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).