The 'terril' effect: Coal mine spoil tips select for collembolan functional traits in post-mining landscapes of northern France

We sampled collembolan communities in 11 coal mine spoil tips (locally called 'terrils') from the North of France, and their surroundings, in various stages of vegetation development (bare soil, herb, shrub and tree vegetation). We wondered whether species, functional trait (Community Weighted Mean trait or CWM) composition and trait diversity (Rao's quadratic entropy) responded to the particular environment of coal mine spoil tips (here called 'terril' effect) and to successional effects and which of these three sets was the best indicator of soil and vegetation effects, taking into account that spoil tips were spatially distant and idiosyncratic (e.g. varying in height, area, past history and surrounding environment). We showed that species, but not traits, were strongly influenced by site effects, while traits were influenced by both 'terril' and successional effects to a significant extent, making traits more robust than species when trying to discern patterns of community response to soil and vegetation changes. Functional diversity did not show any significant multi-trait response to either 'terril' or successional effects, but several functional traits showed an increase in diversity with succession. The overall response of trait diversity was intermediate between species and trait responses. The 'terril' effect was mainly mediated by a coarser soil texture (51% coarse sand compared to 14% in the surrounding environment) which, combined with harsher microclimate conditions, stems in a better representation of functional traits linked to subterranean life, such as shorter body length and increased representation of parthenogenesis. Successional effects (in particular the increase in diversity shown by several functional traits varying with vertical distribution) were mediated by the increasing development of organic horizons but other factors like microclimate were seemingly involved, too. More generally this study showed that shift to subterranean life was an adapted response of collembolan communities to harshness of the environment and that vertically stratified ecosystems allow a more complete expression of their functional diversity.