When ontogenetic niche shifts and dispersal promote coexistence

Elucidating the mechanisms of species coexistence is not only a central problem in ecology but also provides fundamental insights for conservation biology and the management of biological resources. In general, coexistence of two similar species is considered difficult; however, examples are known in which spatiotemporal heterogeneity reduces niche overlap and promotes coexistence. Ontogenetic niche shifts occur in nearly all species, yet their effects are often overlooked in theoretical models. The present study examines the effects of ontogenetic niche shifts on coexistence conditions, both in the presence and absence of patchy environments. In coexistence theory, the Lotka-Volterra (LV) competition model plays a key role, and particular attention here is given to developing an ontogenetic niche shift (ONS) model that can be directly compared with the coexistence conditions of the LV framework. Specifically, starting from a nonlinear integro-partial differential equation model that can be regarded as a state-structured extension of the generalized LV model, a two-species competitive system with ontogenetic niche shift, consisting of immature and mature stages, is analyzed. The results show that coexistence can occur even when the interspecific competition coefficient for immature individuals exceeds the intraspecific competition coefficient. In addition, direct comparison between the ONS model and the corresponding classical LV model showed that the ONS model allowed coexistence in parameter regions where the classical LV model predicted exclusion. A patchy environment can also provide an alternative pathway to coexistence when two patches are weakly connected. These findings indicate that the predictions of the classical LV competition model are not universal and suggest that ontogenetic niche shifts may allow two ecologically similar species to coexist even when interspecific competition exceeds intraspecific competition, without invoking mechanisms such as the storage effect or higher-order interactions.