Competition from ants may drive spatial patterns of songbirds in the Eastern Himalaya

Blog written by Jordan Greer. Read the full article here

The foothills of the Eastern Himalaya are a warm, lush ecosystem with trees that reach over 20 meters in height, and a network of branches woven throughout the canopy. At 1500 meters up the mountain, the landscape transforms into cool cloud forest, with cascades of falling moss and an almost ever-present mist. And despite the immense diversity of plants and animals in the cloud forest, ants are almost non-existent.

Ants are found throughout the world, even in harsh environments like the Arctic Circle. Surprisingly, however, most ant species are unable to establish in the mid elevations of the Eastern Himalaya. Some theories suggest that the consistent cold and wet climate of cloud forests prevent ants from establishing their colonies. In contrast, within the foothills ants are in abundance. The dominant ant species at low elevation is the weaver ant (Oecophylla smaragdina)– an insectivorous species that “weaves” leaves together to create nests in tree branches. Highly aggressive, weaver ants guard the trees in which they dwell, and forage for food both arboreally and on the forest floor.  As these common predators are not present above low-elevation, researchers sought to understand how their presence (or lack of it) may impact the surrounding ecological community.

Songbirds are present at both low and mid-elevations, though within the cloud forests the number of species is roughly double. Guided by Dr. Trevor Price, an expert on Himalayan bird biodiversity and Dr. Corrie Moreau, one of the world’s leading ant experts, first author Supriya wished to investigate whether there could be a link between the burst of songbird species and the lack of weaver ants within the cloud forest region. If songbirds and weaver ants foraged for the same insects, the absence of weaver ants could potentially open niche space for more species of insectivorous songbirds to occupy. This could also explain why in the foothills, where weaver ants are common, there are significantly less species of songbirds—by ants acting as resource competitors, they may influence spatial patterns of songbird diversity.

The fossil record could lend support to this idea. Weaver ant species used to be found globally, including Europe. Interestingly, around the same time weaver ants disappeared from the European fossil record, scientists recorded the earliest songbird fossil in Germany. Though speculative, if both animals occupied the same niche, this could act as an early example of how weaver ant presence could influence the spatial patterns and even diversification of songbird species.

Weaver ants carrying insects into their nest in Chapramari Wildlife Sanctuary, India. (Photo: K. Supriya)

To test whether weaver ants could act as a strong resource competitor to songbirds in the Eastern Himalaya, Supriya first needed to establish that both seek out the same food sources. The researchers extracted animal DNA from songbird fecal matter at both low and mid elevation habitat, and compared that to DNA extracted from weaver ant food removed from their colony. The results demonstrated a significant overlap in the diets of weaver ant and songbirds at both elevations— each with a strong appetite for Coleoptera (beetles) and Lepidoptera (butterflies and moths) in particular.

But even though their diets overlap, that doesn’t necessarily mean weaver ants are so voracious in appetite that they exclude birds from occupying habitats by limiting food. To better address this, Supriya asked whether weaver ants significantly depleted the number of arthropods from the trees in which they lived. In the forests near the lowland village of Panijhora, where much of the field research was carried out, she and her field team took on the grueling task of pairing equally sized trees of the same species with and without weaver ant nests. Then they assessed both the insect abundance and the amount of leaf damage present. They found that the abundance of Lepidoptera and Coleoptera was nearly twice as high in trees without weaver ants.

To make sure this finding wasn’t just the result of ant preference for trees with less insects, they followed up with an ant exclusion experiment. Here, the researchers removed all weaver ant nests from several trees and applied a band of TanglefootTM around the trunk to prevent ants from recolonizing. Measures of arthropod abundance were taken before and one month after weaver ant removal. This allowed investigators to tease out if non-ant arthropods could “bounce back” to greater abundance once ants were removed. On average, after one-month trees with ants removed showed 3 times increase in their insect abundance.

Supriya and her 2016 field team (Amir Chhetri, Jobin Varughese, Priyanka Das, Vinod Sankar) sampling arthropods at a tree (Photo: Corrie Moreau)

Taken together, these results suggest that the absence of weaver ants has a positive impact on arthropod abundance. As such, the absence of ants at mid-elevation may contribute to the high non-ant arthropod density within the cloud forest region. In addition, diet overlap analysis shows birds and ants likely compete for arthropod prey at low elevations in the Eastern Himalaya. The lack of competition between ants and songbirds at mid elevation could be one potential driver for the witnessed increase in insectivorous songbird diversity.

A major takeaway from this study is that it is limiting to only look at competition between two closely related species, as is often done. We must also consider the broader communities and guilds in which species take part—these relationships can be as important or more to ecosystem structure. Further, as community ecology research progresses, we need to better address communities that exist across or over regional or environmental climatic gradients, especially in the face of climate change. Could climate change reshape the weaver ants’ distribution to move into cloud forest? And if so, would they displace the resident songbird species? Many questions are left to be answered, but this study acts as an initial step towards tackling questions about community structure within the Himalayas.

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