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In arthropod community ecology, species richness studies tend to be prioritised over those investigating patterns of abundance. Consequently, the biotic and abiotic drivers of arboreal arthropod abundance are still relatively poorly known. In this cross-continental study, we employ a theoretical framework in order to examine patterns of covariance among herbivorous and predatory arthropod guilds. Leaf-chewing and leaf-mining herbivores, and predatory ants and spiders, were censused on > 1000 trees in nine 0.1 ha forest plots. After controlling for tree size and season, we found no negative pairwise correlations between guild abundances per plot, suggestive of weak signals of both inter-guild competition and top-down regulation of herbivores by predators. Inter-guild interaction strengths did not vary with mean annual temperature, thus opposing the hypothesis that biotic interactions intensify towards the equator. We find evidence for the bottom-up limitation of arthropod abundances via resources and abiotic factors, rather than for competition and predation. publishedVersion

Understanding forest regeneration processes is increasingly important as disturbed forests spread rapidly in tropical landscapes. While successional dynamics are relatively well‐understood for plants, they remain poorly known for tropical insects, particularly in montane rainforests and tree canopies. We studied the effect of montane forest succession on arboreal ant communities in New Guinea near their natural elevational limit (1800 m a.s.l.). We censused ant species and nests in replicated 0.1‐ha plots of primary, old and young secondary forests, destined to be felled for gardens by the local landowners. Overall, 1249 felled trees were dissected for all ant‐associated microhabitats. We used rarefaction‐based models manipulating vegetation structure and multivariate randomizations to assess the effects of tree density, tree size, and nesting microhabitats on the ant communities. We expected increased ant species diversity, microhabitat specialization, and species turnover among trees throughout the succession, with distinct community composition among the stages. In contrast to our expectations, ant species composition and diversity per plot did not change during succession, and species similarity among trees was significantly higher only in old secondary forest. The number of ant species per tree increased with forest age, but trees of similar size hosted only slightly more species in primary than secondary forests. Many ant species were associated with particular nesting microhabitats; however, the same microhabitat types occurred in all stages and thus did not generate successional trends in ant communities. Secondary succession had an unexpectedly low impact on tree‐dwelling ant communities in this montane ecosystem, in contrast with our previous findings from lowland rainforests. These results highlight the need to study successional processes along entire elevational gradients, as montane taxa may react differently to vegetation changes than their lowland counterparts.

JGP was jointly funded by a Natural Environment Research Council studentship, Grant number NE/H524930/1, and the University of St Andrews. Part of this work was funded by the Rural & Environment Science & Analytical Services Division of the Scottish Government. Ant-repellent floral volatiles offer one method through which plants can mediate the detrimental effects of ants on flowers. Although the repellence itself is well-documented, the volatiles involved are less well explored. Here we investigated the floral bouquet of ant-repellent male flowers of Petasites fragrans, identifying 4-methoxybenzaldehyde as the main component. 4-methoxybenzaldehyde significantly repelled ants when presented in isolation in an olfactometer and thus is the likely source of the repellent effect. As 4-methoxybenzaldehyde has previously been shown to attract pollinators, it may therefore have a dual function in P. fragrans, pollinator-attractant and ant-repellent. Additionally, 4-methoxybenzaldehyde is particularly interesting as an ant-repellent as it has been observed in the bouquets of other plant species with specific ant interactions. Publisher PDF

Research on canopy arthropods has progressed from species inventories to the study of their interactions and networks, enhancing our understanding of how hyper-diverse communities are maintained. Previous studies often focused on sampling individual tree species, individual trees or their parts. We argue that such selective sampling is not ideal when analyzing interaction network structure, and may lead to erroneous conclusions. We developed practical and reproducible sampling guidelines for the plot-based analysis of arthropod interaction networks in forest canopies. Our sampling protocol focused on insect herbivores (leaf-chewing insect larvae, miners and gallers) and non-flying invertebrate predators (spiders and ants). We quantitatively sampled the focal arthropods from felled trees, or from trees accessed by canopy cranes or cherry pickers in 53 0.1 ha forest plots in five biogeographic regions, comprising 6,280 trees in total. All three methods required a similar sampling effort and provided good foliage accessibility. Furthermore, we compared interaction networks derived from plot-based data to interaction networks derived from simulated non-plot-based data focusing either on common tree species or a representative selection of tree families. All types of non-plot-based data showed highly biased network structure towards higher connectance, higher web asymmetry, and higher nestedness temperature when compared with plot-based data. Furthermore, some types of non-plot-based data showed biased diversity of the associated herbivore species and specificity of their interactions. Plot-based sampling thus appears to be the most rigorous approach for reconstructing realistic, quantitative plant-arthropod interaction networks that are comparable across sites and regions. Studies of plant interactions have greatly benefited from a plot-based approach and we argue that studies of arthropod interactions would benefit in the same way. We conclude that plot-based studies on canopy arthropods would yield important insights into the processes of interaction network assembly and dynamics, which could be maximised via a coordinated network of plot-based study sites.

Invertebrates are dominant species in primary tropical rainforests, where their abundance and diversity contributes to the functioning and resilience of these globally important ecosystems. However, more than one-third of tropical forests have been logged, with dramatic impacts on rainforest biodiversity that may disrupt key ecosystem processes. We find that the contribution of invertebrates to three ecosystem processes operating at three trophic levels (litter decomposition, seed predation and removal, and invertebrate predation) is reduced by up to one-half following logging. These changes are associated with decreased abundance of key functional groups of termites, ants, beetles and earthworms, and an increase in the abundance of small mammals, amphibians and insectivorous birds in logged relative to primary forest. Our results suggest that ecosystem processes themselves have considerable resilience to logging, but the consistent decline of invertebrate functional importance is indicative of a human-induced shift in how these ecological processes operate in tropical rainforests., Invertebrates are key components in the ecological functioning of tropical forests. Here, Ewers et al. show that, compared to primary forest, logging halves the contribution of invertebrate species to several key ecosystem processes, including litter decomposition.

Both the abiotic environment and the composition of animal and plant communities change with elevation. For mutualistic species, these changes are expected to result in altered partner availability, and shifts in context-dependent benefits for partners. To test these predictions, we assessed the network structure of terrestrial ant-plant mutualists and how the benefits to plants of ant inhabitation changed with elevation in tropical forest in Papua New Guinea. At higher elevations, ant-plants were rarer, species richness of both ants and plants decreased, and the average ant or plant species interacted with fewer partners. However, networks became increasingly connected and less specialized, more than could be accounted for by reductions in ant-plant abundance. On the most common ant-plant, ants recruited less and spent less time attacking a surrogate herbivore at higher elevations, and herbivory damage increased. These changes were driven by turnover of ant species rather than by within-species shifts in protective behaviour. We speculate that reduced partner availability at higher elevations results in less specialized networks, while lower temperatures mean that even for ant-inhabited plants, benefits are reduced. Under increased abiotic stress, mutualistic networks can break down, owing to a combination of lower population sizes, and a reduction in context-dependent mutualistic benefits.

Functional diversity (FD) is an important component of biodiversity that quantifies the difference in functional traits between organisms. However, FD studies are often limited by the availability of trait data and FD indices are sensitive to data gaps. The distribution of species abundance and trait data, and its transformation, may further affect the accuracy of indices when data is incomplete. Using an existing approach, we simulated the effects of missing trait data by gradually removing data from a plant, an ant and a bird community dataset (12, 59, and 8 plots containing 62, 297 and 238 species respectively). We ranked plots by FD values calculated from full datasets and then from our increasingly incomplete datasets and compared the ranking between the original and virtually reduced datasets to assess the accuracy of FD indices when used on datasets with increasingly missing data. Finally, we tested the accuracy of FD indices with and without data transformation, and the effect of missing trait data per plot or per the whole pool of species. FD indices became less accurate as the amount of missing data increased, with the loss of accuracy depending on the index. But, where transformation improved the normality of the trait data, FD values from incomplete datasets were more accurate than before transformation. The distribution of data and its transformation are therefore as important as data completeness and can even mitigate the effect of missing data. Since the effect of missing trait values pool-wise or plot-wise depends on the data distribution, the method should be decided case by case. Data distribution and data transformation should be given more careful consideration when designing, analysing and interpreting FD studies, especially where trait data are missing. To this end, we provide the R package "traitor" to facilitate assessments of missing trait data.