Temporal niche differentiation often leads to priority effects rather than coexistence: Lessons from a marine midge.

While niche differences aid coexistence, the role of temporal niches is complex. A recent study (Stump & Vasseur, 2023) casts doubt on the idea that species coexist easily if they partition abiotic niches that vary in time. The storage effect, which aids coexistence, requires that species differ in what is a 'good year', and that the benefits that the currently common species can draw from its own good year become limited due to intraspecific competition. The recent re-evaluation of temporal niches considered Allee effects only fleetingly. We complement their work by providing a case study of the marine midge Clunio marinus, where coexistence appears to occur in nature, is associated with a strong difference in timing traits, and also features Allee effects because rare timing phenotypes emerge with limited mating opportunities. The larvae develop in the sea, and adults emerge and mate during the lowest low tides. These tides coincide with either the full or the new moon, and genetically determined strains use either one of them, or both, for emergence. A 'good year' in this system translates into a particular low tide. Allee effects create strain-specific good tides if the risk of hybridization is greater for the currently rare strain, which mates more often with another strain, than the currently common strain. We are able to investigate this effect by varying the effects of hybridization in our model of Clunio biology. Temporal niches, mate-finding Allee effects, hybridization possibilities and a potential growth-survival tradeoff do not easily combine to yield stable coexistence. Most factors instead promote positive frequency dependence, leading to priority effects. Ontogenetic niche shifts among larvae deviate from this result: if suitably timed, they are able to concentrate competition in a coexistence-promoting manner. Our study thus complements and strengthens Stump and Vasseur's conclusion that a finding of temporal niche differentiation should not be straightforwardly assumed to be an explanation for the coexistence of two or more morphs or species. We encourage linking temporal niche studies with those of priority effects, as well as the study of other coexistence mechanisms that may operate within systems that feature temporal niches.