Moderators of the dispersal-range size relationship
The final model, including both plants and animals, explained 41% of
the variation (50% considering random effects) and identified four
moderators as main factors explaining the differences in the
dispersal-range size relationship between studies (Fig. 3).
Specifically, studies that included multiple dispersal-related traits,
that were carried out in temperate or subtropical regions or that
included a larger number of species, showed significantly more positive
dispersal-range size relationships than studies including fewer
dispersal-related traits, that were carried out in tropical regions (or
multiple regions), or that included fewer species in the analysis (Fig.
3, Table S2, S3 Suppl. Mat.). Furthermore, studies carried out at higher
taxonomic breadth, such as ‘phylum’ or ‘division’ showed fewer positive
relationships between dispersal and range size than studies carried out
for a particular ‘class’, ‘order’ or ‘family’.
When examining the relationship between dispersal and range size for
animals and plants separately, we found that the factors responsible for
a positive dispersal-range size relationship differed between those
broad clades (Fig. 3, Table S1, S2 Suppl. Mat.). In animals, similar
results as for the global dataset were found, with the number of
dispersal-related traits and number of species positively contributing
to the dispersal-range size relationship. In addition, we found that
studies on endotherms showed fewer significant, positive associations
between dispersal and range size than on ectotherms (Fig. 3, Table S1,
S2 Suppl. Mat.). In plants, taxonomic breadth remained important as a
variable to explain the positive dispersal-range size relationship, with
studies carried out at ‘division’ level or higher showing fewer positive
dispersal-range size relationships than studies carried out at the
‘family’ level (Fig. 3, Table S1, S2 Suppl. Mat.).
It should be noted that a large sample size may increase the probability
of detecting false positives, which could explain our finding that
number of species and number of dispersal-related traits affects the
dispersal-range size relationship. Although, theoretically, this could
go in both a positive or negative direction in terms of dispersal-range
size relationships. The distribution of species numbers used in the
studies is strongly right-skewed: more than 90% of the studied
relationships included less than 500 species (380 relationships, Fig S2
Suppl. Mat.). We performed a sensitivity analysis to explore the effect
of outliers for both the number of species and number of
dispersal-related traits on our findings. Nevertheless, the positive
effect of species number on the dispersal-range size relationship
remains when including only studies with less than 500 species, but not
when only including studies with less than 100 species (Table S3 Suppl.
Mat.). Importantly, the positive effect of including multiple
dispersal-related traits remained after excluding possible outliers
(studies with 10 dispersal-related traits; Table S3 Suppl. Mat.).
Alternatively, our result may reflect serendipity, because the more
traits a study includes, the higher the chance becomes that one of those
traits relates to range size. We tested whether this is the case by
comparing the relationship between the overall dispersal-range size
relationship and the number of dispersal-related traits vs. the
relationship between the overall dispersal-range size relationship and
the number of dispersal ‘factors’ (one factor can be composed of several
dispersal-related traits) included in the models (Table S4 Suppl. Mat.).
Interestingly, although the effect of the number of dispersal-related
traits on the dispersal-range size relationship is significantly
positive, the number of dispersal ‘factors’ included in each
relationship does not have a significant effect (Table S4 Suppl. Mat.).
This means that using multiple dispersal-related traits likely results
in a better approximation of the dispersal process and/or a higher
probability to capture traits that are relevant for dispersal,
increasing the chance to find a positive dispersal-range size
relationship.