Assembly rules: concept and methodology

The term 'assembly rules' was first coined by Diamond in 1975. But the concept of constraints on the (co-)existence of species at a certain spot can be traced back further than that. The problem of 'the constraint on the development of a community' had already been explored by both Clements and Gleason in their pioneer works on succession (Booth & Larson 1999). In the early decades of ecology in the twentieth century most ecologists took, inspired by the work of Clements, for granted that communities exist as natural, repeated, internally organised units with a considerable degree of integration which governed their structure, function, development or succession and even their evolution. Gleason openly attacked them and offered an alternative concept predicated on the individualistic capabilities of species, continuous variation of the environment and diverse probabilities of arrival of propagules (McIntosh 1995). The topic was then not left untouched as argued by May (1984): 'In 1944 [....} Lack, Elton, Varley, and others used various lines of evidence to argue that competition is a major factor in structuring plant and animal communities. Others argued to the contrary'. Still Diamond's book-chapter in 1975 gave a big impulse to the discussion. The discussion around ecological communities and their assembly is littered with side-tracks and confusion. The old controversy between Clements and Gleason is still standing, although not many people will nowadays go as far as Clements in promoting the community as a superorganism. But the definition of the term community and the debate about the mere existence of them will rouse many ecologist to their writing table to produce more articles on that subject (e.g. Wilson 1991, Keddy 1993, Palmer & White 1994, Mirkin 1994, Dale 1994, Wilson 1994, Looijen & van Andel 1999 and Parker 2001). Drake (1990 cited in McIntosh 1995) even discarded most of what has been studied as communities or assemblages as 'not satisfying a community definition' or possessing 'even a hint of community properties', after which he comes with a new definition about which many other workers will have their doubt. 'Community ecology is often seen as being still a soft science dealing primarily with description of plant an animal associations rather than a hard science making accurate predictions about specified state variables' (Keddy 1992). But Drake ci a!. (1999) argue that 'because all biological systems are assembled in a dynamical sense, any generality in the process would prove valuable to our understanding of nature', apparently they do not agree with Keddy's (1999) call that the time of making predictions has come. To make those predictions, more accurate information about the assembly of a community is needed. The 'mechanics of community assembly' are the phenomena that form 'the primary strut in the framework for a general theory of community organisation'(Drake 1990 cited in McIntosh 1995). It is these 'mechanics of community assembly', that this paper is about. To find assembly rules, we should (1) 'formulate hypotheses on what generalised assembly rules might exist,' (2) 'find ecological situations to test those hypotheses and' (3) 'see whether there is any evidence for the rules, subject to the usual statistical tests to ensure that the patterns are not due to chance' (Wilson 1994). These three steps will also be the guidelines in this paper. First we will discuss the different opinions on the definition of assembly rules, trying to get one, or several general definitions of the assembly rules that are commonly accepted. Then we will review the methodologies of several workers in the field of assembly rules, addressing problems of the ecological systems they use and their experimental set-up. After that we will discuss the problems that arise when one is to statistically test spatial distributions of species and the various null-models that have been proposed. In the end we will try to integrate these three steps and discuss the problems that still remain. These topics will give a glance on some 1of the important factors of the discussion on the concept of assembly rules. There are several peripheral problems that have a large influence on the discussion at hand, like the definition of the community itself, the determination of the species pool and the application of assembly rules in conservation biology. Despite their influence and importance for a good view on the following discussion, they fall beyond the scope of this paper.