Insect herbivory, plant defense, and early Cenozoic climate change

Insect damage on fossil leaves from the Central Rocky Mountains, United States, documents the response of herbivores to changing regional climates and vegetation during the late Paleocene (humid, warm temperate to subtropical, predominantly deciduous), early Eocene (humid subtropical, mixed deciduous and evergreen), and middle Eocene (seasonally dry, subtropical, mixed deciduous and thick-leaved evergreen). During all three time periods, greater herbivory occurred on taxa considered to have short rather than long leaf life spans, consistent with studies in living forests that demonstrate the insect resistance of long-lived, thick leaves. Variance in herbivory frequency and diversity was highest during the middle Eocene, indicating the increased representation of two distinct herbivory syndromes: one for taxa with deciduous, palatable foliage, and the other for hosts with evergreen, thick-textured, small leaves characterized by elevated insect resistance. Leaf galling, which is negatively correlated with moisture today, apparently increased during the middle Eocene, whereas leaf mining decreased.

美国落基山脉中部的化石叶片昆虫取食痕迹数据集，记录了古新世晚期（湿润暖温带至亚热带，以落叶植被为主）、始新世早期（湿润亚热带，落叶与常绿植被混合）以及始新世中期（季节性干旱的亚热带，落叶与厚叶常绿植被混合）三个地质时期内，植食性昆虫对区域气候变化与植被更替的响应。在这三个时期中，叶片寿命较短的植物类群均比长寿命类群承受了更高的植食取食强度，这与现存森林的相关研究结论一致——长寿命厚叶植物具备更强的抗虫性。植食取食频率与取食多样性的方差在始新世中期达到峰值，这表明此时两种截然不同的植食取食综合征的占比显著提升：一类对应具有适口性落叶的植物类群，另一类对应宿主为常绿、厚质地小型叶片且抗虫性提升的类群。现今与湿度呈负相关的叶瘿形成现象，在始新世中期明显增多，而潜叶取食行为则有所减少。