Fast-slow traits predict competition network structure and its response to resources and enemies

Plants interact in complex networks but how network structure depends on resources, natural enemies and species resource-use strategy remains poorly understood. Here, we quantified competition networks among 18 plants varying in fast-slow strategy, by testing how increased nutrient availability and reduced foliar pathogens affected intra- and inter-specific interactions. Our results show that nitrogen and pathogens altered several aspects of network structure, often in unexpected ways due to fast and slow growing species responding differently. Nitrogen addition increased competition asymmetry in slow growing networks, as expected, but decreased it in fast growing networks. Pathogen reduction made networks more even and less skewed because pathogens targeted weaker competitors. Surprisingly, pathogens and nitrogen dampened each other's effect. Our results show that plant growth strategy is key to understand how competition respond to resources and enemies, a prediction from classic theories which has rarely been tested by linking functional traits to competition networks. In this paper, we combine ecological theory of biodiversity maintenance and a large field experiment manipulating factorially nitrogen enrichment and foliar pathogen abundance in a perennial grassland composed of 18 species in central Europe (Switzerland) to determine how competition networks within fast and slow growing strategies (defined by their specific leaf area) are modified by resources and leaf fungal enemies. We found that nitrogen and leaf fungal pathogens reshaped the structure of plant interactions, however, fast and slow growing species often responded differently to treatments. Our results show that fast-slow traits are key in determining how species interactions change with resource addition and enemy removal.image