Statistical analysis
During trapping sessions we identified individual melomys that were
captured at each site by their unique microchips. Because melomys on
Indian Island have very small home ranges (tending to be caught in the
same or adjacent traps throughout the trapping period: Jolly et
al. unpub. data) and since we never observed captures of melomys marked
at other sites (Jolly et al. unpub. data), we treated each site
as independent with regard to demographics and behaviour (Fig. 1; Table
S1).
To estimate between-session survival, we analysed the mark-recapture
data to estimate recapture and survival rates using Cormack-Jolly-Seber
models in program MARK (see Supporting Information for details).
To test whether the presence of quolls impacted melomys population size,
we used a hierarchical model in which population size was made a
function of quoll presence/absence, capture session, and the interaction
between these factors. Population size at each site during each session
is estimated in this process, and we fitted this model in a Bayesian
framework (see Supporting Information for details).
To assess whether the introduction of quolls affected the behaviour of
melomys populations, we divided the responses of melomys in open field
tests into two independent response variables: whether individuals
emerged or not during the 0-10 min period (binomial: 0 or 1); and
whether individuals emerged and interacted with the novel object or not
during the 10-20 min period (binomial: 0 or 1). We used generalised
linear mixed-effects models with binomial errors and a logit link to
test the effect of quoll presence (two levels: quolls present and quolls
absent) and trapping session (continuous), with site included as a
random effect, on the behavioural response variables. P -values
were obtained by likelihood ratio tests of the full model with the
effect in question against the model without the effect. This analysis
was performed using R with the lme4 software package (Bateset al. 2007).
To assess whether the numerical impact of quolls on melomys affected the
seed harvesting rate of invaded melomys populations, we first examined
the relationship between melomys population size (estimated above) and
the total number of control (unscented) seeds harvested from each site.
Here we used a simple linear model with number of seeds harvested as a
linear function of population size, quoll presence/absence and the
interaction between these effects. To test whether there was an
additional effect of quoll presence, beyond their effect on population
size, we defined a new variable, Δks, as the difference
in seed take between scented and unscented treatments within each
site.session. Here any effect of melomys density is cancelled out
(because density is common to both treatments within each site.session).
Thus, we fitted a model in which Δks is a function of
quoll presence/absence, session and the interaction between these
effects. All analyses were performed using R version 3.3.2 (R Core Team
2019).