Niche separation of two cryptic species through temperature‐dependent hatching

Abstract Understanding coexistence between species is a central goal in community ecology. In particular, species of cryptic species complexes that often occur in the same habitats and are morphologically almost identical are interesting subjects for the study of eco‐evolutionary processes. In this study, we aimed to detect the underlying mechanism of the coexistence of two rotifer species that belong to the same species complex but differ in their temperature tolerance: Brachionus calyciflorus (heat‐tolerant) and B. fernandoi (heat‐sensitive). First, we used two clones of B. calyciflorus , with different reproductive modes (exclusively asexual, potentially sexual), and one clone of B. fernandoi (potentially sexual). To test whether the heat‐sensitive species was superior to the heat‐tolerant species at low temperatures, we conducted two experiments at temperatures of 12, 15, 18, and 20°C. We found that both heat‐tolerant B. calyciflorus clones developed higher population densities than B. fernandoi at almost all temperatures, and that B. fernandoi was partially excluded when competing with the exclusively asexually reproducing B. calyciflorus clone. In the second experiment, we assessed the propensity for sexual reproduction and found an overall higher investment in sexual reproduction in B. calyciflorus compared to B. fernandoi . In a third experiment, we investigated the hatching pattern of diapausing eggs of the two species using a dynamic temperature regime and found that B. calyciflorus diapausing eggs only hatched at temperatures between 15 and 25°C, while B. fernandoi diapausing eggs hatched between 5 and 15°C. We conclude that the overall superiority of B. calyciflorus over B. fernandoi is avoided by niche differentiation in the temperature‐dependent hatching of diapausing eggs and that B. fernandoi can therefore establish populations in the early months of the year when B. calyciflorus is not present, resulting in competition avoidance. Our results suggest that different temperature‐dependent responses in growth, mixis induction, and hatching act as stabilizing mechanisms facilitating the coexistence of our two species and that these life history differences are one mechanism to enable species coexistence.