  It is well-known that interactions between species determine the population
composition in an ecosystem. Conventional studies have focused on fixed
population structures to reveal how interactions shape population compositions.
However, interaction structures are not fixed, but change over time due to
invasions. Thus, invasion and interaction play an important role in shaping
communities. Despite its importance, however, the interplay between invasion
and interaction has not been well explored. Here, we investigate how invasion
affects the population composition with interactions in open evolving systems
considering generalized Lotka-Volterra-type dynamics. Our results show that the
system has two distinct regimes. One is characterized by low diversity with
abrupt changes of dominant species in time, appearing when the interaction
between species is strong and invasion slowly occurs. On the other hand,
frequent invasions can induce higher diversity with slow changes in abundances
despite strong interactions. It is because invasion happens before the system
reaches its equilibrium, which drags the system from its equilibrium all the
time. All species have similar abundances in this regime, which implies that
fast invasion induces regime shift. Therefore, whether invasion or interaction
dominates determines the population composition.
