- The whistling-thorn acacias of East Africa are famend for internet hosting aggressive species of ants.
- The ants defend the timber in opposition to damaging looking from the likes of elephants and giraffes, in return for the meals and shelter that the timber present.
- A brand new research reveals that tree-nesting birds, like starlings, weavers and sparrows, want to nest in whistling thorns that host probably the most aggressive ant species, to make the most of the ants’ safety in opposition to predators like mongooses.
- This necessary partnership is threatened by invasive African big-headed ants which can be spreading in some elements of Kenya.
The outstanding partnership between East Africa’s whistling thorn timber and their resident ants is well-known, however now a brand new research brings to mild the timber’ relationship with birds.
Acacia ants will swarm over branches and chew elephants or giraffes who attempt to eat the leaves of the whistling thorn (Acacia drepanolobium). As an incentive for this service, the tree offers the ants with meals and shelter: nectar droplets that ooze from leaf glands, and a home contained in the small hole swellings on the base of a few of the timber’ thorns (the timber get their title from the sound of wind passing by holes in these swellings).
The new study, printed in Biotropica, reveals how excellent starlings (Lamprotornis superbus), gray-headed sparrows (Passer griseus) and gray-capped social-weaver birds (Pseudonigrita arnaudi) in whistling-thorn savannas additionally get assist from acacia ants. Researchers learning whistling thorns in Kenya’s Mpala Research Centre and Conservancy discovered the birds selected to nest almost solely in timber occupied by the 2 most aggressive species of ants: Crematogaster mimosae and Crematogaster nigriceps.
They discovered these nests by conducting searches inside grassy glades in Mpala’s whistling-thorn savanna. True to their title, social-weavers had constructed their ball-shaped nests in the identical tree; excellent starlings and gray-headed sparrows have been discovered to have constructed one nest per tree, and though their nests have been the same form, they could possibly be informed aside by the different-sized entrance holes.
The research confirmed that out of 60 nests constructed by the starlings, sparrows, weavers and two unknown species, just one was inbuilt a whistling thorn tree occupied by a much less aggressive species of acacia ant, C. sjostedti.
Thanks to the acacia ants, the birds and their young obtain safety in opposition to predators like snakes and tree-climbing predatory mammals, together with noticed genets (Genetta genetta) and slender mongooses (Herpestes sanguineus).
Wilson Nderitu, a co-author of the research who has researched birds at Mpala in Kenya’s central Laikipia county for 22 years, has witnessed this firsthand.
If mongooses or genets try and raid birds’ nests, the mum or dad birds mob the attackers, the timber shake, and the ants are alerted.
“That will be very helpful for the birds, because then it’s going to make the ants also join in the fight, and fight whatever is coming to the trees,” Nderitu says.
He and others can personally testify to the effectiveness of ant bites in repelling intruders.
“It’s painful, you cannot tolerate them or just sit and wait for them to bite you [or] climb on you,” Nderitu says. “It’s not comfortable.”
Even locusts that land on the timber to feed on the leaves will likely be pushed off by the ants. The birds, nevertheless, are by no means harmed, Nderitu says. “It’s a good relationship and I think it’s more advantageous to the birds.”
The ants present different advantages to birds past this frontline protection. C. nigriceps specifically is a “tree architect,” pruning the buds of the whistling thorns it occupies to stop the branches from extending close to the crowns of neighboring timber and permitting ants from rival colonies to cross over.
The pruning ends in denser leaf cowl that possible provides better safety to the birds’ nests in opposition to predators, the researchers say.
“It is really fascinating to think about how the ant-alteration of canopy connectivity may impact not only ant communities, but communities of other insects, reptiles, carnivores, and even nesting birds,” says Stacy Philpott, a professor of environmental research on the University of California, Santa Cruz.
Philpott, who was not a part of the Kenyan research, has studied ant-bird interactions in Mexico. She says the findings from Kenya are stunning, provided that ants do prohibit chook foraging in different ecosystems by direct assaults, or by limiting the prey availability for birds in tree canopies.
“But it is not surprising if ants indeed are deterring other carnivores that prey on bird adults or nestlings,” she says.
Philpott says she wonders if the ants could possibly be gathering seeds or insect physique elements from chook droppings, or insect or fruit stays from birds’ nests.
Jesse Alston, a quantitative ecologist on the University of Arizona and the corresponding writer of the brand new research, says that whereas he doubts the ants profit from having the birds of their timber, he’s realized to by no means say by no means when talking about ecological techniques.
“Over the very long term, there could be some benefit from nutrient concentration around the tree from birds defecating below the nest,” Alston says, “but I don’t think we know enough about whether birds are nesting in the same exact trees every year.”
In Mpala, it’s not simply the tree-nesting birds that rely on the ants. Others profit not directly.
Among them: the charismatic and extremely photogenic secretary chook (Sagittarius serpentarius).
These long-legged birds of prey with iconic crest feathers stalk throughout the savanna in the hunt for meals that features snakes that they kill with highly effective blows from their toes (their featherless legs are sheathed in thick snake-proof scales).
Snakes are additionally the principle predators of Mpala’s tree-nesting birds. That’s as a result of the ants, regardless of being fearsome protectors of the whistling thorns, aren’t at all times an ideal protection for the birds. If a snake is hungry sufficient, it can endure the discomfort inflicted by the ants to eat birds’ eggs or the nestlings, Alston says. If there have been no birds’ nests and chicks for the snakes to feed on, nevertheless, this could possible have an effect on secretary birds, a species already endangered because of habitat loss.
A deceptively easy ecosystem, dominated almost solely by whistling thorn timber and a small variety of grass species, harbors advanced relationships, Alston says. “Without the ants, the trees, songbirds, and some nest predators wouldn’t be there, and you could imagine a cascade to seemingly unrelated species,” he says.
The lack of acacia ants is an actual risk. African big-headed ants (Pheidole megacephala), now a globally acknowledged invasive species, have arrived at Mpala conservancy, although not but on the research website. It’s nonetheless not clear the place the ant invaders originated; the IUCN’s Invasive Species Specialist Group believes it could possibly be Southern Africa, whereas different sources counsel Madagascar, Mauritius or Ethiopia.
What is for certain is that once they do invade whistling-thorn savannas, the big-headed ants kill the native acacia ants — even the 2 most aggressive species — and render the timber defenseless.
When that occurs, it’s solely a matter of time earlier than the elephants sense the timber are unguarded, at which level they browse, break, and kill them, turning the woodland right into a savanna, the researchers say.
The big-headed invaders at the moment are simply 1 kilometer, or lower than a mile, away from the research website in Mpala, says Todd Palmer, a professor within the biology division on the University of Florida and a co-author of the research.
“Our estimates suggest that the invasion front moves at about 50 meters [164 feet] per year,” he says. “Those rates of spread are likely to depend on environmental factors, like drought, but the possibility that they’ll start wreaking havoc on the acacia ants in this area is a very real one.”
Once that occurs, will probably be “game over” for Mpala’s tree-nesting birds, Alston says: “There just won’t be any nesting habitat for them.”
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Citation:
Lujan, E., Nielsen, R., Short, Z., Wicks, S., Watetu, W. N., Khasoha, L. M., … Alston, J. M. (2023). Symbiotic acacia ants drive nesting conduct by birds in an African savanna. Biotropica, 55(6), 1101-1105. doi:10.1111/btp.13276
Banner picture: Superb starling (Lamprotornis superbus), Western Serengeti, Tanzania. Image by Harvey Barrison through Wikimedia (CC BY-SA 2.0)
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