Plant defence syndromes on islands

Aim: It has been hypothesised that insular plant taxa have evolved reduced defences in response to lower herbivore pressure on islands. However, the few studies testing this hypothesis have addressed variation in individual defensive traits, without paying attention to patterns of correlated trait expression (i.e. defence syndromes). Location: Balearic and Canary Islands. Taxon: Ninety-one woody plant species. Methods: We tested whether plant species with contrasting histories of insularity (namely: endemics, non-endemic natives, and exotics) differed in their patterns of variation for a suite of morphological and chemical traits putatively associated with herbivory. For this, we measured eight leaf traits of 42 endemic, 29 native non-endemic, and 20 introduced species for which specimens were sourced from botanical gardens found in two archipelagos: the Balearic and Canary Islands. Results: We conducted phylogenetic-controlled analyses which showed that, contrary to predictions, insular taxa (endemics and non-endemic natives) across both archipelagos similarly exhibited thicker, smaller, leaves with less nutrients and were physically more defended (especially the Balearic species) than exotic species. There were no differences in chemical defence (phenolic compounds) between endemics, non-endemic natives, and exotics. Finally, we also found what appeared to be differential syndromes between archipelagos: whereas species from the Balearic Islands were more physically defended, on average, those from the Canary Islands had higher chemical defences. Main conclusions: Overall, these results point to a functional-physical defence syndrome characteristic of insular plant taxa that is indistinct for endemic and non-endemic taxa, relative to introduced species, as well as quantitative and qualitative differences in defences between archipelagos owing to differences in species composition and likely also to different histories of biotic or abiotic pressure.研究目的：已有假说提出，海岛植物类群(insular plant taxa)为响应岛上更低的植食压力，演化出了弱化的防御策略。然而，目前为数不多验证该假说的研究仅聚焦于单一防御性状的变异，未关注性状间的协同表达模式（即防御综合征(defence syndromes)）。 研究区域：巴利阿里群岛与加那利群岛。 研究类群：91种木本植物。 方法：本研究检验具有不同海岛演化历史的植物类群（即特有种、非特有乡土种以及外来种）在一系列假定与植食作用相关的形态及化学性状上的变异模式是否存在差异。为此，我们从巴利阿里和加那利两个群岛的植物园采集标本，对42种特有种、29种非特有乡土种以及20种外来种的8项叶片性状进行了测定。 结果：我们开展了系统发育控制分析，结果显示与假说预测相反，两个群岛的海岛类群（特有种与非特有乡土种）均较外来种表现出更厚、更小、养分含量更低且物理防御更强的叶片特征（巴利阿里群岛物种尤为显著）。此外，特有种、非特有乡土种与外来种之间的化学防御（酚类化合物(phenolic compounds)）水平并无显著差异。最后，我们还发现两个群岛间存在明显的防御综合征差异：巴利阿里群岛的物种整体物理防御更强，而加那利群岛的物种则具有更高的化学防御水平。 主要结论：总体而言，本研究结果表明，相较于外来种，海岛植物类群具有典型的功能-物理防御综合征特征，且该特征在特有种与非特有类群间并无明显差异；同时，两个群岛间的防御策略存在定量与定性差异，这源于物种组成的不同，也可能与各自的生物或非生物压力历史差异有关。