Spatially discontinuous relationships between salt marsh invasion and mangrove forest fragmentation

Rapid and large-scale biological invasion results in widespread biodiversity loss and degradation of essential ecosystem services, especially in mangrove forests. Recent evidence suggests that the establishment and dispersal of invasive species may be exacerbated in a fragmented landscape, but the influence of mangrove fragmentation on coastal biological invasion at the landscape scale remains largely unknown. Here, using a derived 10-m-resolution coastal wetland map of the southeastern coast of China, we examine the relationship between the fragmentation of mangrove forests and the magnitude of salt marsh invasion and quantify the geographical variation in this relationship across a climatic gradient. Our results show that mangrove forests with small sizes, large edge proportions, and regular boundary shapes tend to suffer more serious salt marsh invasions than mangrove forests with large sizes, small edges and irregular boundary shapes, indicating a positive correlation between mangrove fragmentation and the magnitude of invasion. In particular, these fragmentation-invasion relationships are shown to be more intensive in the subtropics than in the tropics. Our findings provide the first spatially explicit evidence of the relationships between mangrove fragmentation and biological invasion on a landscape scale and highlight an urgent need for conservation and management actions to improve mangrove connectivity, which will increase the resistance to invasions, especially for small subtropical mangrove forests.