Stage-dependent effects of a non-native invasive saltmarsh plant on the seedling establishment of a native seagrass

Abstract

Concerns on the environmental disruption and ecosystem degradation due to the biological invasions are growing, with far-reaching consequences for the resilience of recipient ecosystems. While numerous studies mostly emphasized the negative impacts of invasive species on native species at a single time point, there is a lack of knowledge whether and how the invasion impacts vary with time. Here, we provide an empirical study on stage-dependent invasive impacts in an intertidal seagrass habitat (Zostera japonica), which is undergoing invasion of an invasive cordgrass (Spartina. alterniflora). Combining results from field surveys and experiments suggested that, impacts of cordgrass on seedling establishment of native seagrass species shift from facilitation at the initial stage of invasion to inhibition at the later stage when the invasive cordgrass become densely vegetated, causing density-dependent effects. In addition, at the later stage, patch expansion of cordgrass triggered degradation of seagrass-dominant habitats, showing the remnant patches of seagrass-dominant habitat have a strong patch-size dependent effect, with an optimal patch size of ~8 m2 and ~1.5 cm detritus to the establishment of seagrass seedlings. These stage-dependent effects strongly linked to water movement and sedimentation rate. Both of them are modified by the abundance of the invasive cordgrass and patch formation through transforming native seagrass habitat with invasion. Our findings highlight the importance of unraveling stage-dependent impacts for assessing the consequences of biological invasion on native ecosystems degradation.