Moderators affecting the dispersal-range size relationship
To investigate how differences between dispersal processes, evolutionary
history, and study design affect the overall dispersal-range size
relationship, we fitted a Generalized Linear Mixed Model (GLMM) with
Binomial error distribution (link probit) and study ID as a random
effect. To test the effect of the 17 moderators (Table 1) on the
presence or absence of a dispersal-range size relationship, we performed
a forward model selection procedure. We started by fitting a model with
only random effects, and sequentially added significant fixed factors
until reaching a final model. To decide which variable to include in the
model in every time step, we tested for the significance of all fixed
factors that can be added to the base model (i.e., all factors that are
not already included in the model at that point) using a log-likelihood
ratio test to identify the most significant variable to add (andp < 0.05). Each model was tested for collinearity using
the vif function from the ‘car’ R package (Fox & Weisberg 2019).
If a new added variable was collinear with one already selected in the
model, the new variable was excluded from the selection procedure. We
retained variables with a generalized variance inflation factor (GVIF)
smaller than 2, which is the square root of the threshold value for the
standard VIF (VIF = 5), and indicates limited collinearity. We used
generalized linear mixed-effects models in a Bayesian setting
(bglmer function from R package ‘blme’; Dorie et al. 2021).
We only included positive and neutral relationships as response
variables, negative relationships were excluded as they were too few to
perform an analytical test on (10 out of 478 relationships in total).
All continuous variables (number of dispersal-related traits,
biogeographical region size and number of species) were rescaled from 0
to 1. Also, 13 relationships for clades that were only assessed in a
single study (mycorrhizal fungal, diatoms and liverworts) were excluded
as they could not be placed in any of the two broad clades (animals,
vascular plants) assessed here. 36 relationships were also excluded as
did not report number of species and 13 were excluded as they missed
information on highest taxonomic level. The final model included in
total 410 relationships from 81 studies.
In addition to a global model including all relationships, we ran
separate analyses for the two broad clades: animals and vascular plants.
In animals, we also included the specific taxon (e.g., birds, mammals,
fish, insects) as a factor related to the departure/transfer dispersal
process, as these clades may differ in their dispersal ability based on
certain biological features (e.g., presence or absence of wings). We
then excluded invertebrates and trematodes, the former because it cannot
be assigned to a specific taxonomic group and the latter, because it
only has two reported relationship from a single study (Thieltges et al.
2011). Because temperature regulation and latitude were correlated, we
ran two separated models including either temperature regulation or
latitude. We recoded ‘tropical’ and ‘multiple latitude’ as
‘tropical-multiple latitude’, and ‘subtropical’ and ‘temperate’ as
‘subtropical-temperate’ for the model including latitude, due to
complete separation. In plants, we excluded factors that lacked
variation within plant studies, such as dispersal type (passive vs.
active), temperature regulation, realm (only studies on terrestrial
plants), species age (only one study included this), taxonomic unit, and
the taxonomic breadth level ‘class’. Latitude was recoded similarly as
done for animals. Model fitting and selection for both subsets was done
as described above for the complete data-set.