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.