4. Discussion
We observed different patterns in species assemblage distribution over
small-scale 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. We found consistently greater horizontal turnover
in ant assemblages than that observed at comparable vertical distances.
However, a distance-decay pattern of ant turnover was only detected
vertically, but not horizontally, with the high species turnover between
horizontal positions being independent of distance. The observed
patterns confirmed our hypotheses, and are likely
associated with continuous
directional changes in environment (i.e., microclimate) and resources
vertically, and stochastic variation in environment and resources, 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 pattern of
microclimate variables in our study plot is in line with other studies
in tropical rain forest: 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 important 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 result not only 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 we 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). 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 Mexican
rain forest, 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 is likely to 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 less common in the Asian
tropics compared with the American tropics (Dial et al. 2004a). Such
lack of connectivity between individual trees means that each tree
canopy might function as an island within the forest (Southwood and
Kennedy 1983, Adams et al. 2017).
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 (Basset et al. 2012).
Our findings need to be interpreted with the caveats of our limited
sampling time and sample size. Microclimate information was only
monitored for 24 hours for each transect, although we feel that this is
representative of longer-term patterns (see above). Future studies
involving long term monitoring of microclimate at fine spatial scales
are needed to understand the distribution of microclimate with the
complex three-dimensional structure of tropical rain forest. In
addition, although our sampling intensity per tree was high, we sampled
across a relatively limited spatial area and some samples were missing.
Future research using greater numbers of replicates at larger spatial
scales are needed to confirm the generality pattern we report here.