3.2 The role of ant competitive hierarchy on network
structure
Dominant ants realized, on average, 3.12 \(\pm\) 2.02 (mean \(\pm\) SD)
interaction and subordinate ants realized 2.47 \(\pm\ \)1.79
interactions with plants along the networks. Dominant ants represented,
on average, 71.62% \(\pm\) 18.1% of all the ant species in the
networks, and this proportion was not affected by variation in the mean
precipitation rate (GLMM; \(\chi^{2}\) = 1.743, df = 1, p = 0.186).
However, the connectance of dominant (\(C_{d}\)) and subordinate
(\(C_{s}\)) ant species varied along the precipitation gradient.
Contrary to our expectations, both increased as the precipitation rate
decreased (\(C_{d}\): 0.38 \(\pm\) 0.16; LMM; \(\chi^{2}\) = 16.209. df
= 1, p < 0.001; Estimate = 1; Fig. 3a; \(C_{s}\): 0.32 \(\pm\)0.22; LMM; \(\chi^{2}\) = 10.441, df = 1, p < 0.001; Estimate
= 0.99; Fig. 3b).
The mean RR for dominant ant species was 0.72\(\ \pm\ \)0.16, and it
was not influenced by the variation in the mean precipitation rate (GLM;\(\chi^{2}\) = 0.093, df = 1, p = 0.173; Fig. 2c). In the same way, the
mean RR of subordinate ants (0.79\(\ \pm\ \)0.21) was similar to the
RR of dominant ants and it was also not affected by variation in the
mean precipitation rate (GLM; \(\chi^{2}\) = 0.149, df = 1, p = 0.149;
Fig. 2d). We also observed no variation in Jaccard index along the
precipitation gradient (0.40 \(\pm\) 0.31; GLS; \(F_{1,34}\) = 0.244, p
= 0624), indicating that the dominant-subordinate overlap in the plant
usage is not influenced by water availability.
We observed a data gap at intermediary levels of precipitation rate
(254.11 to 433.82 mm – see Fig.1). Because of this gap, data from the
wettest networks in our dataset (representing one paper with 12 networks
form the same location, all with mean precipitation rate of 433.82 mm)
could behave as outliers, compromising the fit of our models. To
evaluate this bias, we removed these networks from our dataset and
performed again all analyses described in the Statistical
analysis section. In all cases, there was no qualitative change in any
results described above, indicating that our results were not biased by
the asymmetric distribution of the networks along the precipitation
gradient (see Support information).
DISCUSSION
Our results showed that as the habitats become drier, the number of ant
and plant species interacting with each other decreases, resulting in
smaller networks. However, the mean precipitation rate did not affect
any other metric describing the structure of the ant-plant networks.
Also different from our expectations, the connectance of dominant and
subordinate ants increased as the mean precipitation rate declined.
Considering that connectance is a metric that intuitively accounts for
the probability that any pair of species interact in the network (Landi
et al. 2018), it indicates that the decline in water availability at the
broader scales increases the generalization of ant-plant interactions.
In turn, it increases the chances of both dominant and subordinate ant’s
species interact with species of EFN-bearing plants. Interestingly, the
mean precipitation rate did not affect the RRd,
RRs, or Jaccard similarity index. This lack of effect
suggests that changes in the connectance of dominant and subordinate ant
species are not due to changes in competitive ant behavior towards the
plants but rather a consequence of processes related to the variation in
species richness of the network along the precipitation gradient.
Several studies have shown that ant and EFN-plant communities’ richness
declines along precipitation gradients (Dunn et al. 2009, Luo et al.
2022, Queiroz et al. 2022). For this reason, it is not surprising that
the species richness of ant-plant networks declined with the decline of
the mean precipitation rate. However, we observed no effect of the mean
precipitation rate on any other descriptors of the ant-plant networks.
This is an unexpected result since species richness of networks network
is a crucial trait shaping the structure of ecological networks
(Boccaletti et al. 2006, Minoarivelo and Hui 2016, Mariani et al. 2019)
and, the influence of the mean precipitation rate on it would lead to
modifications in other aspects of the network. However, we controlled,
in our analysis, the effects of species richness on the metrics
describing the network structure (connectance, nestedness, and
modularity), either by using normalized metrics (as zQ and zNODF) or the
species richness as a weighted factor in our models. For this reason,
the lack of precipitation effect on the network structure indicates that
variation in water availability had no additional effect on the network
structure other than those deriving from the variation in the species
richness. This finding directly contrasts with the results from studies
evaluating the role of precipitation in ant-plant networks at the local
scale (Rico-gray et al. 1998, Rico-Gray et al. 2012, Câmara et al.
2018). At local scale, variation in climatic conditions across the
sampled habitats tends to be relatively smaller, resulting in the
observation of ant-plant interactions along a relatively narrower
precipitation gradient than ours. Therefore, it is possible that the
role of water availability in shaping the patterns of ant-plant
interactions at the community level is relatively stronger at the local
scale, with effects at a broader scale being likely an indirect
consequence of its effects on the diversity of ant and plant assemblages
across communities.
Similar to the metrics describing the structure of the ant-plant
networks, the progressive generalization of ant-plant interactions along
our macroecological water gradient is likely a consequence of the
negative effects of water scarcity on the number of ant and plant
species interacting with each other along the precipitation gradient.
Two mechanisms can drive this increased generalization. First, it is
possible that the decline of the richness of plant species available in
the drier environment increases the relative value of any plant partner
to ants, regardless of its quality. In this scenario, interacting with
the maximum of plant species available can be as or more advantageous to
dominant ants than monopolizing the few high-quality plant species
available in drier habitats. By reducing the monopolization strength of
high-quality plants, dominant ants allow the visitation of the
subordinate ones to more plant species explaining why both dominant and
subordinate ants increased their connectance with the decline in the
mean precipitation rate. Alternatively, it is possible that, although
less rich, plant species with EFNs are more abundant in drier habitats.
Since ants and plants are sessile organisms, their interaction depends
on how close the ant nests and plants are (Dáttilo et al. 2013c), and
this proximity should increase as more abundant ant and plant species
populations are. In this case, any increase in the abundance of plants
with EFNs in a community may increase the probability of all plant and
ant species interacting, leading to a more generalized pattern of
ant-plant interaction.
Our results related to the mean RRd,
RRs, and the Jaccard index supports the mechanism
stating the increase in probability of interaction due to abundance and
spatial distribution of ants and plants. The resource range is a
normalization of how many links a given species makes, being not
influenced by the network connectance or species richness (Poisot et al.
2012). Then, if the generalization in the patterns of ant attendance to
EFN plants in drier environments would be driven by changes in the
competitive behavior of ants, we could expect an increase in the mean RR
for both dominant and subordinate ants. Additionally, if both dominant
and subordinate ants use more plant species available as drier the
habitat, we may expect an increase in dominant-subordinate overlap as
the mean precipitation rate decreases. However, we observe no effect of
the mean precipitation rate on the RR values or the degree of
dominant-subordinate overlapping along the gradient. It indicates that
ants and plants interact similarly along the water gradient, and the
generalization in the patterns of ant-plant interaction may be just a
consequence of an increased probability of ant-plant species interaction
in poorer drier communities.
The preponderant role of the species richness on the generalization of
ant-plant interactions may have two significant ecological implications
for the dynamic of these interactions across habitats. First,
theoretical models have shown that connectance tends to beget stability
and persistence of mutualistic networks in space and time (e.g. Thébault
and Fontaine 2010, Sauve et al. 2014). Therefore, more connected
ant-plant networks may be more stable (but see Allesina and Tang 2012)
and persistent than the ones from wetter habitats. In this case, by
increasing the overall connectance and the connectance of subordinate
and dominant ant species in the networks, the decline in water
availability may indirectly increase ant-plant interaction persistence,
including its persistence in the face of environmental disturbances. In
the face of the current biodiversity crisis, it suggests that, at
broader scale, the decline in water availability may be associated with
a decline in the susceptibility of ant-plant interactions to
environmental disturbance. Like the ones driven by human activities and
climate change.
Second, although water availability had no effect on the competitive
behavior of dominant ant species, it is likely that they are the main
ones driving the direct and indirect effects among ant and plant species
with EFNs and the network structure along water gradients. Due to their
numerical and behavioral dominance, dominant ant species are commonly
the most connected species in ant-plant networks worldwide, holding a
higher number of interactions with the plant species available (Dáttilo
et al. 2014b, Costa et al. 2016). Although both dominant and subordinate
ants became more connected to the plants as the precipitation declined,
water availability did not affect the degree of the network nestedness,
suggesting the maintenance of the role of dominant ant species in
regulating this mutualism, regardless of water availability. Finally, it
is important to highlight that the significant role of community
richness in shaping mutualistic networks along environmental
macroecological gradients has already been reported in other studies
using other mutualistic systems as a model, like pollination (e.g.
Devoto et al. 2005, Lance et al. 2017). It suggests that the role of
community diversity in shaping mutualistic networks at a broader scale
is not restricted to ant-plant interactions, being more general than
previously expected.
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