A Primer of Ecology, Fourth Edition, presents a concise but detailed exposition of the most common mathematical models in population and community ecology. It is intended to demystify ecological models and the mathematics behind them by deriving the models from first principles. The book may be used as a self-teaching tutorial by students, as a primary textbook, or as a supplemental text to a general ecology textbook. The Primer explains in detail basic concepts of exponential and logistic population growth, age-structured demography, metapopulation dynamics, competition, predation, island biogeography, succession, and, in a chapter new to this edition, species richness. Each chapter is carefully graded from simple material that is appropriate for beginning undergraduates to advanced material, which is suited for upper-division undergraduates and beginning graduate students. Advanced topics include environmental and demographic stochasticity, discrete population growth and chaos, stage-structured demography, intraguild predation, nonlinear predator-prey isoclines, and passive sampling. Each chapter follows the same structure: model presentation and predictions, model assumptions, model variations, empirical examples, and problems. Essential equations are highlighted for students' use. Intermediate algebraic "expressions" are also illustrated so that students see where the equations came from. New terms are introduced in the text in boldface type to alert students to novel concepts. The Primer contains more mathematical detail than many ecology textbooks, but avoids jargon and mathematical terminology that can intimidate students. Both simple and advanced problems are included, followed by fully worked solutions so that students can gain confidence and a better understanding of the models. Citations are kept to a minimum.