Invader abundance
Results were largely consistent with predictions (Figs 1 & 5). Of the
experimental treatments, fast invader cover was most strongly linked
with successional stage (Fig. 2a), which itself was strongly linked with
extractable soil nitrate (Fig. S6, Table S7), the dominant limiting
resource in these grasslands (Tilman 1990). Limited soil N therefore
likely explained the low invader cover in late successional communities
(Fig. 2c), an effect that was partially alleviated when plots were
disturbed (Fig. 2d), temporarily increasing soil N (Fig. S6). Early
successional plots had consistently high levels of invasion through time
despite marked declines in soil N (Fig. S6), suggesting that soil N did
not limit fast invader cover in early succession, at least not in
undisturbed plots. The non-conditional positive effect of seed dose
demonstrated that seed dose was important across all conditions,
including in early succession where fast invaders were otherwise
unconstrained (Fig. 2a-c). Combined with lack of growth rate- or
resource-limitation, this suggests that seeds were the key limiting
factor in early succession for fast invaders, at least in undisturbed
plots (Fig. 2e). Fast invader cover reached intermediate levels in mid
succession, with trends consistent with seed- and some
resource-limitation.
Slow invader cover was primarily limited by resources in late succession
too, as demonstrated by effects of disturbance and trends across the
three successional stages (Fig. 3c-d). However, unlike fast invaders,
slow invader cover seemed to be limited by species’ growth rates in this
short-term experiment, as the strong link between slow invader cover and
time (Fig. 3a) and its marked temporal increase in early and mid
successional plots demonstrated (Fig. 3c).
Most relationships involved interactions, consistent with effects of an
interplay of limiting factors (Fig. 1b). Unless interactions were
included, successional stage was not selected in the fast invader model
despite it being the most influential treatment for fast invader cover
(Tables S4 & S9, Fig. 2a). A similar situation occurred for slow
invader cover where invader type and disturbance were absent from the
best non-interaction model (Tables S5 & S10). The most marked
interactions for both invader groups were between community successional
stage and disturbance, and successional stage and time (Fig. 2a,c,d,
Fig. 3a,c,d). These interactions reflected the varying importance of
resource- and growth rate- limitation in different contexts (Figs 1 &
5). Invader type (both groups or target group only) was the only
experimental treatment not included in the best model for fast invaders,
which had a marginal R2 of 0.44 and conditional
R2 of 0.80. All four experimental treatments plus time
were included in the best model for slow invaders, which explained
considerable variance (marginal R2 = 0.83; conditional
R2 = 0.92).