Negative biodiversity-ecosystem function relationship in broad but not in narrow functions within pitcher plant microbial communities

The relationship between Biodiversity and Ecosystem Function (BEF) addresses how communities transform their environment. BEF relationships can have positive, neutral, or negative slopes, yet it remains unclear what conditions result in a particular slope. A popular classification in microbial ecology distinguishes broad from narrow functions and we ask whether this distinction improves predictions of BEF relationships. Specifically, we evaluate whether the relationships between broad functions and diversity can be predicted based on (1) the combined slopes from underlying narrow functions, (2) the phylogenetic breadth of associated species and (3) their ecological dominance. We assembled bacterial communities from pitcher plant fluid, using a dilution-to-extinction approach to create a gradient in biodiversity. Darlingtonia californica are carnivorous plants that depend on their bacterial community to degrade insects that supplement their nitrogen requirements. We found a negative BEF relationship between bacterial richness and degradation, while narrow functions had positive and neutral BEF slopes. The narrow functions did not predict the BEF relationship for the broader function. We identified three species statistically associated with degradation: Clostridium sp. had a positive association, while Herbinix sp. and Dyadobacter sp. had negative associations. Clostridium sp. was rare in the dataset and negatively correlated with Herbinix sp., suggesting an antagonism and highlighting important functional contributions of subordinate species. We propose that the negative BEF for degradation is explained by rare key functional taxa that thrive in low-diversity communities. These findings suggest that species abundance distributions outperform function-function relationships in explaining the emergence of complex broad functions. ImportanceIn this study, researchers examined how bacterial diversity shapes ecosystem function, using microbial communities from carnivorous pitcher plants as a model system. Contrary to the common belief that higher biodiversity enhances function, they found that communities with higher bacterial richness were less effective at breaking down insect prey. While specialized microbial activities improved or remained stable with greater diversity, they did not explain the decline in overall degradation. Instead, a rare Clostridium sp. bacterium emerged as a key driver of decomposition in low-diversity communities, while more dominant species were linked to reduced function. These findings reveal that rare microbes can play critical roles in ecosystem processes and suggest that who is present and how abundant they are may matter more than the sum of their potential functions.