Mutualism or parasitism: Partner abundance affects host fitness in a fish reproductive interaction

Mutualisms are ubiquitous in nature but are understudied in freshwater ecosystems. Mutualisms can be unstable, shifting to commensal or even negative outcomes with context. Quantifying context dependency in mutualisms is critical for understanding how biotic interactions will shift along disturbance gradients in freshwater systems. A common reproductive interaction among stream fishes, nest association occurs when individuals of one species spawn in nests constructed by a host fish. Hosts benefit from a dilution effect: high proportions of associate eggs decrease the odds of host brood predation. Thus, partner abundance can be an important source of biotic context influencing the outcome of an association. We conducted a large in situ experiment manipulating abundance of partner yellowfin shiner (Leuciscidae: Notropis lutipinnis) (absent, low, high) at constant abundance of host bluehead chub (Leuciscidae: Nocomis leptocephalus), and quantified chub reproductive success using genetic tools. Evidence suggests that the nest association switched from mutualistic to parasitic outcomes as shiner abundance decreased. Chub reproductive success was highest at high shiner abundances. However, chub reproductive success was actually higher in the complete absence of shiners than at low shiner densities. This study shows that outcomes of biotic interactions in freshwater systems are context-dependent, and that partner abundance can be a key source of context-dependency in nest associations. We encourage future studies on freshwater mutualisms, which are thus far largely overlooked, relative to competition and predation.