Resident population changes did not alter success above the direct effects of treatments
To test if these changes to the resident populations impacted invasion success, we first analysed a model with resident biodiversity and total resident density, plus their interaction, as sole predictors of invasion success. Once again this was done separately for each invader morph. SM invaders were significantly affected by resident density (F1,88=5.03, p=0.028), but not by biodiversity (F1,88=2.64, p=0.11) or an interaction between density and diversity (F1,87=3.06, p=0.084). Conversely, the WS invader was only significantly affected by biodiversity (F1,81=7.07, p=0.010), with density having no significant effect either as a main effect (F1,81=0.67, p=0.42) or as through an interaction with biodiversity (F1,80=1.25, p=0.27). This demonstrates treatments may have indirectly affected the success of both invaders by manipulating resident populations. We therefore tested whether the direct effect of treatments on success remained when these manipulations were considered. SM invaders were still significantly affected by the interaction between disturbance and resources (F2,82=9.27, p<0.001). However, we find the effect of both biodiversity and total resident density to not be significant (biodiversity: F1,82=2.49, p=0.12; Fig. 5A; density: F1,82=0.24, p=0.63; Fig. 5B). When testing resident population effects alongside treatments on the success of the WS invader, we no longer found any significant interactions. Disturbance and resource abundance both significantly affected WS success (F1,75=8.27, p=0.005 and F2,75=27.8, p<0.001, respectively). However, resident population effects did not have a significant effect (biodiversity: F1,75=2.88, p=0.094, total resident density: F1,75=0.006, p=0.94). We therefore show that, although treatments had a significant effect on resident populations, this did not have an effect on success above the direct effects of disturbance and resource abundance.