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\begin{document}

\title{Competition with insectivorous ants as a contributor to low songbird
diversity at low elevations in the eastern Himalaya.}



\author[1]{K. Supriya}%
\author[2]{Trevor Price}%
\author[3]{Corrie Moreau}%
\affil[1]{Arizona State University}%
\affil[2]{University of Chicago}%
\affil[3]{Cornell University}%


\vspace{-1em}



  
  \date{June 03, 2024}


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\selectlanguage{english}
\begin{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.%
\end{abstract}%



\sloppy




\textbf{Introduction}

Biotic interactions such as mutualism, commensalism, competition and
predation, affect diversity and distribution of species at different
scales in different ways (Schemske et al. 2009; Wisz et al. 2013; Ara\selectlanguage{ngerman}újo
and Rozenfeld 2014; Lany et al. 2018; Nelsen et al. 2018). While the
role of these interactions in shaping large-scale biodiversity patterns
is often acknowledged, it is rarely tested (Schemske et al. 2009; McCain
and Grytnes 2010; Wisz et al. 2013). This paucity of studies may be
attributed to the fact that biotic interactions are often difficult to
quantify (McCain and Grytnes 2010) and that cause and effect can be
difficult to distinguish in the relationship between biotic interactions
and species diversity (Fischer 1960; Schemske et al. 2009). However,
there is a growing need to study the role of these interactions in
shaping diversity patterns in the light of predicted shifts in species
ranges due to climate change (Blois et al. 2013; Wisz et al. 2013;
Valiente-Banuet et al. 2015; Anderson 2017; Gavish et al. 2017),
especially as the strength and effects of these interactions are
themselves susceptible to climate change (Tylianakis et al. 2008; Faldyn
et al. 2018).

One of the key biotic interactions that can influence diversity patterns
in various ways is competition. Competition may reduce diversity by
competitive exclusion (Goldberg and Barton 1992; Valone and Brown 1995)
or enhance diversity through increasing specialization of species
(Abbott et al. 1977; Futuyma and Moreno 1988; Emerson and Kolm 2005).
Closely related taxa often compete for similar resources and these
competitive interactions can influence range limits of these taxa
(Terborgh and Weske 1975; Jankowski et al. 2010; Pasch et al. 2013).
However, interactions between distantly related taxa also have major
impacts on diversity patterns. Although a few studies have demonstrated
competition between distantly related taxa (Brown and Davidson 1977;
Eriksson 1979; Palmeirim et al. 1989; Hochberg and Lawton 1990; Jennings
et al. 2010) and the effect of competition between distantly related
taxa on diversity patterns has been inferred from the fossil record
(Jablonski 2008), this subject has not received much attention in
macroecology. In fact, the presence of closely related species in a
community has been used as evidence against the role of competition,
emphasizing instead abiotic filtering (Webb et al. 2002; Gómez Juan
Pablo et al. 2010; Tucker et al. 2017). However, recent advances in
ecological coexistence theory imply that strong differences in
competitive ability between distantly related competitors could cause
competitive exclusion even if niches are substantially different
(Chesson 2000; Mayfield and Levine 2010; Gerhold Pille et al. 2015).
Indeed, many empirical studies support this model for ecological
coexistence (Venail et al. 2014; Germain Rachel M. et al. 2016).

Here, we present observational and experimental evidence that suggests a
role for ecological competition between two distantly related clades in
shaping their complementary diversity patterns: songbirds (Phylum
Chordata, Class Aves, Subfamily Oscines) and ants (Phylum Arthropoda,
Class Insecta, Family Formicidae). While several studies have presented
evidence for competition or amensalism between ants and birds
(Supplementary table S1), they have not assessed the effect of these
interactions on patterns of species diversity. Ants are important
predators of other arthropods, especially in tropical and sub-tropical
lowland forests (Floren et al. 2002; Sam et al. 2014). They have been
experimentally shown to reduce numbers of other arthropods (Piñol et al.
2010; Karban et al. 2017). However, ants are absent or in very low in
abundance in tropical and sub-tropical montane cloud forests across the
world, for reasons that remain unclear (Janzen 1973; Samson et al. 1997;
Longino et al. 2014). The elevation at which cloud forests are found
varies with latitude and other geographic factors, but the lower
elevational limit usually lies between 1500m-2500 m and upper limit
ranges from 2400m-3300m (Stadtmüller 1987). One possible explanation is
that ground-nesting ants cannot persist in places that are wet
throughout the year whereas arboreal nesting ants cannot survive
freezing temperatures (Wheeler 1917; Janzen 1973; Samson et al. 1997).
On the other hand, although capable of living at low and higher
elevations, carabid beetles (Wilson 1987; Maveety et al. 2013),
songbirds (Price et al. 2014) and small mammals (Heaney 2001) are often
very diverse and abundant in these cloud forests. One mechanism for this
diversity and abundance maxima in cloud forests could be the lack of
competition with ants for arthropod prey (Heaney 2001; Price et al.
2014). We investigated this hypothesis in the eastern Himalaya.

In the eastern Himalaya, breeding songbird diversity peaks in the cloud
forests at elevations between 1200m and 2000m. Various historical and
dispersal hypotheses, including greater area, dispersal from both above
and below, and greater time for diversification at mid-elevations
(associated with climatic niche conservation) have little support as an
explanation for the mid-elevation peak in bird diversity (Price et al.
2014). The peak consists largely of small insectivorous bird species and
is associated with greater resource (i.e. arthropod) abundance,
potentially supporting more individuals and hence, more species (Price
et al. 2014; Schumm et al. 2019) (Fig. 1, Fig. S1 and Fig. S2). By
contrast, ants are almost absent in the cloud forests at mid-elevations,
even though they are highly abundant and diverse at the low elevations
(Fig. 1, Fig. S1 and Fig. S2) (Ghosh-Harihar 2013; Price et al. 2014).
Among ants, low elevations are dominated by an arboreal insectivorous
species, the Asian weaver ant \emph{Oecophylla smaragdina} which
disappears at about 900m elevation (K. Supriya pers. obs.). Weaver ants
forage both on the trees and on the ground, move between trees through
canopy connections and are highly aggressive (Basu 1997; Peng and
Christian 2005; Van Mele 2008). We evaluated the possibility that the
dominance of weaver ants at low elevations contributes to the lower
diversity of birds at these elevations due to competition for food
resources.

To test whether weaver ants and birds might compete for resources at the
low elevation, we first assessed if there is dietary overlap between
weaver ants and birds. A necessary pre-condition for competition between
two taxa is significant overlap in the use of the same limiting resource
(Brown and Davidson 1977). Previous research suggests that arthropods
are a limiting resource for songbirds in the eastern Himalaya (Price et
al. 2014) (also see Fig.1 and Fig. S2). Here, we compared the diet of
weaver ants and birds at the low elevation where they co-occur, and at
higher elevations where weaver ants are absent, to assess dietary
overlap. Next, we compared arthropod abundance and leaf damage due to
insect herbivory on trees with and without weaver ants and conducted a
weaver ant removal and exclusion experiment using a paired design to
assess whether weaver ants significantly reduce arthropod abundance on
trees.

\textbf{Methods}

\textbf{Study sites} All fieldwork was conducted over April-June of
2014-2016. Ants and birds were studied at the moist subtropical
broadleaved forests at low elevations in Chapramari Wildlife Sanctuary
(26.8858°N, 88.8341°E, \textasciitilde{} 200m) in West Bengal, India.
Bird diets were studied at Chapramari and in the temperate broadleaved
cloud forests at the mid-elevation site of Neora Valley National Park
(27.0611°N, 88.7707°E, \textasciitilde{} 2000m) in West Bengal. Bird
diets were also studied at 1200m elevation in Neora Valley National Park
(27.03°N, 88.78°E) and at 2300m (27.42°N, 88.20°E), 2700m (27.43°N,
88.19°E), 3200m (27.45°N, 88.17°E) elevations in Khangchendzonga
National Park in Sikkim, India.

\textbf{Molecular diet analysis}

\emph{Field and lab methods} To examine bird diets, between April-June
2016 we caught 103 birds and successfully collected 41 faecal samples at
200m and 2000m elevations at the Bengal sites (Supplementary table S2).
We caught an additional 107 birds and collected 82 faecal samples at
1200m, 2300m, 2700m and 3200m elevations in May 2017. All faecal samples
were stored in 95\% ethanol until DNA extraction. We used Qiagen QIAamp
DNA Stool Mini kit to extract DNA with some minor modifications in the
protocol, following Zeale et al. (2011).

To examine weaver ant diet, we collected items of food that the weaver
ants were carrying to their nest between April-June 2016. We collected
food items from 25 different colonies, either for an hour or until we
had collected 10 items, whichever happened first. We extracted DNA from
these samples by taking a small part of each of the prey items, crushing
it and then using the Qiagen DNeasy blood and tissue kit and following
the manufacturer's protocol for DNA extraction. We carried out a 25\selectlanguage{greek}μ\selectlanguage{english}L
PCR (polymerase chain reaction) to amplify a segment of 16S rDNA using
the lns16S-short primer pair (5'-3' TRRGACGAGAAGACCCTATA;
ACGCTGTTATCCCTAAGGTA) described by Clarke et al. (2014). We used this
primer set because it is invertebrate specific, so successful DNA
amplification indicates presence of prey DNA in the bird faecal DNA
extract. We sent successful PCR products to the Sequencing core at the
University of Illinois at Chicago, Chicago, Illinois, for barcoding,
pooling, size selection and sequencing on an Illumina Miniseq platform.
All PCR products had strong primer dimer bands that were removed at the
sequencing core before sequencing with a procedure that selected for
fragments longer than 150 bp. (see Supplementary methods for further
details)

\emph{Data analysis} We followed the dada2 pipeline (Callahan et al.
2016) to construct an amplicon sequence variant (ASV) table and then
created a FASTA file with all the unique sequences recovered from our
samples. We used blastn in blastplus to BLAST this FASTA file against
the NCBI nr database and used the NCBI-taxcollector script (Dias et al.
2014) to get detailed taxonomic information for the top hit of our BLAST
results. After filtering out bacteria and other contaminating sequences,
we estimated overlap between the diets of weaver ants and birds at low
and mid-elevations at various taxonomic levels using EcoSimR (Gotelli et
al. 2015) and visualized the overlap using venn diagrams and non-metric
multidimensional scaling (NMDS) in R (R Core Team 2014). EcoSim R is
used to estimate niche overlap between species and compare it to a null
distribution of niche overlap given information on resource utilization
(in columns) by each species (rows). The algorithm randomizes resource
utilization for each species by reshuffling the row values and generates
a null distribution of niche overlap. In our case, we considered birds
at low elevations (N = 18), birds at middle elevations (N = 15) and
weaver ants (N = 25) at low elevations as ``species'' and used the
frequency of occurrence of different orders or families in diets as the
``resources''. All the analyses were done in the R programming
environment and in the shell using the R packages dada2 (Callahan et al.
2016), DECIPHER (Wright 2016), VennDiagram (Chen and Boutros 2011),
ggplot2 (Wickham 2011), vegan (Oksanen et al. 2018) and EcoSimR (Gotelli
et al. 2015). All scripts and details of the steps are available as
supplements to this paper.

\textbf{Effect of weaver ants on arthropod abundance}

\emph{Field Methods} In June 2014 and May 2015, we measured arthropod
abundance at 17 pairs of trees with weaver ants present and trees
without weaver ants, where the trees were paired by species, height and
girth. To measure arthropod abundance, we beat the foliage of a tree
with a stick, and collected all the insects that fell on an upturned
umbrella ([?] 100cm diameter), which is similar to the method used by
Pi\selectlanguage{ngerman}ñol et al. (2012). We beat the foliage three times before collecting
arthropods in vials containing 95\% ethanol and repeated the process at
another part of the tree. Later in the field camp, we counted all the
collected arthropods, measured body length to the nearest mm, and
classified them to taxonomic order. To control for observer bias, the
person counting the arthropods was unaware of the presence or absence of
weaver ants on the source tree. We also estimated leaf damage on the
pairs of trees as a longer-term proxy of insect herbivore abundance,
with greater arthropod abundance implied by higher leaf damage. We used
two methods to assess leaf damage: (1) visual estimation of \% absent on
10 leaves from different parts of each tree, and (2) visual estimation
of \% absent on each leaf on a short (\textasciitilde{}0.5m) clipped
branch of the tree.

In April-June 2015 we carried out a weaver ant removal and exclusion
experiment using 15 pairs of trees, paired by species, height and girth.
On trees in the experimental treatment group, we removed all weaver ant
nests using a tree pruner and then applied a band of
Tanglefoot\textsuperscript{TM} around the trunk of the tree at about 1 m
height from the base of the tree. We plugged all the holes in the trunk
crevices under the Tanglefoot band with cotton. We were unable to
completely remove or exclude weaver ants from 6 out of 15 treatment
trees because of the nature of the bark of the tree trunk or the canopy
of the tree. On trees receiving the control treatment, we attached a
band of brown paper to make them appear similar to trees receiving the
experimental treatment and pruned a few branches to mimic the
disturbance caused by the removal of weaver ant nests. We measured
arthropod abundance at each of these trees using beating and
branch-clipping (Ozanne 2005) before and one month after the
experimental treatments. We also recorded leaf damage as an index of
insect herbivory on the clipped branch and on 16 random leaves at each
tree. We recorded arthropod abundances one year later in April-June
2016, but by that time a number of trees had gained or lost weaver ants
to an uncertain degree; results are presented in the supplement.

\emph{Data analysis} We used paired t-tests to compare arthropod
abundance and leaf damage between trees with or without weaver ants.
Arthropod abundance data were log-transformed with one added to all
values to avoid zeroes in the data. We also used paired t-tests to
compare the change in arthropod abundance and leaf damage over the
period of one month and over the period of one year between trees in the
weaver ant exclusion and control treatments. Since we were interested in
the effect of weaver ants on other arthropods, we removed the number of
all ants from our arthropod count. We also removed the number of insects
belonging to the suborder Homoptera, because weaver ants form
mutualistic associations with Homopterans (Peng and Christian 2005;
Crozier et al. 2009). All analyses were done in the RStudio programming
environment (R Core Team 2014).

\textbf{Results}

\textbf{Molecular diet analysis} Of the 41 faecal samples collected at
200m and 2000m elevations, we were able to amplify and sequence prey DNA
from 33 samples in total, from five bird species at low-elevations and
nine different bird species at mid-elevations (Supplementary table S2)
and all 25 weaver ant food samples. We were also able to amplify and
sequence prey DNA from 36 additional bird faecal samples at higher
elevations in Sikkim and 2 additional samples at 1200m elevation. Even
though the primer set we used was supposed to be invertebrate-specific,
it amplified some vertebrate taxa such as Squamata as well
(Supplementary table S3). We recovered 1331 amplicon sequence variants
(ASVs) from the dada2 pipeline. Of these, 1072 sequences yielded BLAST
matches which reduced to 980 sequences after filtering for bacteria and
contaminants (162 from birds at the 200m elevation, 224 from birds at
the 2000m elevation, 325 from weaver ants and 333 from birds at 1200m,
2300m, 2700m and 3200m elevations; note that this is greater than 980
because of overlap in ASVs among these groups).

The most frequent orders in the bird diet at all elevations were
Lepidoptera and Coleoptera (Fig. S3 \& Supplementary table S3). At 200m,
Lepidoptera was detected in 94\% and Coleoptera in 88\% of the samples.
Molecular diet analyses confirmed that Lepidoptera (69\%) and Coleoptera
(48\%) are common in weaver ant diet, in addition to Blattodea,
Hemiptera and Hymenoptera (all 61\%). Each of the orders present in
weaver ant diet was also found in bird diets (Fig. 2 \& Fig. S4,
Supplementary tables S3-S6) but birds had consumed animals in nine
additional orders. These included larger animals such as centipedes
(order Scolopendromorpha, 33\%), annelids (order Haplotaxida, 11\%),
molluscs (class Gastropoda, 5.5\%) and lizards (order Squamata, 5.5\%)
as well as some small arthropods such as springtails (order
Entomobryomorpha, 11\%), stoneflies (Plecoptera, 22\%), booklice
(Psocoptera, 5\%), thrips (Thysanoptera, 5\%) and earwigs (Dermaptera,
11\%). At elevations between 2000m and 3200m (where weaver ants are
absent), we identified all but one of the 18 orders present at the low
elevation, plus three more {[}lacewings (order Neuroptera, 44\%),
nemertean worms (order Monostilifera, 6\%) and mayflies (order
Ephemeroptera, 6\%){]}.

Our EcoSimR results showed that the diets of birds at low elevations and
weaver ants overlapped significantly more than expected from random
resource utilization at all taxonomic levels, albeit not at the ASV
level. Diet of birds from low and mid-elevations overlapped
significantly at order, family and ASV levels. Diet of birds from
mid-elevations and the diet of low elevation weaver ants (weaver ants
are absent at mid-elevation) did not overlap significantly at finer
taxonomic scales (Supplementary Table S7). Lower diet overlap at finer
taxonomic scales could be a function of sampling as indicated by the
absence of asymptote in accumulation curves at finer taxonomic levels
(Fig. S5) or the difference in arthropod species across the elevational
gradient.

\textbf{Effect of weaver ants on arthropod abundance} We found no
significant difference between number of arthropods on trees with or
without weaver ants (Fig. S6a). However, the number of insects belonging
to orders Coleoptera and Lepidoptera, the two most common orders in bird
diet at all elevations, was 1.7 x higher on trees without weaver ants
than on trees with weaver ants (Fig. 3a). Leaf damage estimated from 10
leaves on each tree was significantly greater on trees without weaver
ants than on trees with weaver ants (Fig. 3b) and showed a
non-significant trend in the same direction on the clipped branches from
these trees (Fig. S6b).

\textbf{Weaver ant removal and exclusion experiment} One month after
weaver ant removals and exclusion, the numbers of arthropods(excluding
ants and homopterans) had increased from 4.67 ± 0.95 SE to 12.73 ± 1.26
SE, N = 15, while controls showed no significant change (before: 7.73 ±
1.06 SE, after one month: 9.8 ± 1.51 SE, N = 15). Overall, the
difference between the change over time in treatment and control trees
was significant (paired t-test, N = 15, P = 0.018) (Fig. 4). In the
following year, the increase in number of arthropods on treatment trees
was greater than control trees, but the difference was not statistically
significant (Figs. S7, S8). Some of the experimental trees had been
recolonized by ants over the course of the year, which might contribute
to the reduced effect (Figs. S7, S8).

In the experimental treatments, two taxonomic orders of insects were
responsible for the increase on experimental trees with respect to
controls (Fig. 5a). Lepidoptera increased significantly in abundance
(Lepidoptera: treatment 1.0 ± 0.5 SE, control -0.5 ± 0.3 SE, N=15,
paired t-test P = 0.01) and Coleoptera showed a large increase that was
close to significance (treatment 1.3 ± 0.9 SE, control -0.3 ± 0.4 SE, P
= 0.13). These orders are also the most frequent components of bird
diets at all elevations (Fig. 5b \& Fig. S3). On the other hand,
Hemiptera decreased after weaver ant exclusion (paired t-test N=15;
treatment -8.8 ± 5.5 SE, control 4.3 ± 4.9 SE, P = 0.09). This decrease
is expected given that it contains the suborder Homoptera and weaver
ants form mutualistic associations with Homopterans (Peng and Christian
2005; Crozier et al. 2009).

\textbf{Discussion}

In this study, we set out to ask if weaver ants were likely to compete
with birds for arthropods at low elevations in eastern Himalaya. Our
results show significant diet overlap between weaver ants and birds.
Moreover, we found lower arthropod abundance on trees with weaver ants
and confirmed experimentally that weaver ant removal led to increased
abundance of arthropods. This is important because a greater abundance
of arthropods at mid-elevations correlates with the presence of many
small insectivorous bird species at these elevations (Price et al.
2014). Further, the two most common arthropod orders in bird diets at
low elevations were the ones to increase most in abundance after weaver
ant removal. Since weaver ants are found only at low elevations, they
could contribute to the presence of fewer arthropods at these elevations
rather than higher up. Together, these results suggest that weaver ants
reduce food availability for birds at low elevations in the eastern
Himalaya.

Our molecular diet analyses showed overlap in the diet of weaver ants
and low elevation birds at all taxonomic levels and overlap in the diet
of weaver ants and mid-elevation birds at higher taxonomic levels. While
many previous studies have presented evidence for competition between
birds and ants (Supplementary table S1), this appears to be the first
study that has quantified overlap in diet between an ant species and
insectivorous songbirds. Our work also demonstrates the utility of
molecular diet analyses to examine dietary niche overlap between
distantly related organisms. Studies of dietary niche partitioning among
species are increasingly using molecular tools to get fine-scale
taxonomic information on diet composition (Razgour et al. 2011; Brown et
al. 2014; Kartzinel et al. 2015; Arrizabalaga\selectlanguage{english}-Escudero et al. 2018). Due
to PCR and sequencing biases, these methods may not give accurate
information on the quantity of dietary items (Pompanon et al. 2012;
Shokralla et al. 2012). Still, we think that information on the identity
and frequency of dietary items obtained using these methods can be very
useful to understand the diversity patterns of dietary guilds along
environmental gradients.

Reduced herbivory and reduced abundance of large arthropods on trees
with weaver ants have been demonstrated previously. Asian weaver ants
have been long used as a biological control agent and are still used to
control pest populations in orchards (Way and Khoo 1992; Peng and
Christian 2005; Van Mele 2008; Thurman et al. 2019). A recent review
found that weaver ants significantly reduce pest populations on tropical
tree crops (Thurman et al. 2019). Offenberg et al. (2004b) showed that a
leaf beetle species avoided eating leaves with weaver ant pheromones on
them. In mangrove forests, the presence of weaver ants is negatively
correlated with leaf damage due to herbivores (Offenberg et al. 2004a),
as we also found in this study. More broadly, many plant species form
strong mutualistic associations with ants to reduce the risk of
herbivory by offering them rewards such as food bodies, extra-floral
nectaries and domatia (i.e. nesting sites) (Janzen 1966; Fiala et al.
1989; Webber et al. 2007; Chomicki et al. 2016). Even facultative or
opportunistic ant-plant interactions are known to reduce herbivory and
deter other arthropods from plants occupied or visited frequently by
ants (Bentley 1977; Fiala et al. 1994; Chamberlain and Holland 2009;
Rosumek et al. 2009; Trager et al. 2010).

We suggest that the relatively high arthropod abundance at
mid-elevations in the eastern Himalaya is partly a consequence of
reduced ant predation, but this does not exclude contributions from
other factors, including differences in primary productivity (Acharya et
al. 2011), plant diversity and density (Acharya et al. 2011) and higher
seasonality (Supriya et al. 2019). Overall, our results lend support the
idea that competition from ants could contribute to mid-elevational peak
in songbird diversity in the eastern Himalaya. A similar link between
competition with ants and diversity patterns was recorded by Brown \&
Davidson (1977) who found complementary diversity patterns in response
to annual precipitation in seed-eating ants and rodents along a
north-south gradient in the US. Likewise Heaney (2001) suggested
competition with ants may be responsible for the peak in small mammal
diversity in cloud forests in the Philippines. Other studies have shown
patterns of complementary diversity patterns between ants and other
arthropod groups, such as staphylinid beetles, carabid beetles and
spiders (Holldobler and Wilson 1990; Halaj et al. 1997; Noreika and
Kotze 2012). More generally, the near-absence of ants in cloud forests
(Longino et al. 2014) could be an important explanatory factor for the
high diversity of many other taxa there.

A recent meta-analysis of ant species diversity patterns along
elevational gradients found some support for a model whereby temperature
and precipitation interact to affect ant diversity, i.e. there is a
significant relationship between temperature and ant diversity on 83\%
of wet mountains compared to only 25\% of arid mountains (Szewczyk and
McCain 2016). Still, the question of why ants are so rare in montane
cloud forests remains largely unanswered. Previous studies have
suggested that the combination of year-round cool temperatures and
humidity make cloud forests unsuitable for ants (Wheeler 1917; Janzen
1973; Samson et al. 1997). However, competition from endotherms such as
birds and mammals might also contribute to limiting ant distributions,
in much the same way that we postulate ants affect birds in the warm and
wet lower elevations. Experimental tests to compare the importance of
abiotic versus biotic effects in shaping the pattern of ant abundance
along elevational gradients are much needed.

Overall, our observational and experimental data suggest that birds and
ants compete for arthropod prey at low elevations in the eastern
Himalaya. As diversity patterns of taxa shift due to climate change, it
is important to monitor these patterns and compare patterns of such
distantly related but potentially competing taxa. We advocate for more
studies on ecological interactions between distantly related taxa in
shaping diversity patterns, because these interactions could dampen
(Suttle et al. 2007) or enhance the effect of climate change on species
abundance and range distributions, depending on the actors involved
(Davis et al. 1998).

\textbf{Acknowledgements}

We thank members of the Moreau lab, Price lab and Jablonski lab for
discussions, David Jablonski and Cathy Pfister for their comments on an
earlier version of the MS, and Daniel Hooper for his original insights
into the possible competitive effects of ants on birds. We would also
like to thank Amir Chhetri, Suresh Brar, Suresh Rana, Deepika Karmarkar,
Jobin Varughese, Vinod Shankar, Priyanka Das, Ritobroto Chanda, Pranav
Balasubramanian, Sumit Bawalia and Abhijit Dey for assistance in the
field. We are grateful to Amruta Varudkar, Prachi Thatte, Krishnapriya
Tamma, Jyothi Nair, Anubhab Khan, Kevin Feldheim, Brian Wray and Stefan
Green for help with lab work. Data collection was funded by a Young
Explorer grant from National Geographic Society, a Rufford Small Grant
and Henry Hinds Fund for Graduate Student Research in Evolutionary
Biology to K. Supriya. K. Supriya was supported by a Field Museum Women
in Science Fellowship at the time of data collection and a Faculty for
the Future fellowship from the Schlumberger Foundation at the time of
writing this manuscript. We would also like to thank two anonymous
reviewers for their comments on a previous version of this manuscript.

\textbf{Ethics}

This study was approved by Institutional Animal Care and Use Committee
at the University of Chicago (ACUP \# 71393). We also received
permission for our fieldwork from the West Bengal Forest Department
(1423/WL/4R-1(Part-XII)/2014 and 2702/4R-6/2015).

\textbf{Data, code and material}

All data and code used for analyses is available on a github repository
(https://doi.org/10.5281/zenodo.2651358)

\textbf{Competing interests}

The authors have no competing interests

\textbf{Authors contributions}

All authors conceived and designed the study. K.S. carried out
fieldwork, molecular lab work, data analyses and drafted the manuscript.
C.S.M. and T.D.P critically revised the manuscript. \emph{All authors
gave final approval for publication and agree to be held accountable for
the work performed therein.}

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\textbf{Figure legends:}

\textbf{Figure 1: a.} Correlation between number of songbird species and
individuals along an elevational gradient in the eastern
Himalaya\textbf{b.} Correlation between number of songbird species and
arthropod abundance along the same gradient. Site at 200m and 2000m
elevation examined further in this study shown in blue and yellow
respectively. Based on data in (Price et al. 2014)

\textbf{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 phylopic.org. Anthony Caravaggi produced the bird
silhouette.

\textbf{Figure 3:} \textbf{(a-b)} Data from 34 trees paired for size and
species, where one tree had weaver ants and the other did
not.\textbf{a.} Number of arthropods belonging to the orders Lepidoptera
and Coleoptera \textbf{b.} Leaf damage estimated on 10 leaves
distributed around the tree.

\textbf{Figure 4:} Number of arthropods (excluding ants and suborder
Homoptera) before and one month after weaver ant removal and exclusion
experiment. Data from 30 trees paired for size and species, with one
member of each pair subject to weaver ant removal and exclusion. Grey
lines and dots show control trees and purple lines and dots show
treatment trees. Darker dots and lines indicate multiple overlapping
points.

\textbf{Figure 5} \textbf{a.} Difference in mean abundance of major
arthropod orders in control and treatment trees one month after ant
exclusion and removal (derived from the data in Fig. S7)
\textbf{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)\selectlanguage{english}
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