DOI: 10.5061/dryad.t1g1jwtbn
title: Testing the contribution of vertebrate predators and leaf traits to mainland-island differences in insect herbivory on oaks

description: Ecological theory predicts that herbivory should be weaker on islands
 compared to mainland based on the assumption that islands have lower
 herbivore abundance and diversity. However, empirical tests of these
 predictions are rare, especially for insect herbivores, and those few
 tests often fail to address the mechanisms behind island-mainland
 divergence in herbivory. In particular, past studies have not addressed
 the relative contribution of top-down (i.e., predator-driven) and
 bottom-up (i.e., plant-driven) factors in these dynamics. To address this,
 we experimentally excluded insectivorous vertebrate predators (e.g.,
 birds, bats) and measured leaf traits associated with herbivory in 52
 populations of 12 oak (Quercus) species in three island-mainland sites:
 The Channel Islands of California vs. mainland California, Balearic
 Islands vs. mainland Spain, and the island Bornholm vs. mainland Sweden (N
 = 204 trees). In each site, at the end of the growing season, we measured
 leaf damage by insect herbivores on control vs. predator-excluded branches
 and measured leaf traits, namely: phenolic compounds, specific leaf area,
 and nitrogen and phosphorous content. In addition, we obtained climatic
 and soil data of island and mainland populations using global databases.
 Specifically, we tested for island-mainland differences in herbivory, and
 whether differences in vertebrate predator effects or leaf traits between
 islands and mainland contributed to explain the observed herbivory
 patterns. Supporting predictions, herbivory was lower on islands compared
 to mainland, but only in the case of Mediterranean sites (California and
 Spain). We found no evidence for vertebrate predator effects on herbivory
 on either islands or mainland in any study site. In addition, while
 insularity affected leaf traits in some of the study sites
 (Sweden-Bornholm and California), these effects were seemingly unrelated
 to differences in herbivory. Synthesis: Our results suggest that
 vertebrate predation and the studied leaf traits did not contribute to
 island-mainland variation patterns in herbivory, calling for more nuanced
 and comprehensive investigations of predator and plant trait effects,
 including measurements of other plant traits and assessments of predation
 by different groups of natural enemies.
description: We experimentally excluded insectivorous vertebrate predators
 (e.g., birds, bats) and measured leaf traits associated with herbivory in
 52 populations of 12 oak (<em>Quercus</em>) species in three
 island-mainland sites: The Channel Islands of California vs. mainland
 California, Balearic Islands vs. mainland Spain, and the island Bornholm
 vs. mainland Sweden (N = 204 trees). In each site, at the end of the
 growing season, we measured leaf damage by insect herbivores on control
 vs. predator-excluded branches and measured leaf traits, namely: phenolic
 compounds, specific leaf area, and nitrogen and phosphorous content. In
 addition, we obtained climatic and soil data of island and mainland
 populations using global databases.
description: # Testing the contribution of vertebrate predators and leaf traits to
 mainland-island differences in insect herbivory on oaks
 [https://doi.org/10.5061/dryad.t1g1jwtbn](https://doi.org/10.5061/dryad.t1g1jwtbn) ## Description of the data and file structure We experimentally excluded insectivorous vertebrate predators (e.g., birds, bats) and measured leaf traits associated with herbivory in 52 populations of 12 oak (*Quercus*) species in three island-mainland sites: The Channel Islands of California vs. mainland California, Balearic Islands vs. mainland Spain, and the island Bornholm vs. mainland Sweden. Specifically, we included six conspecific island-mainland comparisons consisting of local oak species that were present both in islands and in mainland, and three congeneric comparisons consisting on insular endemic species (i.e., species only present in islands). For each of the nine oak species that were present on islands, we selected two or three populations in one or two islands and three paired conspecific or congeneric mainland populations, for a total of 52 populations. At each population, we selected three to four adult trees (N = 204) and experimentally excluded vertebrate predators (e.g., birds, bats) from two opposing, low-hanging accessible branches. At the end of the growing season, we measured leaf damage by insect herbivores on control vs. predator-excluded branches in all trees, and measured leaf traits, namely: phenolic compounds, specific leaf area, and nitrogen and phosphorous content (nutrient content has been measured at the population level). ### Files and variables #### File: Vázquez-González\_et\_al\_2024\_-\_Herbivory\_data.csv **Description:** Branch-level data of average herbivory ##### Variables * SP: Species code; Quercus ilex -ILEX, Q. coccifera - COCC, Q. agrifolia - AGRI, Q. chrysolepis - CHRY, Q. tomentella - TOME, Q. suber - SUBE, Q. berberidifolia - BERBE, Q. pacifica - PACI, Q. robur - ROBU, Q. lobata - LOBA, Q. × macdonaldii - MACD and Q. petraea -PETR * SITE: Sites where the study was conducted; USA for Channel Islands of California vs. mainland California, SPAIN for Balearic Islands vs. mainland Spain and SWEDEN for the island Bornholm vs. mainland Sweden * I.M: Island vs. Mainland * POP: Population name (ID) within each site and for each species * PLANT: Tree ID for each population and species * EXCLUSION: Whether control or predator excluded branch (CT = Control branch; EX = Predator-excluded branch) * REP: branch replicate; 1 or 2 * Mean_G: average branch new growth expressed in cm - not available in all cases * Herb: Average herbivory measured on 10 randomly selected leaves per branch, expressed as a percentage (%) * POP_ID: Unique population name (ID) * PLANT_ID: Unique tree ID * PAIR: comparison pair coded as numbers * comp: Type of comparison; LOCAL or ENDEMIC * PAIR_ID: ID for each camparison pair - i.e., what species have been paired in islads vs. mainland #### File: Vázquez-González\_et\_al\_2024\_-\_nutrient\_content\_data.csv **Description:** Population-level data of nutrient content considering only control branches ##### Variables * SP: Species code; Quercus ilex -ILEX, Q. coccifera - COCC, Q. agrifolia - AGRI, Q. chrysolepis - CHRY, Q. tomentella - TOME, Q. suber - SUBE, Q. berberidifolia - BERBE, Q. pacifica - PACI, Q. robur - ROBU, Q. lobata - LOBA, Q. × macdonaldii - MACD and Q. petraea -PETR * SITE: Sites where the study was conducted; USA for Channel Islands of California vs. mainland California, SPAIN for Balearic Islands vs. mainland Spain and SWEDEN for the island Bornholm vs. mainland Sweden * I.M: Island vs. Mainland * POP:  Population name (ID) within each site and for each species * N: Nitrogen content, in mg/g * P: Phosphorous content, in mg/g * POP_ID: Unique population name (ID) * PAIR: comparison pair coded as numbers * PAIR_ID: ID for each camparison pair - i.e., what species have been paired in islads vs. mainland #### File: Vázquez-González\_et\_al\_2024\_-\_Phenolic\_content\_data.csv **Description:**  Tree-level data of phenolic content considering only control branches ##### Variables * SP: Species code; Quercus ilex -ILEX, Q. coccifera - COCC, Q. agrifolia - AGRI, Q. chrysolepis - CHRY, Q. tomentella - TOME, Q. suber - SUBE, Q. berberidifolia - BERBE, Q. pacifica - PACI, Q. robur - ROBU, Q. lobata - LOBA, Q. × macdonaldii - MACD and Q. petraea -PETR * SITE: Sites where the study was conducted; USA for Channel Islands of California vs. mainland California, SPAIN for Balearic Islands vs. mainland Spain and SWEDEN for the island Bornholm vs. mainland Sweden * I.M: Island vs. Mainland * POP: Population name (ID) within each site and for each species * PLANT: Tree ID for each population and species * SLA: Specific leaf area, in mm2/mg * HA: hydroxycinnamic acids in mg/g * CT: condensed tannins in mg/g * HT: hydrolysable tannins in mg/g * FL: Flavonoids in mg/g * POP_ID: Unique population name (ID) * PLANT_ID: Unique tree ID * PAIR: Comparison pair code as numbers * PHEN: total phenolics, the sum of subgroups, in mg/g * PAIR_ID: ID for each camparison pair - i.e., what species have been paired in islads vs. mainland #### File: Vázquez-González\_et\_al\_2024\_-\_population\_data.csv **Description:**  Population-level data used to conduct the PSEM ##### Variables * POP_ID: Unique population name (ID) * Dim.1: PC 1 traits associated with nutritional value of leaves * Dim.2: PC 2 traits associated with phenolic compounds * island.num: dummy variable; 1 = island; 0 = mainland * SITE: Sites where the study was conducted; USA for Channel Islands of California vs. mainland California, SPAIN for Balearic Islands vs. mainland Spain and SWEDEN for the island Bornholm vs. mainland Sweden * PAIR: Comparison pair code as numbers * Herb: Average herbivory across populations, expressed as a percentage (%) * cli.1: PC 1 of climate (unitless) * cli.2: PC 2 of climate  (unitless) * soil.1: PC1 of soil  (unitless) * soil.2: PC 2 of soil  (unitless) * SMD: Hedge's g as a measure of predation effects (unitless) * lower.ci: lower confidence interval for SMD (unitless) * upper.ci: upper confidence interval for SMD (unitless)