DOI: 10.5061/dryad.rbnzs7hg3
title: Data from: Are all forms of defense lost on islands? Persistence of an indirect defensive trait in six island colonists from New Zealand

description: The loss of defense hypothesis posits that island colonizers experience a
 release from predation on the mainland and subsequently lose their
 defensive adaptations. However, while support for the hypothesis from
 direct defensive traits is abundant, far less is known about indirect
 defensive traits. Leaf domatia are cave-like structures produced on the
 underside of leaves that facilitate an indirect defensive interaction with
 predaceous and microbivorous mites. I tested the loss of defense
 hypothesis in six domatia-bearing taxa inhabiting New Zealand and its
 offshore islands. No support for the loss of defense hypothesis was found.
 Changes in domatia investment were instead associated with changes in leaf
 size – a trait that has been repeatedly observed to undergo rapid
 evolution on islands. Overall results suggest that not all types of
 defense are lost on islands.
description: (a) Data collection Data
 collection was conducted between June 2018 and April 2019. The six study
 taxa (<em>Coprosma
 repens </em>[Rubiaceae]<em>, C. rhamnoides, C.
 robusta, C. lucida, Elaeocarpus dentatus </em>[Elaeocapraceae]
 and<em> Vitex lucens</em> [Lamiaceae]) were chosen as they are
 the most widely distributed across New Zealand’s north-eastern islands.
 All taxa produce pit domatia, except <em>E. dentatus</em>
 which produces tent domatia. Mainland field sampling was conducted in the
 Kaimai-Mamaku Forest Park in Tauranga (37°41ʹS, 175°45ʹE). This site was
 chosen because the Kaimai Ranges span a large latitudinal extent of the
 north-eastern corner of New Zealand, and therefore represent the probable
 source pool for many island populations. Island field sampling was
 conducted on seven islands: Cuvier, Great Mercury, Red Mercury, Otata,
 Ruamahuaiti, Ruamahuanui, and Waiheke. Individuals were chosen haphazardly while
 walking through easily accessible forest sections. Only fully expanded,
 mature leaves were measured (1-3 leaves per individual from at least 30
 individuals per site per species). Leaf length was measured as the longest
 linear distance from the most proximal to the most distal point of the
 leaf lamina using a digital calliper. Leaf width was measured as the
 widest distance across the leaf lamina perpendicular to the leaf length
 measurement. Leaf area was calculated as the product of leaf length and
 leaf width. Although more accurate methods of estimating leaf area exist
 [e.g., leaf scanners, image recognition software, or using ad hoc leaf
 shape correction factors 43], leaf <em>x</em> width
 calculations sufficed for the purpose of this analysis as it is not
 concerned with among species differences in leaf size <em>per
 se</em>. Couched in other terms, any errors associated with leaf
 area estimates are consistent between island-mainland comparisons. In a
 prior study incorporating the same species. I trialled ad hoc correction
 factors and found they provided little utility. Further, a leaf scanner
 could not be used as most of the islands included in this study are
 protected by the Department of Conservation, which does not allow
 destructive sampling protocols. Domatia were counted systematically in a
 basipetal direction with the aid of a USB microscope (Toolcraft DigiMicro
 2.0 Scale). It should be noted that while this study quantifies defense
 investment by domatia count, experimental work with Grape cultivars has
 demonstrated that mite abundance on the phylloplane can scale with domatia
 size. To expand upon data gathered in the field,
 pressed herbarium specimens from a further 53 islands and 58 mainland
 sites from the Auckland War Memorial Museum Herbarium (AK) were measured.
 The Herbarium houses an extensive collection of high-quality, preserved
 specimens of both indigenous and exotic plants spanning the full
 geographic extent of the New Zealand landmass – including its offshore
 islands. To standardize sampling across specimens, 3 leaves per specimen
 were measured using the same methodology outlined above. Leaves were
 chosen haphazardly, and care was taken to not damage specimens (i.e.,
 gloves and minimal calliper–specimen contact). 1,129 observations from 60
 islands and 59 mainland sites comprised the final dataset.
 (b) Data analysis To test
 for mainland-island differences in domatia production, while
 simultaneously accounting for leaf size and exploring differences in
 domatia–leaf size scaling, I performed a linear mixed effects models of
 number of domatia production (total count per leaf) against leaf area
 (cm<sup>2</sup>), insularity (island, mainland), and their
 interaction. Both domatia count and leaf area were logarithm transformed
 to conform to linearity. One was added to counts of domatia before
 transformation to avoid biologically nonsensical values (i.e., negative
 infinity). Species was included as a random effect permitting both
 intercept and slope to vary among species. Analysis and data visualization
 were conducted in R version 4.2.2 (R Core Team 2022) using the
 ‘<em>lme4</em>’ and ‘<em>tidyverse</em>’ packages.
description: RStudio