  We use the optimal foraging theory to study coexistence between two plant
species and a generalist pollinator. We compare conditions for plant
coexistence for non-adaptive vs. adaptive pollinators that adjust their
foraging strategy to maximize fitness. When pollinators have fixed preferences,
we show that plant coexistence typically requires both weak competition between
plants for resources (e.g., space or nutrients) and pollinator preferences that
are not too biased in favour of either plant. We also show how plant
coexistence is promoted by indirect facilitation via the pollinator. When
pollinators are adaptive foragers, pollinator's diet maximizes pollinator's
fitness measured as the per capita population growth rate. Simulations show
that this has two conflicting consequences for plant coexistence. On the one
hand, when competition between pollinators is weak, adaptation favours
pollinator specialization on the more profitable plant which increases
asymmetries in plant competition and makes their coexistence less likely. On
the other hand, when competition between pollinators is strong, adaptation
promotes generalism, which facilitates plant coexistence. In addition, adaptive
foraging allows pollinators to survive sudden loss of the preferred plant host,
thus preventing further collapse of the entire community.
  Keywords: mutualism, competition, optimal foraging, evolutionarily stable
strategy, coexistence, adaptation rate
