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‐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 (Hölldobler 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).