The taxonomic effect on the dispersal-range size relationship
We found that studies that included species that can only be grouped
into a high taxonomic level such as ‘phylum’ or ‘division’ found less
often positive dispersal-range size relationships than studies that
examined species within lower taxonomic levels (‘family’, ’order’,
’class’). Our separate analyses for plants and animals show that this
effect is primarily driven by studies on plants, in which there was a
clear distinction between e.g. ‘angiosperm-wide’ studies (126/159
relationships, in contrast to 10/298 relationships in animals that was
at ‘phylum’ or higher taxonomic level), versus those performed for a
specific ‘family’ or ‘genus’. This suggests that the disparity of
dispersal-range size relationships when including plant (or animal)
lineages that are phylogenetically distantly related and may therefore
differ to a large extent in the traits that capture their dispersal
ability (e.g., small seeds for wind-dispersed taxa, large seeds for
animal-dispersed taxa, Table 2), will obscure or erase a relationship
between dispersal and range size. In addition, including a clade in
which many species are missing may also simply bias the initial dataset
towards e.g. well-sampled species with relatively large range sizes.
This means that a clear hypothesis and expectation on how dispersal
affects range size in a studied clade is essential. In addition,
including phylogenies to control for clade differences could provide a
solution. We found that only 24% and 44% of dispersal-range size
relationships that have been performed on animals and on plants,
respectively, explicitly considered phylogenetic relationships. Even
though phylogenetic correction was not selected in our final statistical
model, we should be aware that we may not have had the statistical power
to detect its real effect.