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 invasion impact. Demographic models estimate the strength of
species interactions but have several shortcomings, often ignoring
positive interactions 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 variation. Positive effects of
exotic species played an integral role in the invaded community, but
demographic variation caused many species interaction outcomes to vary
from positive to negative, regardless of abiotic conditions. Our
approach reveals variation that could be responsible for the diverse and
unexpected impacts of exotic species on recipient communities.