Plant dispersal characteristics shape the relationship of diversity with area and isolation

Aim This study disentangles how plant dispersal syndromes influence the
relationship of species richness with area and isolation while also
accounting for the human impact on island biodiversity. It builds on the
potential of islands at the mesoscale and of similar origin to contribute
to the ongoing discussion in island biogeography on what determines
species richness and filtering. Location Denmark, 54 saltwater and
brackish water islands in the North and Baltic Sea Taxon Vascular plants,
including pteridophytes (ferns, clubmosses and horsetails) Methods
Generalized linear models (GLMs) and linear regressions are used to
analyse how dispersal syndromes influence the relationships of species
numbers with island area and isolation, as well as island inhabitation and
human density, respectively. Results Species numbers, as well as mean seed
mass and the proportion of zoochore and anemochore species, are positively
related to island area while the share of water-dispersed species
decreases with increasing area. The slope of the regression line
representing the species-area relationship (SAR) was 0.34 and lies within
the common range for this relationship. Isolation is weakly related to
mean seed mass but has no explaining power for species numbers and the
presence of specific dispersal syndrome on the target islands. Species
richness and seed mass was positively related to human presence. Main
conclusions Human impact for centuries has not overwritten the strong
relationship of species richness with area on the Danish Islands but is
affecting the shape of this relationship. Island area constitutes a strong
filter for different dispersal syndromes and leads to the assumption that
heavier and animal-dispersed seeds are positively related to island area
due to the presence of more bird and mammal species. Human-induced loss of
isolation caused by ongoing traffic and the connection of landmasses by
bridges and ferries may be a reason for the overall low explanatory power
of island isolation. Higher species richness on inhabited islands may
further be linked to higher habitat diversity in human modified
landscapes.