Community dynamics of macro- and micro-organisms in pitcher plant systems
Ecological communities are complex systems comprised of organisms interacting via mutualism, competition, predation and parasitism. Studies of community ecology generally incorporate only organisms from one trophic or phylogenetic group, such as plants or lizards. Carnivorous pitcher plants are self-contained microcosms, housing aquatic arthropods, protists, fungi and bacteria. They provide an opportunity to analyze community dynamics of many different taxa, across multiple domains of life and trophic levels. We used Illumina amplicon sequencing of 16S and 18S rDNA to characterize the bacteria and eukaryotes living in three species of Nepenthes pitcher plants: N. gracilis, N. rafflesiana, and N. ampullaria, in each of three different parks in Singapore. We examined community diversity patterns among the different taxa within pitcher plants, and across different host species, collecting sites, and environmental variables, using multivariate statistics and network analyses. We also collected data from a convergently evolved system, the Sarraceniapitcher plants in the United States, in order to examine potential convergence of associated organisms.
Both deterministic and stochastic processes shape assembly of communities in pitchers. Multivariate analyses reveal that bacterial and eukaryotic community composition is most influenced by host species and pH, though site and volume also have significant effects. Phylogenetic diversity and Unifrac dissimilarity matrices of bacteria and eukaryotes are strongly correlated. Bacteria may be introduced into pitchers primarily via arthropod guts; however, if this were the case we would expect a higher correlation between bacterial and arthropod diversity. Instead, we suggest that the correlations are driven by predator-prey dynamics between bacteria and protists. Network analyses indicate significant specialization of certain taxa with different pitcher plant hosts.
Comparisons of Southeast Asian and American pitcher plant communities point to a pattern we name convergent interactions, also apparent in many other systems. Recognition of convergent interactions can facilitate the characterization of evolutionarily distinct multispecies interactions sharing common features, and thereby help to illuminate functions within poorly described or newly discovered systems. Furthermore, the convergent interactions framework can improve our understanding of how selective forces can lead to similar species interactions with important ecosystem effects.