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Islands as areas for the study of community assembly Compared with theoretical or laboratory studies on community assembly, studies of natural microcosms such as tree holes, or studies of the recovery of devastated continental areas, studies of the rebuilding of true island biotas on a macro-scale have both advantages and disadvantages. Island studies are often more informative than studies of the recovery of continental areas simply because the devastated area is clearly defined and separated from its source, so that colonizing organisms can usually be clearly identified as such, and not confused with transients. Moreover, the encircling barrier reduces the likelihood of a ‘rescue effect’ (declining populations being ‘rescued’ from extinction by infusions of individuals from populations outside the area in question), thus allowing extinctions to occur and to be recognized. With both immigrants and extinctions clearly identifiable, turnover rates can be estimated, at least in theory, with some precision. The most obvious disadvantage of such studies is that they are seldom possible. Destruction of an island's biota followed by natural reassembly is a rare occurrence, and if such natural experiments are to be exploited fully by scientists they must be recognized and acted upon quickly. The rarity of such events means that each must be analysed and studied; scientists cannot afford to let one pass unstudied. Each is fundamentally one of a small number of parallel ‘replicate’ natural experiments with considerable importance in helping to develop any general picture that exists.

One of the central themes of ecology is the origin of communities. This theme comprises a number of subsidiary questions, such as how primary plant and animal colonists become established on abiotic substrates, and how the various links and dependencies that go to make up a functioning community arise, and in what order; in fact, how community complexity develops and whether there are any ‘rules’ underpinning this. The natural reassembly of a biota following an extirpating eruption provides biologists with a rare opportunity to monitor the process, if they are lucky, from the beginning, allowing them to attempt to answer the first of these subsidiary questions. There have been a number of studies of the recovery of areas that are not in themselves islands, which have been devastated by fire, flood, clear-felling, or by being covered with ash or lava. In such cases, the reservoir of species for recolonization is contiguous with the devastated area, so that it is sometimes difficult to distinguish true immigrants from the many transients. For the same reason, the ease with which individuals can enter the devastated area from adjacent unaffected species pools, recolonization occurs so readily that species extirpations are often precluded, or may occur without being noted by observers. Commonly, it is not known whether any remnant biota persist in the devastated areas – for example, as seed banks in the soil. Primary colonists commonly have traits that maximize dispersal but allow growth on dry infertile substrates.

New or recently sterilized islands (for example through volcanic activity), provide ecologists with natural experiments in which to study colonization, development and establishment of new biological communities. Studies carried out on islands like this have provided answers to fundamental questions as to what general principles are involved in the ecology of communities and what processes underlie and maintain the basic structure of ecosystems. These studies are vital for conservation biology, especially when evolutionary processes need to be maintained in systems in order to maintain biodiversity. The major themes are how animal and plant communities establish, particularly on 'new land' or following extirpations by volcanic activity. This book comprises a broad review of island colonization, bringing together succession models and general principles, case studies with which Professor Ian Thornton was intimately involved, and a synthesis of ideas, concluding with a look to the future for similar studies.