Can interspecific competition explain the absence of saline species in freshwaters? A test using water beetles

Abstract The habitats a species occupies are not determined solely by its environmental tolerance, because biotic interactions can further restrict its realised niche. This mismatch between fundamental and realised niches is well documented in saline-associated aquatic insects, many of which tolerate freshwater conditions in the laboratory yet are largely confined to saline waters in nature. We tested whether interspecific competition could contribute to the absence of the halophilic diving beetle Nebrioporus baeticus in freshwater. We compared its response to restricted food availability with that of the freshwater-associated N. clarkii under freshwater conditions, using single- and mixed-species treatments. Survival was monitored over a 30-day period, while feeding behaviour was assessed on days 1 and 7 based on feeding time and the number of contacts with the food resource. Contrary to our expectations, the halophilic beetle was not competitively disadvantaged by the freshwater congener. Interspecific coexistence instead had a transient negative effect on the feeding behaviour of the freshwater species, but not of the halophilic species. These findings suggest that competition alone cannot explain the absence of halophilic species from freshwater habitats, indicating that other factors are likely involved (e.g., predation pressure, parasitism, or other biotic interactions).