The aim of this special issue is to highlight the critical importance of biological soil crusts to ecosystem structure and functioning, with contributions from a wide range of approaches and disciplines, including taxonomy, molecular biology, community ecology, and functional ecology.

Ecological units, such as population, community, or ecosystem are pivotal concepts in the ecological sciences; this holds for theoretical, practical, as well as institutional aspects. Some of these concepts have been developed in the very beginning of ecology as a scientific endeavour. In the end of the 1860s, Karl August Möbius conducted a research project on the oyster banks of the German coasts, describing them as communities and coining the term "biocoenosis" (Möbius 1877). Other concepts were introduced much later, such as the neologism "ecosystem" (Tansley 1935) that mainly served to demarcate a theoretical difference, in particular against the organismic views of Frederic Clements and his followers, and an overemphasis on the biological elements which Tansley perceived. Being the major elements of ecological theory building, all of these concepts are neither immobile in their use and meaning nor exclusively fixed to ecology. They have experienced historical and epistemic shifts in their interpretation and application and they have moved back and forth between different contexts: they were adopted (metaphorically or literally) from other scientific fields, as is the case for the term "population" that was conceptualized in statistics and economics since about mid-19th century. Other ecological concepts were picked up from everyday language, such as the "niche", which is still in use today, or also the famous metaphor of "the lake as a microcosm", being one of the first conceptualizations of a lake as a biological system – and thus as a unit. The development of the concept "ecosystem" is certainly one of the most convincing examples for a highly fertile extension beyond its original use, not only in management and policies but also in medicine and economics: human bodies or parts of it are looked at as ecosystems and a national economy might well be described as an ecosystem.

We have seen that concepts of ecological units, as concepts in general, are moving concepts: highly dynamic tools that afford and structure thoughts and action. This dynamic quality is worthwhile to be noted, as it allows ecological knowledge to develop and to accommodate to new challenges. It leads, however, also to problems and ambiguity, a complaint that accompanies ecology from its very beginning. Thus it is important to – time and again – revisit conceptual tools and to discuss them both in their historical and recent context as well with respect to their epistemological, political and ethical challenges (Schwarz & Jax 2011). This is the very subject of the current special issue. A basic thesis, and a reason why we deal with the different concepts that we assemble here under the heading of "ecological units" together is that the different concepts are not isolated from each other but that they form what we have called a "conceptual cluster" (Jax & Schwarz 2011). A conceptual cluster assembles concepts with common properties in terms of their epistemology, their meaning and function in ecology, and the phenomena they describe. This is the case with ecological units. Thus, for example, it is common to all these concepts that they describe units relevant to ecological research containing (usually) more than a single individual organism and that they have all been subject to the same questions during their histories, such as how to conceptualise and describe their boundaries.

The purpose of this special issue is to investigate the conceptualisation of ecological units in their historical and recent context. Because this context is necessarily a dynamic one, historically and epistemically, it suggests itself that it is a matter of concern for a vital science such as ecology, to also include historical and philosophical considerations in the constant efforts of developing better theories and good practice in applications. Looking at concepts as tools allows both to discuss theoretical problems as well as scientifically informed practices. We hope that the contributions to this special issue will help to encourage attention to the conceptualization of ecological units and the role of these concepts as conceptual tools in the building of ecological knowledge.

Jax, Kurt & Schwarz, Astrid (2011): Structure of the handbook, in: Astrid E. Schwarz & Kurt Jax (eds.), Ecology revisited: reflecting on concepts, advancing science, pp. 11-17. - Dordrecht (Springer).

Möbius, Karl August (1877). Die Auster und die Austernwirtschaft. 126 pp. Berlin (Wiegandt, Hempel & Parey).

Schwarz, Astrid E. & Jax, Kurt (eds.) (2011): Ecology revisited. Reflecting on concepts, advancing science. 444 pp. - Dordrecht (Springer).

Tansley, Arthur G. (1935): The use and abuse of vegetational concepts and terms. - Ecology 16, 284-307