Impacts of invasion on resident diversity and other invaders
Resident diversity, as indicated by effective species richness, was negatively related to invader cover (Fig. 4b). The relationship did not depend on invader type, time, nor any other experimental treatment (Fig. 4, Tables S6 & S11). Resident diversity was highest in late succession and lowest in early succession, but these differences declined over time as richness decreased slightly in late succession and increased in mid and, to a lesser extent, early succession (Fig. 4c). Resident diversity was higher in disturbed than undisturbed plots (Fig. 4d). In general, resident diversity in plots with little or no invader cover was stable over time, whereas resident diversity in plots with high invader cover declined over time (Fig. S9). These trends are consistent with impacts of invasion on resident diversity rather than effects of resident diversity on invasion, though the relationship can go in both directions (Catford et al. 2020; MacDougall et al. 2014). Resident diversity was most strongly related to invader cover and community successional stage (Fig. 4a, marginal R2 = 0.50, conditional R2 = 0.78).
Evidence suggested that fast invaders limited cover of slow invaders (Fig. 3b) but co-occurring slow invaders did not affect fast invader cover (Fig. 2, Tables S9-10). Plots sown with only fast invaders had similar total invader cover to plots sown with both fast and slow invaders (Fig. S5d, Tables S3 & S8). Plots with only slow invaders had lower cover (Fig. S5d).