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.