Mean and maximum dispersal distance produced distinctive
results. Plant height and seed mass were more strongly correlated
with mean dispersal distance than with maximum
dispersal distance. This is probably because maximum
distances are outliers which are notoriously difficult to measure
(Nathan 2006). The accuracy of all dispersal distance data
depends on sample size and the inclusion of methods to detect
rare long-distance dispersal events. It is important to remember
that our data are maximum observed dispersal distances,
rather than species’ true maxima. Nonetheless, data for maximum
distance were included because they are an important
part of the dispersal process, as long-distance dispersal can be
critical for plant migration and colonization rates (Cain, Damman
& Muir 1998; Levin et al. 2003). Furthermore, seed
dispersal distance varies so much among species (maximum
dispersal distance in our study ranged from just 15 cm to
400 km) that even if researchers’ best efforts in the field do
yield imperfect measures, the bigger picture from a compilation
like this is still expected to be informative. The only potential
bias would be if long-distance dispersal events were harder
to track for small-seeded than large-seeded species. However,
as researchers tend to use methods appropriate to the size of
the seeds that they are tracking (e.g. threads on large seeds and
seed traps for small seeds), we do not expect this to be a serious
problem for our analysis.