Nest microhabitats and tree size mediate shifts in ant community structure across elevation in tropical rainforest canopies

Declines or mid-elevation peaks in invertebrate diversity with elevation
are often attributed to climate and geometric constraints. However,
vegetation structure may also drive diversity patterns, especially for
tree-dwelling species, via its effects on microhabitat use and competitive
interactions. Here we investigate these effects on the diversity and
community structure of tree-nesting ants over elevation. We exhaustively
sampled ant nests in 1254 trees within continuous plots of primary
rainforest at low (200 m a.s.l.), mid (900 m a.s.l.) and high (1800 m
a.s.l.) elevation in Papua New Guinea. Ant diversity, nest abundance and
tree occupancy peaked at mid-elevation. Although host tree diversity also
peaked at mid-elevation, there was low specialisation of ant species to
tree species at all elevations. Mid-elevation trees hosted more species,
more nests and a greater diversity of nest types than trees of a similar
size at low or high elevation. Tree size and nest microhabitat use were
the strongest predictors of species composition, explaining twice as much
of the variability in the communities than elevation. At mid to high
elevation there were proportionally fewer large nests than in the
lowlands, with an increase in smaller nests in live hollow twigs and
epiphytes. There was high species turnover between elevations, and between
trees within elevations. Species co-occurrence patterns within trees
differed with tree size, and with elevation. In large trees species tended
to co-occur at random at low and high elevation, but co-occurred more
often than expected by chance at mid elevation, indicating an elevational
shift in competitive interactions. We conclude that the more extreme
diurnal temperatures at higher elevations, combined with increased
epiphyte availability, drive ants to nest in more insulated microhabitats.
This results in smaller colony sizes, and a decrease in interspecific
competition, thereby boosting species co-existence at mid elevation.