Varying thermal exposure, host-plant traits and oviposition behaviour across vegetation ecotones

Abstract

Vegetation cover generates local microclimatic gradients in the understorey, being especially pronounced at narrow ecotones linking open and forested habitats (open-closed ecotones). They provide key habitats for multiple insect communities and may largely determine the exposure of herbivorous insects to the increasing impacts of climate change. We report parallel measurements of microclimatic variables, multiple host-plant traits, and oviposition behaviour in Mediterranean populations of two Pieris butterflies across ecotones of vegetation cover. Open microhabitats were significantly warmer, drier, and more exposed to thermal amplification, which increased temperatures to values affecting insect larval survival. Host plants advanced their reproductive phenology and were shorter. Open microhabitats also inhibited the development of shade-adapted plants (e.g. Alliaria petiolata), decreasing fruit production. In contrast, the reproduction of sun-adapted host plants (e.g. Lepidium draba) was vigorous in the open microhabitats and completely inhibited in closed microhabitats, which were exclusively inhabited by non-reproductive ramets. Key plant traits for the selection of oviposition sites by butterflies, such as foliar water and chlorophyll contents, varied significantly across the open- closed ecotones. Foliar water content was always lower in the open microhabitats, whereas foliar chlorophyll gradients differed between sun- and shade-adapted plants. The oviposition behavior of Pieris butterflies across the ecotones differed significantly between the thermotolerant species (P. rapae, preferentially selecting open microhabitats) and the thermosensitive species (P. napi, selecting microhabitats protected by vegetation cover), matching the values of thermal susceptibility estimated from parallel heat tolerance assays of the populations. The larvae of the thermotolerant Pieris species grew under completely different thermal conditions due to differential microhabitat selection, indicating marked interspecific differences in thermal exposure (5-10 {degrees}C). These results suggest that the impacts of global warming in these communities will likely be mediated by open-closed ecotones, which determine pronounced local variability in thermal exposure, oviposition placement, and host-plant traits affecting larval performance in summer.