A key question of theoretical ecology is which properties of
ecosystems favour
their stability and help maintaining biodiversity. This qu
estion recently reconsid-
ered mutualistic systems, generating intense controversy
about the role of mutu-
alistic interactions and their network architecture. Here
we show analytically and
verify with simulations that reducing the effective intersp
ecific competition and the
propagation of perturbations positively influences struct
ural stability against envi-
ronmental perturbations, enhancing persistence. Notewor
thy, mutualism reduces
the effective interspecific competition only when the direct
interspecific competition
is weaker than a critical value. This critical competition i
s in almost all cases larger
in pollinator networks than in random networks with the same
connectance. Highly
connected mutualistic networks reduce the propagation of e
nvironmental perturba-
tions, a mechanism reminiscent of MacArthur’s proposal tha
t ecosystem complexity
enhances stability. Our analytic framework rationalizes p
revious contradictory re-
sults, and it gives valuable insight on the complex relation
ship between mutualism
and biodiversity.