Fascinating research from the Indian Institute of Science reveals how the aggressive Oecophylla ants are shaping the diversity of bird species in mountainous regions. By competing with insect-eating birds for food at lower elevations, these ants are pushing montane birds higher up the mountains. This discovery could have significant implications as climate change alters the ranges of these influential ants, potentially impacting the delicate balance of high-altitude ecosystems. The findings highlight the intricate web of interactions that underpin the remarkable biodiversity found in Earth’s mountainous regions.
Ants Outcompeting Birds
Published in Ecology Letters, the study is centered around the effects of Oecophylla ants — which are aggressive and dominant “weaver” ants on the assembly rules governing insect-eating birds at low elevations.
Previous work had suggested their presence at the base of the eastern Himalayas brought insect densities down, and this might influence food availability for bug-hunting birds. The present study extends this pattern to other insect-eating bird species.
Looking at databases of bird species seen at different elevations and across several mountain ranges, the researchers built a model. The scientists broke the birds down into dietary guilds, such as insectivores and omnivores, and then they compared the numbers of species in mountain ranges with and without Oecophylla ants at their bases.
Forcing Birds to Slightly Higher Elevations
There was an unambiguous signal in the data: where Oecophylla ants occurred at lower altitudes, the richness of insectivorous birds would be decreased. This indicates that the ants are outcompeting these bird species for their staple food, which is insects.
This pushed the insect-eating birds higher up in elevation to nearly 1000 meters, an altitude where species diversity peaked. By contrast, most other groups of birds such as nectar-eaters or fruit-eaters that do not compete with the ants showed a decline in species diversity with elevation.
This discovery emphasizes the potentially important role of biotic interactions, such as between ants and birds, in determining species distributions/diversity in mountain ecosystems. This finding also highlights the need to take into account the complexity of these ecological relationships, in particular when looking at mountain habitats and efforts for their conservation.
Likelihood of Effects of Climate Change
The implications of this size difference are huge, according to the researchers potentially even more so in a future shaped by climate change. The rages of Oecophylla ants extend to higher altitudes, where they overlap with those of insectivorous birds which have already been driven upwards by warmer temperatures.
That could unbalance central mountain ecosystems even more, wreaking havoc on the broad biodiversity and ecosystem function. The researchers hope the study will alert ecologists to pay more attention to how biotic interactions, such as ants helping birds, can increase the resilience of mountain habitats against environmental changes.
With mountain regions of the world under increasing threat from climate change, habitat fragmentation, overgrazing, and human activities, this research suggests that conserving even those organisms not directly impacted by a particular intervention can have unforeseen benefits by helping maintain species’ roles and relationships in mountain ecosystems. These findings offer a new perspective on the complex nest-ant-plant interactions in these forests and will help us to further understand the mechanisms by which ants affect bird community structure and diversity, reinforcing what we already know about how important ants are to the functioning of rainforests.
