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).