Functional traits predict competitive outcomes in fungi
Fungi are the dominant decomposers of organic matter in many ecosystems worldwide, yet how fungal communities respond to environmental change – both ecologically and biogeochemically – is an important outstanding question. Abiotic factors are often assumed to be the predominant drivers of fungal activity at large spatial scales, but a growing body of evidence suggests that fungal interactions are integral for understanding ecosystem-level processes. Fungi compete through a wide variety of mechanisms, including direct combat, volatile production, nutrient acquisition, and growth rate; all of which can directly affect organic matter decomposition. Studies have shown that competitive outcomes among fungi differ across environmental conditions, yet an understanding of the biological, ecological, or biogeochemical mechanisms linking competitive ability to environmental conditions remains unclear. Here, we investigate the mechanisms underlying fungal competitive outcomes by linking competitive ability to functional trait expression across environmental gradients. Twenty-two wood-rot fungi were isolated from North American temperate forests, and trait expression (enzyme production, growth rate, density) and ecological performance (moisture and temperature niche breadth) were measured for each fungus. All 231 unique pairs of fungi were competed in a nutrient-rich media at optimal temperature and moisture conditions, and their outcomes were tracked over the course of 8 weeks.
Competitive outcomes between fungi were best predicted by difference in growth rate, difference in phosphatase enzyme production, and difference in moisture niche width (R2adj=0.28). In particular, pairwise competitive ability was positively associated with higher relative growth rate (F1,208=38.94, p<0.001) and negatively associated with increased phosphatase production (F1,208=8.49, p=0.003). These results suggest that competitive ability under optimal nutrient conditions is driven by a fungus’ ability to down-regulate enzyme production or produce low levels of enzymes in response to substrate conditions. Presumably, the cost of producing unnecessary enzymes limits the extent to which a fungus can invest in energy-intensive combative compounds and strategies, putting it at a competitive disadvantage relative to a fungus whose enzyme production more closely matches the substrate. Somewhat counterintuitively, even after controlling for growth rate and enzyme production, those fungi with wider moisture niche widths were relatively more competitive under optimal conditions (F1,208=15.23, p<0.001), indicating that the fundamental niche space of fungi (i.e., where they can maintain positive growth) is not indicative of their competitive ability. Additional research is therefore needed to understand how growth and enzyme plasticity across low moisture and nutrient conditions affect fungal competitive ability under suboptimal conditions.