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

Abstract

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.

Spatial covariance of herbivorous and predatory guilds of forest canopy arthropods along a latitudinal gradient

“…Plowman et al . (2019) reported a greater importance of nesting sites (microhabitat diversity and composition) as opposed to elevation (temperature) in a whole‐forest study of arboreal ants from PNG, which suggests that nest space, the availability of sugar‐rich resources (Davidson et al ., 2003), and the lack of an unfavourable season could support a greater abundance of ants in tropical canopies.…”

“…This non‐linearity can probably be attributed to the distribution of microhabitats, some of which were found only on large‐canopy trees, or trees with numerous lianas or epiphytes (Klimes, 2017). Indeed, microhabitat richness was found to be the main driver for the pattern in arboreal ant communities in other extensive whole‐forest studies (Klimes et al ., 2015; Mottl et al ., 2019; Plowman et al ., 2019). A similar pattern of tree biomass affecting the abundances of arboreal arthropods has also been observed in specialist herbivores feeding only on a single tree genus or species (Novotny et al ., 2006, 2010).…”

“…For ants, we hand‐collected foraging individuals during a standardised search immediately upon felling, beginning with the base of the trunk and working up to the top of the canopy (Klimes et al ., 2015). In addition, we sampled all ant nests found during the destructive sampling (including cryptic nests inside tree tissues and the attached epiphytes and lianas, see details in Klimes et al ., 2015; Mottl et al ., 2019; Plowman et al ., 2019). The size of nests was visually estimated on a three‐level categorical scale: (1) <100, (2) 100–1000 and (3) >1000 individuals.…”

Environmental drivers of ant dominance in a tropical rainforest canopy at different spatial scales

“…According to our results, tree size combined with a higher abundance of climbing plants results in higher species richness than either of these two factors alone (Adams et al ., 2017). Interestingly, a similar pattern was found in a lowland forest of Papua New Guinea (Plowman et al ., 2020), where small and medium trees host significantly over‐dispersed ant communities, while large trees harbour a random distribution. On the other hand, the increased richness of epiphyte/climbing plants was associated with higher levels of ant dominance, revealing an unpredictable pattern.…”

Ground-dwelling arthropods of pinyon-juniper woodlands: Arthropod community patterns are driven by climate and overall plant productivity, not host tree species

“…How isolated a tree is may affect what GDAs occur under it. These effects may be exacerbated in species such as ants, which typically stay close to nests [51]. Our trees varied in their level of isolation, as the densities of trees varied across sites (Fig 2).…”