Biological invasions have long fascinated ecologists as they
fundamentally alter ecological communities, often in surprising ways.
The demography of interacting native and exotic populations are core
drivers of invasions. Demographic models estimate the strength of
species interactions but have several shortcomings, including
disregarding facilitation and focusing only on competition, disregarding
individual-level variance in demographic parameters, and focusing on one
exotic species at a time. In this study, we investigate the fitness
outcomes of eleven native and exotic species from a diverse annual plant
community in Western Australia. We use a Bayesian demographic modelling
approach that integrates demographic stochasticity and facilitation.
Facilitation mediated by exotic species played an integral role in the
invaded community, but demographic stochasticity caused many species
interactions to vary from facilitative to competitive, regardless of
abiotic conditions. Our approach reveals variation that could be
responsible for the diverse and unexpected impacts of exotic species on
recipient communities.