Density dependence of seed dispersal and fecundity profoundly alters the spread dynamics of plant populations

Plant population spread has fundamental ecological and evolutionary
importance. Both determinants of plant population spread, fecundity and
dispersal, can be density-dependent, which should cause feedback between
population densities and spread dynamics. Yet it is poorly understood how
density-dependence affects key characteristics of spread: spread rate at
which the location of the furthest forward individual moves, edge depth
(the geographical area over which individuals contribute to spread) and
population continuity (occupancy of the spreading population). We present
a general modelling framework for analysing the effects of
density-dependent fecundity and dispersal on population spread and
parameterize this framework with experimental data from a common-garden
experiment using five wind-dispersed plant species grown at different
densities.  Our model shows that density-dependent fecundity and
dispersal strongly affect all three population spread characteristics for
both exponential and lognormal dispersal kernels. Spread rate and edge
depth are strongly correlated but show weaker correlations with population
continuity. Positive density-dependence of fecundity increases all three
spread characteristics. Increasingly positive density-dependence of
dispersal increases spread rate and edge depth but generally decreases
population continuity. Density-dependent fecundity and dispersal are
largely additive in their effect on spread characteristics. For population
continuity, the joint effects of density-dependent fecundity and dispersal
are somewhat contingent on the dispersal kernel. The common-garden
experiment and the experimentally parameterized mechanistic dispersal
model revealed density-dependent fecundity and dispersal across study
species. All study species exhibited negative density-dependent fecundity,
but they differed qualitatively in the density-dependence of dispersal
distance and probability of long-distance dispersal. The negative
density-dependence of fecundity and dispersal found for three species
reinforced each other in reducing spread rate and edge depth. The
positively density-dependent dispersal found for two species markedly
increased spread rate and edge depth. Population continuity was hardly
affected by population density in all study species except Crepis sancta
in which it was strongly reduced by negatively density-dependent
fecundity. Synthesis. Density-dependent fecundity and seed dispersal
profoundly alter population spread. In particular, positively
density-dependent dispersal should promote the spread and genetic
diversity of plant populations migrating under climate change but also
complicate the control of invasive species.