opportunistic ant that doesn't defend the tree as effectively against its real threats. It's this loss of the protector ant, whatever the cause, that doubles the mortality of the trees it used to occupy.
If this same mutualism breakdown repeats over a century, everywhere large herbivores like the elephant are genuinely declining in whistling-thorn savannas — not just on an experimental plot — the protector ant's disappearance could end up marking the landscape: more acacias dying before maturity, across whole savannas rather than a single site.
ExtrapolatedDetail
Extrapolation à partir de Palmer et al. (2008), combinée aux données de déclin réel des grandes populations d'herbivores comme l'éléphant de savane.
Palmer et al.'s experiment artificially controls herbivore presence on a single plot; it doesn't measure whether the same pattern plays out, over a century and across whole savannas, in areas where elephants are declining for real-world reasons (poaching, habitat loss) rather than experimental exclusion.
What's missing to confirm it: A replication of the exclusion experiment at other whistling-thorn savanna sites, along with monitoring over timescales longer than ten years, would confirm whether this mutualism breakdown generalizes at the landscape scale wherever large herbivores are genuinely declining.
Over a thousand years, the protector ant likely wouldn't return to resettle these acacias the same way: other, less effective ant species would durably occupy its swollen thorns instead. The savanna would still have acacias, but more vulnerable trees, never quite as well defended as with their original partner.
HypothesisDocumentation
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Palmer et al. (2008) — Breakdown of an ant-plant mutualism follows the loss of large herbivores from an African savanna, Science
Accessed August 7, 2026
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AntWiki — Crematogaster mimosae, répartition et écologie
Accessed August 7, 2026
Last verified: August 16, 2026