Opposing Effects of Succession on Bacterial Diversity and Function within Pitcher Plant ( Sarracenia purpurea ) Leaves

How biodiversity and ecosystem functions change with succession has proven to be difficult to predict. Generally, it is thought that species accumulation over time should increase function, yet other successional trajectories can have alternative effects on diversity and function. We hypothesize that community diversity and function may respond in opposite ways to successional drivers such as nutrient availability, species interactions, or abiotic stress. The microbial communities within Sarracenia purpurea leaves perform degradation functions, providing essential nutrients to the plant, but we know little about how succession within the leaf influences bacterial diversity and degradation. We collected pitcher plant fluid from leaves aged 2–24 weeks to use in microcosm experiments. We used amplicon sequencing and a degradation assay to quantify diversity and ecosystem function. Because bacterivore activity increases with leaf age, we hypothesize that bacterial diversity will decrease over time, enhancing function if functionally important species are tolerant to predation. We thus added a common bacterivore to half of the replicated microcosms. We found that succession had opposite effects on diversity and function in pitcher plant bacteria but was unrelated to predator activity. As the leaves aged, bacterial degradation increased while diversity declined, with no significant effects from predator addition. This negative relationship between biodiversity and function likely results from functional traits associated with low nutrient availability or poor competitive ability. By broadening the landscape of successional scenarios and identifying their underlying mechanisms, we can advance our ability to predict diversity and functional dynamics in natural communities.