A quantitative approach to assessing aboreal plant metacommunities in two New Zealand forests

Abstract

Identifying patterns and processes generated from species interactions is fundamental to community ecology. Metacommunities have recently been investigated using a network approach to identify patterns of species co-occurrence. This study uses the network approach and species interaction matrices to understand liana-host interactions in Otari-Wiltons Bush, on the southern tip of the North Island, New Zealand. Network properties of liana metacommunities were compared with mistletoe and epiphyte metacommunities. Host specialisation, nestedness and negative co-occurrence were searched for using species- and individual-based null models. Network properties were predicted to differ according to the type of interaction between climbing plant and host tree (i.e. commensal or parasitic relationship). In addition, interactions between species and individual host trees were investigated on the following three metacommunity scales. The (1) forest scale, (2) host species scale, and (3) guild scale were assessed with a c-score index using (1) fixed-fixed and (2) fixed-equiprobable row and column constraints to search for patterns of negative co-occurrence. Results from the network approach showed lianas, mistletoe and epiphytes had very different properties. Liana degree distributions exhibited fewer links than expected under the null model, indicating strong host preferences. Furthermore, lianas were less nested than null model expectations and instead showed support for negative co-occurrence patterns, meaning liana species tended to have mutually exclusive host preferences. Liana network properties were very similar to mistletoe networks and contrasted sharply with epiphyte networks. Epiphytes were more nested than expected by chance and showed positive co-occurrence patterns. Degree distributions were consistent with randomised expectations. Results for species interactions at each scale showed differences in negative co-occurrence patterns. The forest scale showed a strong pattern of negative co-occurrence for the fixed-fixed and fixed-equiprobable null models, indicating deterministic structure at the community level. The host species scale showed no evidence of negative co-occurrence indicating (1) strong host preferences or (2) habitat partitioning. The guild scale showed a strong pattern of negative co-occurrence for all three liana guilds when assessed using relaxed column constraints. This result was not consistent with the fixed-fixed null model. Overall results indicate that plant-plant interactions exhibited by different types of arboreal plants have very different network properties. These results are hypothesized to occur from 1) differences in coevolutionary dynamics between arboreal plants and their hosts, which range from parasitic (mistletoes) to commensal (epiphytes), and 2) biotic interactions among arboreal plant species for access to host trees. More specifically, the pattern observed at the forest scale is hypothesized to be a reflection of competitive interactions between liana species using similar regions of host space.