Analysis of range expansion in two species undergoing character displacement: why might invaders generally ‘win’ during character displacement?

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Ecological character displacement occurs when interacting species diverge in resource use and associated traits in response to selection to minimize resource competition between them. Yet, when resource quality is asymmetric, the species that monopolizes the more profitable resource following character displacement may have higher fitness and therefore be deemed the 'winner'. Here, we ask: does the winner tend to be the resident species (i.e. the earlier inhabitant of the geographic region where character displacement occurred) or the invader (i.e. the subsequent inhabitant of the region)? We focus on two spadefoot toad species that have undergone character displacement. Previous studies revealed that Spea bombifrons gains the higher quality resource following character displacement; consequently, Spea multiplicata must use the lower quality resource, and as a result, experiences negative fitness consequences. Where the two species have undergone character displacement, three lines of evidence implicate S. bombifrons as the invader: S. bombifrons possess lower haplotype and nucleotide diversity; they do not exhibit isolation by distance (in contrast to S. multiplicata); and they display much higher population growth rates. We hypothesize that historical patterns of selection in its ancestral range pre-adapted S. bombifrons to evolve phenotypes capable of monopolizing the superior resource. Generally, because superior competitive abilities may facilitate successful invasions, invaders may be well positioned to win during character displacement.