Seed size–number trade‐offs are absent in the introduced range for three congeneric plant invaders

Abstract

Abstract Seed size and seed number are key functional traits that can affect invasion success and evolve in the introduced range. With finite resources, seed size and number are expected to trade off. If trade‐offs are relaxed in the introduced range, this could promote invasion. Few studies have compared life‐history trade‐offs between ranges, so these dynamics are still poorly understood. Using three Rumex spp. we asked: (a) Is there evidence for differences in seed size, number or trade‐offs in response to climate in the native and introduced ranges? (b) Do trade‐offs differ between provenances when grown in common gardens? and (c) What is the relative contribution of environment and maternal line to seed size variation? We quantified seed size and fecundity of ~160 individual plants across 16 populations per species in each range and grew around 100 maternal lines per species across common gardens in New Zealand. We modelled seed size, seed number and their trade‐off, accounting for climate, range and plant height. We also examined seed size in common gardens as a function of maternal seed size to assess the relative contribution of environment and maternal line. Two widespread invasive alien species, R. crispus and R. obtusifolius, had greater seed production in the introduced range. Although R. conglomeratus and R. crispus showed seed size–number trade‐offs in the native range, none of the three species showed trade‐offs in the introduced range. Maternal seed size explained ≥35% of the variation in seed size in common gardens for R. conglomeratus and R. crispus. In R. obtusifolius climate was more important, where seed size increased in response to precipitation in the introduced range. Synthesis. For the first time, we document changes in seed size–number trade‐offs between the native and introduced ranges for two alien plant species. Greater seed production in the introduced range for plants from both ranges suggests that abundant resources, rather than evolution, allowed these species to avoid trade‐offs in the introduced range. Heritable variation in seed size and different climate responses between ranges suggest that seed size may evolve as plants adapt to their introduced range.