Invaders impact community diversity and other invaders via resource competition
The observed decline in resident diversity with increasing invader cover is consistent with effects of resource competition and resource limitation. The invasion-diversity relationship did not vary with invader type, despite slow invaders being better able to persist in the more diverse late successional communities and their potential to reduce soil N and water to lower levels than fast invaders. Somewhat surprisingly, fast invaders appeared to limit cover of slow invaders when the groups were sown together. This trend likely reflects niche pre-emption where the more abundant fast invaders colonised safe sites more rapidly than slow invaders (Catford et al. 2012a; Wandraget al. 2019). With more time, and as their temporal trends suggest (Figs. 2c & 3c), slow invaders will likely replace or displace shorter-lived fast invaders (Lauenroth & Adler 2008) and have a greater impact on community diversity than fast invaders, provided that disturbance is rare and no more seeds are sown (Tilman 1990).
The negative relationship between invader cover and resident diversity was not conditional on invader type or local conditions and suggested that resource limitation was important under all conditions examined here. Our experimental design (including its temporal and spatial scale) meant that we were more likely to detect effects of resource limitation on resident diversity than effects of other factors that might be limiting. In other situations, invader impacts may be more variable, reflecting a greater diversity of factors that limit resident species and how invaders may affect those limiting factors (e.g. via enemies, mutualists, abiotic conditions) (Kempel et al. 2013; Levineet al. 2004; Petruzzella et al. 2020).