Seed density is as important as limiting similarity, diversity effect, and propagule pressure in plant restoration to control invasion of Sicyos angulatus.

Chaeho Byun, Department of Biological Sciences and Biotechnology, Andong National University, Andong, Korea, Republic of (South), Minwoo Oh and Eun Ju Lee, School of Biological Sciences, Seoul National University, Seoul, Korea, Republic of (South), Hojeong Kang, School of Civil and Environmental Engineering, Ecological Engineering Lab, Seoul, Korea, Republic of (South)

Background/Question/Methods
Restoring invasion-resistant plant communities is critical for the successful control of invasive plant species. It is based on ecological principles, such as limiting similarity, and the diversity–invasibility hypothesis, which can be used to select optimal combinations of species and determine appropriate plant density for the effective suppression of invasion due to propagule pressure. However, no attempt has been made to combine these factors in a single research framework. Here, we show for the first time the relative importance of all significant factors, including seed density, limiting similarity, diversity effect, and propagule pressure, in the invasion mechanism of Sicyos angulatus, an invasive plant species in riverine.
Results/Conclusions
In monoculture treatments, the biotic resistance of resident plants to S. angulatus was primarily related to their functional group identity, whereas the species identity effect was redundant within each functional group, except FG3. The average RCI (RCIavg) of 15 resident plants varied significantly among the three FGs (F2,40 = 12.83; P < .001). FG1 (annual plants) showed the highest RCIavg value, followed by FG2 and FG3 (RCIavg= 0.649, 0.314, and 0.188, respectively). The RCIavg of mixed treatments was significantly higher than that of monocultures (F1,71 = 10.92; P = .0015), indicating that mixed treatments (four species) were more resistant to S. angulatus invasion than monocultures. In density treatment, two-way analysis of variance (ANOVA) of shoot numbers (log-transformed data) revealed significant effects of propagule pressure (F2,22 = 49.13; P < .001) and seed density (F3,22 = 16.96; P < .001). In the structural equation model, while the direct positive effects of three main factors (seed density, limiting similarity, and diversity effect) on the performance of resident plants (in terms of biomass and shoot number) were significant, biotic resistance of resident plants had negative effects on invasion success, and propagule pressure always had positive effects on invasion success. Our results suggest that seed density, rarely explored in previous studies, is as important a determinant of invasion success as limiting similarity, diversity effect, and propagule pressure. Thus, the density-mediated mechanism must be given careful consideration for the restoration of strong invasion-resistant native plant communities.