Priority effects determine how dispersal affects biodiversity in seasonal metacommunities

The arrival order of species frequently determines the outcome of their
interactions. This phenomenon, called the priority effect, is ubiquitous
in nature and determines local community structure, but we know
surprisingly little about how it influences biodiversity across different
spatial scales. Here, we use a seasonal metacommunity model to show that
biodiversity patterns and the homogenizing effect of high dispersal depend
on the specific mechanisms underlying priority effects. When priority
effects are only driven by positive frequency dependence,
dispersal-diversity relationships are sensitive to initial conditions but
generally show a hump-shaped relationship: biodiversity declines when
dispersal rates become high and allow the dominant competitor to exclude
other species across patches. When spatiotemporal variation in
phenological differences alters species’ interaction strengths
(trait-dependent priority effects), local, regional, and temporal
diversity are surprisingly insensitive to variation in dispersal,
regardless of the initial numeric advantage. Thus, trait-dependent
priority effects can strongly reduce the effect of dispersal on
biodiversity, preventing the homogenization of metacommunities. Our
results suggest an alternative mechanism that maintains local and regional
diversity without environmental heterogeneity, highlighting that
accounting for the mechanisms underlying priority effects is fundamental
to understanding patterns of biodiversity.