DOI: 10.5061/dryad.4f4qrfjhb
title: Dispersal ability and niche breadth influence interspecific variation in spider abundance and occupancy

description: The relationship between species local abundance and their regional
 distribution (occupancy) is one of the most extensively recognised and
 investigated patterns in ecology. While exceptions exist, the generally
 held model is that locally abundant species also tend to be more
 widespread geographically. However, there is only a limited understanding
 of both the mechanisms driving this relationship, and their scale
 dependency. Here we use occupancy and abundance data for 123 species of
 spider from across the Canary Islands to understand how both dispersal
 ability and niche breadth might mediate variation among species for local
 abundance and occupancy. We test the predictions that (i) dispersal
 ability explains variation among species for both abundance and occupancy,
 and (ii) species with a higher degree of habitat specialisation,
 reflecting more limited niche breadth, will have both higher occupancy and
 abundance. We find no evidence within habitat patches for an effect of
 dispersal ability on either local abundance or site occupancy, while
 across all patches species with higher dispersal ability tend to occupy
 more sites. Species largely restricted to laurel forests have higher
 abundance than species with broader niche breadth, but similar
 occupancy. The study revealed that dispersal ability and niche
 breadth were significant predictors of the abundance-occupancy
 relationship, highlighting the importance of both factors for understating
 patterns of abundance and occupancy among spider species.
description: The 5' region (658 bp) of the mtDNA COI gene was amplified
 using the primers LCO1490 and HCO2198 in order to identify presumed
 biological species (PBS). A custom R script was used to produce an
 unweighted pair group method with arithmetic mean (UPGMA) tree from
 pairwise K2P distances.<br>PBS were then taxonomically assigned to
 species or genus level based on GenBank and BOLD Systems search
 results.<br>PBS (hereafter species) were categorised for dispersal
 ability, considering the potential for juvenile stages to be passively
 dispersed by air currents, while suspended from silk threads, henceforth
 referred to as 'ballooning', which has been broadly used as a
 proxy for dispersal ability.<br>A two-step process was used to
 categorise species as either ‘non-specialist’ or ‘specialist’, based on
 their affinity to the laurel forest habitat, i.e., whether they are
 present in a broad range of habitats or restricted mainly to laurel
 forest. As a first step, the species list was assessed by five Canary
 Island spider specialists, ), to categorise species as either laurel
 forest specialists or non-specialists based on their knowledge of the
 biology of the species.<br>In a second step, species association to
 the laurel forest was quantified using distribution records within the
 Biodiversity Data Bank of the Canary Islands
 (https://www.biodiversidadcanarias.es/biota/). The total number of 500 x
 500 m cells occupied by a species in the archipelago was quantified and
 the percentage of those cells corresponding with laurel forest habitat was
 then calculated. <br>For the subset of species that were categorised
 by the panel of experts as laurel forest specialists, species were ranked
 from highest to lowest percent laurel forest occupancy, and the percentage
 corresponding to the 50% quantile was used for the categorisation of the
 species non-characterised in the first step (i.e., above this percentage
 species were considered as ‘specialists’, whereas below it, species were
 categorised as ‘non-specialist’). Additionally, 10%, 25% and 75% quantiles
 were also used for niche breadth categorisation to explore the robustness
 of further inferences. <br>For each species, local abundance was
 calculated as i) the mean site abundance (i.e., the sum of all individuals
 divided by the number of occupied sites at both island and archipelago
 scales, so with species as the replicate unit), and ii) the individual
 site abundance (i.e., the sum of all individuals within each site, so with
 site as the replicate unit). <br>Occupancy was calculated both
 across islands (i.e., presence or absence of each species within each of
 the islands) and across sites (i.e., presence or absence of each species
 within each site). <br>The latter was again calculated both at the
 archipelago (across all sites) and at the island (across all sites within
 a given island) scales.<br>AORs were expressed as a logistic
 regression between the occupancy and the log of the abundance per species
 (mean site abundance per species).