Community successional stage and disturbance reveal
importance of resource limitation on invader abundance
Consistent with resource competition and succession theory (Huston &
Smith 1987; Tilman 2004), the less diverse early successional
communities experienced higher levels of invasion than mid or late
successional communities, and their levels of invasion increased with
time, especially for cover of slow invaders. Invasion trends in mid and
late successional communities were more complicated, with their ranks
changing depending on invader type and time. Collectively, these
dynamics reflect invader life histories and the strength of competition
in the recipient communities. Other studies have also found that
invasion levels decline as the diversity of recipient communities
increases (Beaury et al. 2020; Hector et al. 2001;
Petruzzella et al. 2018) (although not universally, MacDougallet al. 2014) and as limiting resources become scarcer (Catfordet al. 2020; Fargione & Tilman 2005; Seabloom 2011). Fast
invaders were able to establish quickly, but their cover declined
notably in late successional communities, presumably as resource
limitation took effect (Figs S6 & S8). Slow invaders are more resource
competitive and conservative and have slower rates of growth than fast
invaders (Lauenroth & Adler 2008), attributes that enabled them to
persist in late successional communities and gradually increase in cover
in mid successional communities. The importance of resource limitation
was further highlighted by interactive effects of disturbance and
community successional stage on invader cover (Figs 2d & 3d).
Temporal trends indicated that disturbance facilitated invasion in late
successional communities but inhibited invasion in early successional
communities (Figs 2d & 3d). We expected that the faciliatory role of
disturbance on invasion would strengthen with succession reflecting
greater resource limitation in later stages of succession (Catfordet al. 2012a). However, the negative disturbance-invasion
relationship in early succession indicates that disturbance can hinder,
as well as help, invasion depending on local conditions. Disturbance is
usually hypothesised (and often found, Kempel et al. 2013;
Seabloom 2011) to facilitate invasion because it decreases the biomass
of competing established plants and because it increases resource
availability (Catford et al. 2009; Davis et al. 2000), as
was the case with soil N in mid and late succession. Resource
competition tends to be weaker earlier in succession (Catford et
al. 2012a; Clark et al. 2019; Lohbeck et al. 2014) and in
less diverse communities (Catford et al. 2020; Fargione & Tilman
2005), which may restrict the positive effects of disturbance on
invasion, enabling negative effects to be seen. By destroying standing
biomass, disturbance can alter microclimates, potentially exposing
seedlings to dry and hostile conditions and reducing the number of safe
sites for recruitment (Wandrag et al. 2019). Indeed, disturbed
plots had higher light and lower surface soil moisture than undisturbed
plots. However, disturbance reduced soil N in our early successional
communities in the peak growing season (June and July), which could
plausibly be the explanation for the negative effect of disturbance on
invader cover in early succession. We posit that disturbance had a
largely neutral effect in mid successional communities because of its
opposite effects on soil N and soil moisture (Figs S6 & S7, Table S7c).
The benefit of increased N availability was annulled by reduced water
availability (which was already very low in the mid successional plots),
marking a shift from N- to water-limitation with disturbance (though
water-limitation can itself cause N-limitation because of reduced access
to nutrients in the soil, Bloom et al. 1985). Light is rarely
limiting in unfertilised grasslands that occur on the sandy,
low-nitrogen soils of Cedar Creek (Tilman 1990; Wilson & Tilman 1991),
so any benefits of increased light availability with disturbance in this
experiment were likely to be secondary to benefits of increased N
availability.
Had we only considered main effects or only examined invasion in early
successional communities, the effect of disturbance in our study would
appear to contradict ecological theory when, in fact, our findings are
consistent with hypothesised disturbance-invasion mechanisms. The
succession-dependent effects of disturbance illustrate the importance of
explicitly examining interaction effects (Catford et al. 2022)
and considering how limiting factors will likely vary from situation to
situation – in this case, when resource limitation, and thus a positive
disturbance-invasion relationship, would be likely or unlikely.