  We study the dynamical evolution of superradiant instabilities of rotating
black holes for multiple axion fields with comparable masses, motivated by
string theory constructions, which typically exhibit a large number of light
axions, with a broad range of masses. We show, in particular, that even though
superradiant clouds for the heavier axion species grow faster, they are
eventually reabsorbed by the black hole as the latter amplifies the lighter
axion field(s), analogously to the dynamics of different species competing for
the same resources in an ecosystem. We also incorporate in our study the
effects of accretion and gravitational wave emission. We further demonstrate
that the existence of multiple axion species with comparable masses may have a
substantial impact on the stochastic gravitational wave background produced by
axion clouds around black hole binary merger remnants, which could be probed
with planned detectors.
