The outstanding partnership between East Africa’s whistling thorn timber and their resident ants is well-known, however now a brand new examine brings to mild the timber’ relationship with birds.
Acacia ants will swarm over branches and chunk elephants or giraffes who attempt to eat the leaves of the whistling thorn (Acacia drepanolobium). As an incentive for this service, the tree supplies the ants with meals and shelter: nectar droplets that ooze from leaf glands, and a house contained in the small hole swellings on the base of among the timber’ thorns (the timber get their title from the sound of wind passing by means of holes in these swellings).
The brand new examine, revealed in Biotropica, reveals how very good 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 Analysis Centre and Conservancy discovered the birds selected to nest virtually 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; very good starlings and gray-headed sparrows had been discovered to have constructed one nest per tree, and though their nests had been an identical form, they may very well be informed aside by the different-sized entrance holes.
The examine confirmed that out of 60 nests constructed by the starlings, sparrows, weavers and two unknown species, just one was in-built a whistling thorn tree occupied by a much less aggressive species of acacia ant, C. sjostedti.
Due to the acacia ants, the birds and their younger 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 examine 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 probably be very useful for the birds, as a result of then it’s going to make the ants additionally be a part of within the combat, and combat no matter is coming to the timber,” Nderitu says.
He and others can personally testify to the effectiveness of ant bites in repelling intruders.
“It’s painful, you can not tolerate them or simply sit and anticipate them to chunk you [or] climb on you,” Nderitu says. “It’s not comfy.”
Even locusts that land on the timber to feed on the leaves will probably be pushed off by the ants. The birds, nonetheless, are by no means harmed, Nderitu says. “It’s a superb relationship and I believe it’s extra advantageous to the birds.”
The ants present different advantages to birds past this frontline protection. C. nigriceps particularly is a “tree architect,” pruning the buds of the whistling thorns it occupies to forestall the branches from extending close to the crowns of neighboring timber and permitting ants from rival colonies to cross over.
The pruning leads to denser leaf cowl that probably provides better safety to the birds’ nests in opposition to predators, the researchers say.
“It’s actually fascinating to consider how the ant-alteration of cover connectivity might affect not solely ant communities, however communities of different bugs, reptiles, carnivores, and even nesting birds,” says Stacy Philpott, a professor of environmental research on the College of California, Santa Cruz.
Philpott, who was not a part of the Kenyan examine, has studied ant-bird interactions in Mexico. She says the findings from Kenya are shocking, on condition that ants do prohibit hen foraging in different ecosystems by means of direct assaults, or by limiting the prey availability for birds in tree canopies.
“However it’s not shocking if ants certainly are deterring different carnivores that prey on hen adults or nestlings,” she says.
Philpott says she wonders if the ants may very well be gathering seeds or insect physique components from hen droppings, or insect or fruit stays from birds’ nests.
Jesse Alston, a quantitative ecologist on the College of Arizona and the corresponding creator of the brand new examine, 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 run, there may very well be some profit from nutrient focus across the tree from birds defecating beneath the nest,” Alston says, “however I don’t suppose we all know sufficient about whether or not birds are nesting in the identical actual timber yearly.”
In Mpala, it’s not simply the tree-nesting birds that depend upon the ants. Others profit not directly.
Amongst them: the charismatic and extremely photogenic secretarybird (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 ft (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’s going to 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, nonetheless, this could probably have an effect on secretarybirds, a species already endangered because of habitat loss.
A deceptively easy ecosystem, dominated virtually solely by whistling thorn timber and a small variety of grass species, harbors complicated relationships, Alston says. “With out the ants, the timber, songbirds, and a few nest predators wouldn’t be there, and you would think about 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 examine web site. It’s nonetheless not clear the place the ant invaders originated; the IUCN’s Invasive Species Specialist Group believes it may very well be Southern Africa, whereas different sources counsel Madagascar, Mauritius or Ethiopia.
What is for certain is that after 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 massive-headed invaders at the moment are simply 1 kilometer, or lower than a mile, away from the examine web site in Mpala, says Todd Palmer, a professor within the biology division on the College of Florida and a co-author of the examine.
“Our estimates counsel that the invasion entrance strikes at about 50 meters [164 feet] per yr,” he says. “These charges of unfold are more likely to depend upon environmental components, like drought, however the risk that they’ll begin wreaking havoc on the acacia ants on this space is a really actual one.”
As soon as that occurs, it is going to be “sport over” for Mpala’s tree-nesting birds, Alston says: “There simply received’t be any nesting habitat for them.”
Quotation:
Lujan, E., Nielsen, R., Brief, 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
This article by Ryan Truscott was first revealed by Mongabay.com on 27 December 2023. Lead Picture: Excellent starling (Lamprotornis superbus), Western Serengeti, Tanzania. Picture by Harvey Barrison through Wikimedia (CC BY-SA 2.0).
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