High specialization and limited structural change in plant-herbivore networks in tropical montane forest

Figure 1. Representative bipartite networks from each of the three stages of succession- young secondary, mature secondary and primary forest. Only two plots per stage are shown, each represents 0.2ha of sampled forest. Lower trophic level is coloured according to host genus, and the width of the bars is proportional to host biomass. Hosts are ordered by most basal to most recent (left to right). Only hosts with herbivore interactions are included. See paper for details of labelled species.

Secondary succession is becoming increasingly prominent in tropical landscapes, arising through major land use changes and forest disturbance. Despite this, we know relatively little regarding the effects of succession on plant-herbivore interaction networks. We utilise a succession series in montane forest in Papua New Guinea to tackle this unknown. Taking a ‘whole forest’ plot-based approach, we sample a total of 1.8 ha of tropical forest exhaustively, documenting trophic interactions between larval Lepidoptera and their host plants above 5 cm dbh. This was performed by felling the forest, working alongside the local community ensuring only forest which they had previously earmarked for clearance was felled. We successfully identified 12,357 trophic interactions, involving 292 herbivore species and 89 host plant species. This enabled us to reconstruct interaction networks throughout three stages of succession, namely young secondary forest, mature secondary forest and pristine primary forest. From this, we carried out a quantitative analysis of network metrics in combination with a newly developed, phylogenetically based measure of host specialization. Using established successional theory as a framework, we expected notable differences between each stage of succession. However, we found that these stages were surprisingly similar. Our results suggest that herbivores show a preference for earlier succession stages reflected in higher numbers of herbivores per host and greater abundance, in line with predictions based on the resource availability hypothesis. However, host phylogenetic specialization was consistently high, and several network parameters changed little over the course of succession. These trends emerge despite extremely high turnover of network components (network β-diversity). This may indicate the existence of some general rules of network assembly. Uncovering these rules would represent a milestone in ecological studies.

Redmond C. M., Auga J., Gewa B., Segar S. T., Miller S., Molem K., Weiblen G., Butterill P. T., Maiyah G., Hood A.S.C., Volf M., Jorge L.R., Basset Y. F., Novotný V. (2018) High specialization and limited structural change in plant-herbivore networks along a successional chronosequence in tropical montane forest. Ecography 42: 162-172. DOI: 10.1111/ecog.03849