Counting animals and plants is so fundamental to conservation biology that it is often taken for granted. When dealing with rare and elusive species (and as this book points out, ‘sparse’, ‘rare’ and ‘elusive’ are quite different concepts), counting individuals can be very difficult, and field biologists often resort to using whatever measure of abundance is intuitively and logistically appealing. As this book emphasizes, such measures may not be biologically meaningful. This is because simple counts do not take into account spatial and temporal variation in the detectability of the organisms concerned. The literature on the statistical models that can estimate and control for variation in detectability was once beyond the grasp of the average field biologist. This propagated a gulf in communication between those dealing with theory and those dealing with practice. Recent years have seen this gulf bridged, and as Kenneth P. Burnham points out in his foreword, this has been driven by both practical needs for better methodologies coupled with increasing cooperation between statisticians and field biologists. The publication of this book, which neatly summarizes where the field has reached in terms of both theory and practice, is therefore very timely.

The book is sensibly organized into five sections. Part one, ‘Overview and basic concepts’, contains three chapters that provide important background for the later sections. Lyman McDonald's entirely non-mathematical essay in chapter 2, provides a survey of views on sampling rare populations. This is a refreshingly reassuring account as it embraces successes, failures and warts-and-all case studies trawled from a wide range of researchers. The remaining two chapters in this section go on to introduce the idea of detectability and how it affects estimates of population parameters. Part two, ‘Sampling designs for rare species and populations’, describes various sampling designs for estimating the abundance of rare species, including some, such as adaptive cluster sampling, that have not yet been widely applied to field data. For some species, estimating occupancy, rather than abundance, may be the prime goal of the exercise. However, the problem of incomplete detectability applies to presence-absence studies as well as to those seeking to estimate population parameters; apparent absence may be due to actual absence or to a low probability of detection. Part three, ‘Estimating occupancy’, reviews and develops current models in this area in a further three chapters. These models would seem to have enormous potential for countries such as the UK, which rely on a voluntary workforce of naturalists to gather data on the presence-absence of a variety of species in different parts of the country. Part four, ‘Estimating abundance, density, and other parameters’, consists of six chapters that cover non-invasive genetic sampling, camera trapping, use of animal tracks, adaptive sampling and bioacoustics, survival estimation and model evaluation. The case study on the Mexican owl provides a salutary lesson for the government agencies who commission population assessments and the field biologists who execute them. Spatial and temporal variation in abundance affects the precision of parameter estimates and the power to detect population trends. The sample sizes required to achieve sufficient power to detect the trends sought by conservation agencies may require resources and logistics that may be prohibitive. In the final, single-chapter section of the book, the editor of the volume looks to the future. His take-home message is clear. Future directions in this area can be summarized in three words, innovation, technology and software. He points out that many of the traditional sampling designs were developed for moderately abundant species and that the biodiversity crisis will be a strong driving force to develop further designs and counting techniques for rare and elusive species.

The major strength of this book is its well-balanced integration of statistical models with practical case studies. The case studies cover a wide variety of taxa, and are neatly integrated with discussions of model development. The information is presented in an unfussy typeface with clear diagrams and useful referencing throughout. These features should give the volume wide appeal. As the book deals with a topical subject that is currently undergoing rapid development, I fully expect that further books on the subject will start to appear within the next few years as the demand for emerging tools in this area grows. Nevertheless, wildlife biologists who are engaged in developing monitoring programmes for rare or elusive species will find this book hard to beat for a user-friendly, comprehensive and wide-ranging introduction.