Interpopulation Plasticity in a Darkling Beetle Life-History along a Whole Oceanic Island Altitudinal Gradient

SIMPLE SUMMARY: Young oceanic islands often harbour poor biodiversity because they need time to be colonised. However, some animal species can be abundant, presenting high densities and being widely distributed through different altitudinal habitats of high oceanic islands. This is the case of the darkling beetle Pimelia laevigata costipennis Wollaston, 1864 on El Hierro, the westernmost island of the Canary archipelago. Therefore, this kind of framework is ideal for studying the variation of one insect species in body size, reproductive strategies, and populations in different environments. This study shows how environmental factors related to rainfall and temperature affect the different populations of this flightless beetle. Furthermore, in the more favourable habitats, located at higher altitudes, these beetles were larger in body size, laid clutches with smaller eggs, the reproduction occurred later, and showed the highest population in abundance. Lastly, this is the first study that assesses the changes of an insect along a whole insular altitudinal gradient, demonstrating apparent modifications to each habitat condition. ABSTRACT: Insects show remarkable phenotypic plasticity in response to changing environmental conditions. The abiotic factors that determine their phenotypes often vary in time and space, and oceanic islands harbour ideal environments for testing predictions on this matter. The ubiquitous beetle Pimelia laevigata costipennis Wollaston, 1864 (Tenebrionidae) is distributed over the entire altitudinal gradient of the island El Hierro (Canary archipelago), from 0 to 1501 m above sea level. Here, we examine how environmental factors (i.e., rainfall and temperature), associated with the altitudinal gradient, affect the body size, reproductive phenology, clutch size and egg volume, and population dynamics of this ectothermic flightless insect. Pimelia l. costipennis populations inhabiting upland localities, typified by lower temperatures, and greater precipitation and vegetation cover, were larger in body size and laid larger clutches with smaller eggs than those in the lowlands. Moreover, reproduction occurred earlier in the year at lower sites and later at higher sites, whereas activity density was highest in the uplands where it increases with temperature. This study first explores the changes in life history patterns along a whole insular altitudinal gradient, and finds interpopulation plasticity. It confirms that environmental factors associated with species spatial distribution act additively as drivers of phenological and phenotypic expression.