IMPACTS OF DENSITY‐INDEPENDENT MORTALITY AND PRODUCTIVITY ON THE STRENGTH AND OUTCOME OF COMPETITION

Nonselective, density‐independent mortality (DIM) is commonly thought to reduce the importance of competition, promoting coexistence of ecologically similar species. Yet, recent theory suggests that competition intensity and niche differences remain important under high DIM, even when competitive dominance is modified. Productivity is also thought to interact with DIM, potentially enhancing species coexistence. I explored these issues using a laboratory system composed of a diverse assemblage of bacteria as prey, and four species of competing protozoan bacterivores. Productivity manipulations were crossed with varying levels of DIM imposed on the bacterivores (in both polyculture and monoculture). DIM altered patterns of competitive dominance. At low DIM, Paramecium aurelia and Paramecium caudatum dominated, while Colpoda inflata (a species absent at low DIM) was able to invade and dominate at high DIM. Productivity did not promote shifts in dominance and hindered Colpoda performance. Examination of competition effect sizes provided clear evidence of competitive reversals and the importance of competition under low and high DIM. Variable competitive outcomes appeared to be mediated by changes in the strength of apparent competition effects on bacterial diversity, in accordance with theory. Bacterial diversity was low at low levels of DIM, due to strong consumer effects. High DIM weakened consumer effects and increased bacterial diversity, likely facilitating invasion and coexistence.