  Recent work draws attention to community-community encounters ("coalescence")
as likely an important factor shaping natural ecosystems. This work builds on
MacArthur's classic model of competitive coexistence to investigate such
community-level competition in a minimal theoretical setting. It is shown that
the ability of a species to survive a coalescence event is best predicted by a
community-level "fitness" of its native community rather than the intrinsic
performance of the species itself. The model presented here allows formalizing
a macroscopic perspective whereby a community harboring organisms at varying
abundances becomes equivalent to a single organism expressing genes at
different levels. While most natural communities do not satisfy the strict
criteria of multicellularity developed by multi-level selection theory, the
effective cohesion described here is a generic consequence of division of
labor, requires no cooperative interactions, and can be expected to be
widespread in microbial ecosystems.
