Linking demographic transitions to population dynamics in a fluctuating environment

Recruitment has been linked to decreases in the ratio of age-specific mortality (M') to mass-specific growth (G'), and year-class strength may be predicted by the age when M'/G' = 1. Hydrological stress adversely affects these parameters for species inhabiting floodplains; however, the relationship between M' and G' in hydrologically variable environments is poorly understood. We evaluated age-specific mortality for six species from a 20-year time series and growth curves from otolith length-at-age data. We assessed the effect of hydrology on the transitional age (age M'/G' = 1) at 21 sites representing a hydrological gradient. Disturbance intensity influenced age-specific mortality but had no effect on mass-specific growth. The transitional age was inversely correlated with annual density, but weakly associated with population biomass. Hydrological disturbance shifted the transitional age to older ages, reducing recruitment overall. We demonstrated that the M'/G' transition was affected adversely by hydrological stress and can be applied to a diverse group of taxa. Growth, survivorship, and the transitional age should be evaluated to improve population modelling efforts used to predict the influence of future restoration actions.