DOI: 10.5061/dryad.wwpzgmsfs
title: Direct and indirect effects of island size and wave exposure on shoreline arthropod diversity

description: Aim: Available theories suggest that the species number increases with
 island size, but the relationship between island size and species density,
 the number of species per unit area, is less understood. Moreover,
 mechanisms connecting island size and species density are not always
 evident because small and large islands differ in physical characteristics
 other than island size. The purpose here was to disentangle direct and
 indirect effects of island size and wave exposure on the species density
 of arthropods from herb-dominated shore habitats. Location: Islands in the
 Stockholm Archipelago, Sweden Taxon: Spiders (Araneae) and beetles
 (Coleoptera) Methods: We collected spiders and beetles on vegetated shores
 on islands of varying sizes, and used structural equation models to
 disentangle direct and indirect island size and wave exposure effects on
 species density. Results: Island size affected diversity of web-building
 spiders, predatory beetles and other beetles (detritivores and omnivores),
 direct and indirectly. Indirect effects arose because large islands have
 more trees and steeper slopes than small islands, both of which affected
 species occurrence. Because height above the sea level reduces the effect
 of wave exposure, there were effects of these variables on the forest
 cover and on the diversity of web-building spiders. Finally, surrounding
 forest cover and diversity of flowering plants within sites increased
 diversity of herbivore beetles and cursorial spiders, likely because these
 sites provide more diverse resources. Main conclusions: The result that
 predictors for species density vary between functional groups, with
 mechanisms that may or may not relate to island size, suggest potential
 developments in island biogeography theories. Future studies of island
 size effects should also attempt to quantify indirect effects, and not
 only total effects.
description: Methods are described in the paper R code was
 used for the Structural Equation Models in the paper
  
description: 'island data.txt': This file contain all data used for
 the SEM. Metadata: Site - Letter refers to island in
 Fig. 1 in the paper; Coordinates - position of the site; Island group -
 see Fig. 1; Island name - see Fig. 1; Date - Collection date; Year -
 Collection year; islsize - Size of island in ha; open_area - area of open
 habitats (m<sup>2</sup>) in the buffer (diameter 50m);
 forest_area - area of forest habitats (m<sup>2</sup>) in the
 buffer (diameter 50m); rock - area of rocky habitats
 (m<sup>2</sup>) in the buffer (diameter 50m); dem - elevation
 above sea level as estimated from a digital elevation model; slope - unit
 is degrees; herb_sp - the number of herbs; meanh - mean height of
 vegetation (cm); exposure - wave exposure as estimated using a model which
 calculates exposure based on fetch and prevailing wind conditions, as
 described in Sundblad et al. (2014); total_beetles - the total number of
 beetle species; herbivore_beetles - the number of herbivorous beetle
 species; predatory_beetles - the number of predatory beetle species;
 other_beetles - the number of other beetle species; total_spiders - the
 total number of spider species; cursorial_spiders - the number of
 cursorial spider species; web_spiders - the number of web
 spiders Area of each sample - 3.5
 m<sup>2</sup> Missing values indicated with
 NA. 'Beetle occurrence data.txt': This file
 contain the number of collected beetle individuals per site and species.
 Site identities as in 'island data.txt'
 'Spider occurrence data.txt': This file contain the
 number of collected spider individuals per site and species. Site
 identities as in 'island data.txt'