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Direct and indirect effects of island size and wave exposure on shoreline arthropod diversity

Citation

Hambäck, Peter; Strandmark, Alma; Aggemyr, Elsa; Cousins, Sara (2019), Direct and indirect effects of island size and wave exposure on shoreline arthropod diversity, Dryad, Dataset, https://doi.org/10.5061/dryad.wwpzgmsfs

Abstract

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.

Methods

Methods are described in the paper

R code was used for the Structural Equation Models in the paper

 

Usage Notes

'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 (m2) in the buffer (diameter 50m); forest_area - area of forest habitats (m2) in the buffer (diameter 50m); rock - area of rocky habitats (m2) 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 m2

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'