Time since first record and population density influence range sizes of non-native plants, but also of native plants, in a chronically overgrazed island

Abstract

Background and aims − Humans are increasingly introducing species to new regions. It is necessary to understand the processes that drive the expansion of non-native species into these new habitats across multiple spatiotemporal scales.Material and methods − We studied the spatial distribution of the non-native flora (39 species) of Guadalupe Island (246 km2) in the Mexican Pacific. We analyzed how residence time (time since first report in historical sources, 1875–2004) and species attributes (population density, flowering phenology, and individual height) are related with range sizes of non-native plants. To test whether the residence time – range size relationship of non-native plants can result from other factors besides time since their arrival, we compared it to the residence time – range size relationship of native plants. Range sizes were obtained using herbarium data and a systematic field sampling of 110 transects (50 × 2 m) throughout the entire island. We used beta regression to analyze the relationship of range sizes with residence time and species attributes.Key results − Range sizes of non-natives showed a positive relationship with residence time, flower phenology, and notably with plant density, but not with individual height. However, similar relationships were found for native species, casting doubts on whether our results reflect the range expansion rates of non-native species. Conclusions − Our results suggest that the production of large numbers of propagules, both as a result of long reproductive periods and large population sizes, determines to a large extent the rates of range size expansion of non-native species. However, the relationship we found between time since discovery and range size may arise from sampling biases, biological processes, or – most likely – both. This highlights the need for new approaches that allow us to discern the relative contributions of bias and process in our study of non-native species expansion.