4 Discussion
We observed different patterns in species assemblage distribution over
vertical and horizontal gradients in a complex habitat. The high local
diversity of arboreal ants can be explained by high environmental and
habitat heterogeneity along both horizontal and vertical dimensions. The
replacement of individuals and species, rather than their loss is the
major component of total assemblage dissimilarity both vertically and
horizontally, indicating a potential environmental filtering process in
maintaining high biodiversity across small spatial scales (Baselga
2010). We found consistently greater horizontal turnover in ant
assemblages than that observed at comparable vertical distances.
However, increase in turnover with distance was only detected
vertically, but not horizontally, with the high species turnover between
horizontal positions being independent of distance. The observed
patterns are likely associated
with continuous directional changes in environment (i.e., microclimate)
and resources vertically, and stochastic variation in environment and
resource, as well as poor connectivity horizontally.
We found the spatial distribution of ant assemblages was associated with
heterogeneity in air temperature and relative humidity within the
three-dimensional structure of the forest. The spatial distribution
pattern of microclimate variables in our study plot is in line with
other studies: air temperature increased with height above ground, while
relative humidity decreased (Davis et al. 2019, De Frenne et al. 2019).
Horizontal variation in microclimate was also high but without
directional change, presumably relating to variation in vegetation
structure within and between trees (Fetcher et al. 1985, Scheffers et
al. 2017). Such non-directional horizontal variation in microclimate may
contribute to the high turnover of ant assemblages and lack of
distance-decay pattern in this dimension at the scales we sampled. Given
that turnover at very short horizontal distances was already nearly
maximal, it would not have been possible for turnover to then be even
greater with increasing horizontal distance. Tropical arboreal ants show
thermal adaptation to their vertical habitat use through their
physiology (Kaspari et al. 2015), morphology (Law et al. 2020) and
nesting site selection (Plowman et al. 2019). The high variance in
microclimate generates diverse thermal niches for ant species with
different thermal tolerances, and hence can facilitate co-existence of
multiple species at small spatial scales (Lessard et al. 2009). For tiny
ectotherms like ants, fine-scale environmental heterogeneity can play an
essential role in defining their distributions, probably due to the
small foraging range and small body size of ants, and the thermal
diversity of the environment (Ribas and Schoereder 2007, Fayle et al.
2010, Klimes et al. 2012, Kaspari et al. 2015, Bütikofer et al. 2020).
The association between microclimate pattern and ant assemblage
composition may not only result from direct effects through
environmental preferences of ant species, but can also be a consequence
of indirect biotic effects, for example if microclimate influences ant
food availability. However, the relative importance of direct and
indirect microclimate influences on ant assemblage composition remains
to be investigated.
In addition to microclimate, biotic influences such as resource
limitation and vegetation structure may also contribute to the high
horizontal turnover observed. The decline in ant richness and abundance
with height in the canopy could be due to reduction in leaf area which
limits foraging range and nest site availability (Adams et al. 2019,
Plowman et al. 2019). However, partitioning analyses of vertical
pairwise dissimilarity demonstrated that the changes in ant assemblages
was mostly driven by the turnover of the assemblage rather than the
nestedness of assemblage, which implicated that environmental filtering,
rather than resource limitation is a more plausible explanation (i.e.
different resources, rather than just less of the same resources). Ant
assemblage composition can be affected by vegetation structure such as
tree size, number of branches, and cavity diversity (Powell et al. 2011,
Yusah and Foster 2016, Adams et al. 2019, Plowman et al. 2019). Hence,
we can expect distinct ant communities to be hosted by different
individual trees and that the distance between trees per se might not be
driving assemblage dissimilarity. The lack of distance effects on high
horizontal turnover arboreal ants in our study is consistent with that
found in canopy ant assemblages across greater horizontal distances
(100-700 m) in rain forest of Mexico, whereas a distance-decay pattern
was observed in ground ant assemblages (Antoniazzi et al. 2021). For
social insects like ants where workers are wingless, vertical movement
of workers within the colony tree can be less challenging than movement
between trees, especially without vegetation connections such as lianas
(Yusah and Foster 2016, Adams et al. 2019), which may be lessened in
Asian versus American tropics (Dial et al. 2004a). Such barriers between
individual trees could make each tree canopy function as an island
within the forest (Southwood and Kennedy 1983, Adams et al. 2017).
We did not find any evidence to support the existence of ant mosaics at
the scales sampled, since turnover was not higher in the upper canopy,
where ant mosaics are more likely to influence community structure
(Ribeiro et al. 2013, Yusah et al. 2018). However, previous work
conducted close to our study site has found evidence for segregation
between species in the high canopy (Yusah et al. 2018). This could be
because our horizontal sampling grain was 20 m, and hence most adjacent
horizontal samples might already fall in different territories, on
different trees, with the exception of species with very large
territories such as Dinomyrmex gigas (Pfeiffer and Linsenmair
2001). This is supported by the high horizontal turnover in species
composition that we observed, regardless of distance across all heights
in the canopy.
Our findings offer important insights into the way in which biodiversity
is maintained at fine scales in complex three-dimensional habitats. The
high species richness discovered within the 130 m long by 70 m high
vertical plot in our study site represents a high proportion of ant
diversity at larger scales. The number of species that we detected using
precision fogging across vertical strata among 11 trees reached about
40% of the number of species sampled in a similar forest habitat
elsewhere in Sabah from 99 trees (Floren et al. 2014). This finding of a
relatively large proportion of regional biodiversity being sampled from
small plots within rain forest is in line with patterns for herbivorous
insects (Novotny et al. 2007), birds (Huang and Catterall 2021) and
butterflies (Daily and Ehrlich 1995), and is likely driven by high
structural complexity at small scales. The high turnover of the ant
assemblages across short horizontal and vertical distances explains how
a high proportion of local biodiversity of arthropods can be detected by
comprehensive sampling of the ground and canopy at small spatial scales
(Basset et al. 2012). While previous studies have pointed out the
importance of including both canopy and understory communities when
examining assemblage composition at large spatial scales (Ashton et al.
2016), our results suggest that comprehensive sampling covering multiple
horizontal and vertical planes may allow detection of a large proportion
of certain species that utilize multiple dimensions of the space, even
when sampling a relative small area of the habitat. Future sampling
across multiple vertical and horizontal replicates at microhabitat
scales for various taxa, particularly for groups that are sensitive to
microclimate and with limited dispersal abilities such as non-flying
arthropods, are needed to test the generality of high community
dissimilarity over small horizontal distances.
Our study highlights the great conservation value of primary tropical
rain forest in supporting high diversity of arboreal arthropods.
Compared with disturbed forest, primary tropical rain forest exhibits
higher complexity in vegetation structure, greater level of vertical
stratification and larger vertical distances due to the presence of
taller trees (Klimes et al. 2012, Liu et al. 2016). For instance, in oil
palm plantation, which has displaced large areas of forest in tropical
Asia (Fitzherbert et al. 2008), canopy ant species richness is 52%
lower compared with primary rain forest, possibly due to the
simplification of canopy structure (Fayle et al. 2010). The observed
high beta diversity supported by vertical forest layers in our study
indicates a potential risk of biodiversity loss among arboreal insects
with the reduction of these important vertical niches through logging
and conversion to plantation. The heterogeneity in microclimates of rain
forest may also increase the climate change resilience of tropical
species by providing them high microclimate heterogeneity to track their
optimal climatic niche for activity (Scheffers et al. 2013, Pincebourde
and Suppo 2016).