Native seed density and non-native invasive species arrival order independently affect biotic resistance to invasion.
Abstract
An essential objective in ecological restoration is enhancing the biotic resistance of native plant communities by promoting competition between invasive and native species. However, little is known about how the sowing density of native species and the arrival order of invasive species affect the biotic resistance of native plant communities. We established a competition experiment in the Republic of Korea involving a mix of four native species and an invasive species, leveraging the concepts of density and priority effects. The native seeds of four species were sown at three relative densities (low, medium, and high), and the targeted invasive species (Ageratina altissima L.) was introduced at different time points (2 weeks ahead of the native species, simultaneously, and 2, 4, and 6 weeks after the native species). The biotic resistance of native species was computed by the average relative competitive index (RCIavg) using the density, cover, and height of invasive species. The biotic resistance of native species increased with increasing seed densities. The RCI values were 0.43, 0.64, and 0.78 for low, moderate, and high seed densities, respectively. Furthermore, delaying invasion enhanced the resistance of native species and competitively excluded invasion if invasive species arrived 4 and 6 weeks after native species. These findings underscore the interactions between density and priority effects, revealing that sowing native species early at high densities improves the resistance of native communities to invasion. Based on Pearson correlation coefficients (r), the RCIavg of resident plants was influenced by plant density, cover, and height. Monthly observations of plant cover clearly showed a consistent increase in the cover of native species in high-density plant communities, and this was sustained for the entire duration of the experiment. Synthesis and applications. This study reveals that sowing diverse native species at high seed densities and priority effects can promote biotic resistance and competitively exclude invasive species from ecological restoration sites.