Competition with insectivorous ants as a contributor to low songbird diversity at low elevations in the eastern Himalaya

Abstract

Competitive interactions between distantly related clades could cause complementary diversity patterns of these clades over large spatial scales. One such example might be ants and birds in the eastern Himalaya; ants are very common at low elevations but almost absent at mid-elevations where the abundance of other arthropods and insectivorous bird diversity peaks. Here, we ask if ants at low elevations could compete with birds for arthropod prey. Specifically, we studied the impact of the Asian weaver ant (Oecophylla smaragdina), a common aggressive ant at low elevations. Diet analysis using molecular methods demonstrate extensive diet overlap between weaver ants and songbirds at both low and mid-elevations. Trees without weaver ants have greater non-ant arthropod abundance and leaf damage. Experimental removal of weaver ants results in an increase in the abundance of non-ant arthropods. Notably, numbers of Coleoptera and Lepidoptera were most affected by removal experiments and were prominent components of both bird and weaver ant diets. Our results suggest that songbirds and weaver ants might potentially compete with each other for arthropod prey at low elevations, thereby contributing to lower insectivorous bird diversity at low elevations in eastern Himalaya. Competition with ants may shape vertebrate diversity patterns across broad biodiversity gradients.

Keywords: ant exclusion; inter‐clade competition; macroecology; molecular diet analyses.

FIGURE 1

(a) Correlation between number of songbird species and individuals along an elevational gradient in the eastern Himalaya. (b) Correlation between number of songbird species and arthropod abundance along the same gradient. Site at 200 and 2,000 m elevation examined further in this study shown in blue and yellow, respectively. Based on data in Price et al. (2014)

FIGURE 2

Venn diagram showing diet overlap at multiple taxonomic levels as assessed with molecular metabarcoding between weaver ants, birds at low elevations, and birds at mid‐elevations. Bird and ant silhouettes taken from http://phylopic.org. Anthony Caravaggi produced the bird silhouette

FIGURE 3

(a–d) Data from 34 trees paired for size and species, where one tree had weaver ants and the other did not. The gray line shows a slope of 1.0, that is, points on this line indicate pairs that do not differ. (a) Number of arthropods (excluding ants and suborder Homoptera). (b) Number of arthropods belonging to the orders Lepidoptera and Coleoptera. (c) Percent leaf damage estimated on 10 leaves distributed around the tree. (d) Percent leaf damage estimated on a clipped branch

FIGURE 4

Data from trees paired for size and species, with one member of each pair subject to weaver ant removal and exclusion. Gray lines and dots show control trees and purple lines and dots show treatment trees. Darker dots and lines indicate multiple overlapping points. (a) Number of arthropods (excluding ants and suborder Homoptera) before and 1 month after weaver ant removal and exclusion experiment (N = 30 trees). (b) Number of arthropods (excluding ants and suborder Homoptera) before and 1 year after weaver ant removal and exclusion experiment (N = 28 trees)

FIGURE 5

(a) Difference in mean abundance of major arthropod orders in control and treatment trees 1 month after ant exclusion and removal (derived from the data in Figure S7, the error bars indicate standard errors). (b) Frequency of different arthropod orders in bird diets at low elevations (proportion of individual birds with at least one sequence from that order, N = 18 individuals)