Is hybridization evil? --Competition generates alternative stable states of coexistence of hybridizing species

Abstract

Predicting ecological outcomes of hybridizing species remains challenging because of complex interactions between genetic and ecological processes. Previous studies have mainly focused on exclusion scenarios driven by genetic mechanisms, whereas few have explored the impact of competitive interactions among parent species and their hybrids on generating multiple coexistence regimes. Here, we develop a population dynamics model that explicitly couples hybridization genetics with population dynamics driven by competitive interactions among two parent species and their hybrids. Our analysis reveals that backcrossing can fundamentally reshape coexistence regimes, generating five alternative outcomes: (i) alternative stable states of coexistence of both parent species and hybrids, (ii) frequency-independent coexistence, (iii) frequency-dependent coexistence, (iv) competitive exclusion of a rarer species, and (v) extinction of all populations. These outcomes arise from the balance between two key processes: regeneration of parental genotypes through hybrid x hybrid mating and competitive exclusion mediated by hybrids. Thus, hybridization can generate multiple stable coexistence regimes even when species would otherwise be excluded. This eco-genetic mechanism provides a unifying theoretical basis for predicting the ecological consequences of hybridization, highlighting the crucial role of competitive interactions in determining coexistence outcomes.