Spoil tip production is one of the most extreme means of soil destruction, replacing the native soil with a coarse substrate. In this paper, we aim to determine the colonization of soil biota in new substrates, using collembola assemblages as an indicator. In Northern France, we sampled collembola communities in 11 coal mine spoil tips and their surroundings divided in four stages of vegetation development: bare soil, meadow, shrub and tree covers. We demonstrated that collembola assemblages of spoil tips were different from those observed in the surrounding native soil. Collembola communities on bare soil were characterized by pioneer (based on the Indval index) or exotic species (new in Northern France). However, homogenization occurred with development of vegetation cover. Indeed, our data showed no difference in springtail diversity between spoil tips and their corresponding environments regarding the tree vegetation cover. Using the Indval method, we defined pioneer, colonizing, opportunist or stenoecious species as a function of substrate affinities. Using the same method, we defined specialists, elective, preferring or indifferent species as a function of vegetation cover affinities, showing similarities with previously published surveys. Hence, our results were obtained by a focused analysis of species and their particularity. Finally, we discussed the interest in and the complementarity between the species analysis approach and the methodology dealing with functional traits and of its importance in the decision process of restoration and/or conservation of nature.

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.

Soil invertebrates are poorly studied in an urban context. Here we examine soil compaction in an urban environment, focusing on its persistence and its impact on biodiversity. This is achieved by studying a sensitive taxon, the collembolan (springtails). Soil core samples were taken in different location of a residential neighbourhood under construction, and collembolan species were extracted using a Berlese-Tullgren funnel. Soil compaction was determined by measuring bulk density. We documented soil compaction, ranging from 1 g/cm3 to 1.6 g/cm3. While collembolans are normally affected by compaction, such an effect was not observed in terms of species diversity, which remains relatively constant under all compaction levels. However, compaction was correlated with differences in the structure of collembolan communities. Indeed, we were able to discriminate different levels of compaction using collembolan community data alone. Moreover, this study confirms that compaction decreases with time. However, differences in species community structure can be observed one year after the end of the disturbance. Finally, we provide some insights into the reality of soil compaction in an urban environment, and highlight that the classical biodiversity indices may be insufficient for assessing the impact of an environmental disturbance, and that more detailed analyses at the community level may be very useful, or even indispensable.