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The rediscovery of a species that was putatively considered to be extinct can provide valuable data to test biogeographical hypotheses about population decline and range collapse. Moreover, such rediscoveries often generate much-needed publicity and additional funds for the conservation of rare species and habitats. However, like extinction, rediscovery is challenging to define. In this perspective we argue that the 'loss' of a species and its subsequent rediscovery can be understood in terms of the interplay among four socio-ecological factors: (1) the state of knowledge of species loss and rediscovery; (2) the presence of people and/or organizations with the interest, motivation, resources, skills and technology to find target species; (3) the accessibility of the areas, habitats or sites where the species are thought to survive; and (4) the ease with which a species can be located when it is present within a habitat. Thus, species are 'lost' from scientific knowledge for different reasons and, consequently, not all rediscoveries are equally significant for biogeographical research or conservation. Indeed, rediscoveries of species that underwent a well documented decline and disappearance-and are therefore of greatest potential importance for both conservation and biogeographical research-appear to be poorly represented in the literature compared to rediscovered species that were only known from a handful of museum specimens. Thus, carefully distinguishing between the causes of temporal gaps in zoological records is essential for improving the utility of rediscovery data for biogeographical research and conservation practice.

The Amazon drainage basin (Amazonia) contains the largest continuous area of tropical rainforest in the world and is the most species-rich terrestrial ecosystem on Earth. In biogeographical terms, the Amazon rainforest is still somewhat of a mystery, beset by data shortfalls in many taxonomic groups, lacking systematic surveys and faced with the challenge of collecting and collating data over a vast area. Nevertheless, considerable progress has been made over the last 20 years, leading to new insights from diverse fields of study. One of the most exciting developments has been the creation of large international research networks which are collating and synthesizing information from widely scattered permanent botanical plots. Data from these networks and other studies are providing valuable new insights on contemporary biodiversity patterns and processes in Amazonia. Here we review the major findings of these networks and discuss the factors that correlate with and may explain the spatial distribution of Amazonian tree species and the factors that may underpin the emergent patterns of functional traits and diversity across the Amazon Basin.