Testing for latitudinal gradients in defense at the macroevolutionary scale

Abstract

Plant defenses against herbivores are predicted to evolve to be greater in warmer climates, such as lower latitudes where herbivore pressure is also thought to be higher. Instead, recent findings are often inconsistent with this expectation, suggesting alternative hypotheses are needed. We tested for latitudinal gradients in plant defense evolution at the macroevolutionary scale by characterizing plant chemical defenses across 80 species of the evening primroses, spanning both North and South America. We quantified phenolics in leaves, flowers, and fruits, using advanced analytical chemistry techniques. Dominant individual ellagitannin compounds, total concentrations of ellagitannins, flavonoids, total phenolics, and compound diversity were quantified. Variation in these compounds was predicted with latitude, temperature, precipitation, and continent using phylogenetic generalized least squares (PGLS) multiple regression models. Latitude did not strongly explain variation in chemical defenses. Instead, fruit total ellagitannins, oenothein A, and total phenolics were greater in species inhabiting regions with colder climates. Using analytical chemistry and 80 species in two continents, we show that contrary to classic predictions, concentrations of secondary metabolites are not greater at lower latitudes or in warmer regions. We propose higher herbivore pressure in colder climates and gradients in resource availability as potential drivers of the observed patterns in Oenothera.

Search for top-down and bottom-up drivers of latitudinal trends in insect herbivory in oak trees in Europe

“…Latitudinal and climatic gradients in insect herbivory, plant chemical traits and bird attack rates -Variation in the incidence of gall-inducers and leaf-miners was associated with variation in temperature and precipitation, rather than with latitude per se (Anstett et al, 2018;Moreira et al, 2018;Loughnan & Williams, 2019). Climatic effects on herbivory were, however, contingent on herbivore feeding guild, with significant effects of climatic conditions detected for gallinducers and leaf-miners, but not for leaf herbivory (leaf chewers and skeletonizers).…”

Multiple metrics of latitudinal patterns in insect pollination and herbivory for a tropical‐temperate congener pair

“…The opposite pattern is seen in Oenothera , with stronger chemical defenses at higher latitudes across the genus (Anstett et al. ), but at lower latitudes within O. biennis (Anstett et al. ).…”

“…Their shared evolutionary history allows for comparisons of shared traits such as secondary metabolites with known defensive function (Rasmann and Agrawal , Pearse and Hipp , Anstett et al. ). However, measuring interaction importance across a genus trades breadth for detail, as measuring interactions in the field across many species is infeasible.…”

“…For example, observed trends across Asclepias of more toxic, diverse, and inducible cardenolides at low latitudes (Rasmann and Agrawal 2011) contrasts with greater herbivore resistance observed in higher-latitude populations of Asclepias syriaca (Woods et al 2012). The opposite pattern is seen in Oenothera, with stronger chemical defenses at higher latitudes across the genus (Anstett et al 2018), but at lower latitudes within O. biennis (Anstett et al 2015). In both of these systems, the conflicting results at different scales are complicated by the breadth-depth trade-off, as the single-species studies used herbivory in common outdoor gardens to quantify resistance and identify important defensive traits, while the genus-level studies compared secondary metabolites that are typically defensive in the genus, without the context of herbivore pressure, other defensive traits that could be important in each species, or herbivory tolerance.…”

“…Studies of wide-ranging genera can address some of the limitations of one-species studies by spanning a wider geographic range than any one species and including species with different range sizes. Their shared evolutionary history allows for comparisons of shared traits such as secondary metabolites with known defensive function (Rasmann and Agrawal 2011, Pearse and Hipp 2012, Anstett et al 2018). However, measuring interaction importance across a genus trades breadth for detail, as measuring interactions in the field across many species is infeasible.…”