Larry L. Rockwood would like to thank his graduate students for keeping him intellectually rejuvenated.

Thanks especially to Christine Bozarth, Jennifer Mickelberg, Jennifer Sevin, and Ryan Valdez. Special thanks to Lorelei Crerar and Naomi Coles who read and gave me feedback on all of the chapters as they were being written. He would also like to thank his office staff, Rebekah Flis, Joanne Zimmerman, and Angel Washington, whose efficiency allowed him time to work on this book.

Jonathan W. Witt would like to thank Kristen Baker, Christopher Clark, and Marlene Cole, who read and provided valuable feedback on Chapter 12. The views expressed in this book are those of the authors and do not necessarily reflect the views or policies of the US Environmental Protection Agency.

Finally, this book is dedicated to Jane Rockwood for her patience, wisdom, support and love.

This book is accompanied by a companion website:

www.wiley.com\go\rockwood\populationecology

The website includes:

• Pdfs of all figures from the book for downloading
• Powerpoints of all tables from the book for downloading

Why population ecology? What distinguishes the study of populations from the study of landscapes and ecosystems? The answers lie in scale, focus, and traditions. In population ecology the scale is a group or groups of taxonomically or functionally related organisms. The emphasis is on fundamental properties of these populations: growth, survivorship and reproduction. The tradition is based on the interplay of theory, laboratory testing and, ultimately, field work. The competition and predator–prey equations of Lotka (1925) and Volterra (1926; 1931) stimulated the laboratory work of Gause (1932; 1934); Park (1948; 1954), Huffaker (1958) and others. But Elton (1924), Errington (1946); Lack (1954); Connell (1961a; 1961b); Paine (1966); Krebs et al. (1995) and many others have brought population ecology into the field, where its theoretical underpinnings are constantly tested. In the age of personal laptop computers and the Internet, data can now be analyzed, sent around the world, and experiments redesigned without ever leaving the field site. Increasingly sophisticated experimental design and statistical rigor constantly challenge new generations of scientists. Indeed much of the training of modern ecologists is in methodology.

Yet why do we become ecologists in the first place? Is it because of our love of computer programs and statistics? For most of us, that would be, “No.” More likely it is because of a love of the organisms that we find in natural (“wild”) places. We love the sounds, the smells, the feel, the being in nature. Perhaps it is also because of our love of the idea of nature and of places not yet under the total domination of man. Nothing quite matches a day (or night) in the field for an ecologist, and we are usually eager to communicate these experiences to other people. Contrast an ecologist to a typical urban dweller like Woody Allen. In one of his movies Woody complains that he hates spending nights in the country because of the “constant noise of the crickets.” Undergraduate students at George Mason University often approach the first field trip of the semester with fear and trepidation. Yet, such individuals have little fear of automobile traffic and find traffic noises normal and even soothing. Obviously an ecologist has a different orientation to the world.

Population ecology is, in a primitive sense, an organized way of communicating our ideas about nature to others. Population ecology, with its emphasis on groups of individuals and their survival and reproduction, their relationships with their competitors and their predators, is rooted in both field work and in natural history. As such it appeals to us at a very fundamental level. Instead of (or perhaps in addition to) swapping tales around the campfire at night, we communicate by publishing in journals or books.

Furthermore, without the basic data from population studies, most landscape and ecosystem studies would either be impossible to carry out, or would lack fundamental meaning. The advantage of ecosystem studies is the comprehensiveness of the approach. However, the disadvantage is the complexity of interactions among species and our lack of understanding of community organization. Everyone can agree, I believe, that we need a better understanding of interspecific interactions, and this is the role of population ecology. To develop laws of ecosystem functioning, we need to comprehend how individual populations behave. From there we can develop an understanding of interactions among populations. Therefore it seems to me that studies at the landscape and ecosystem level must be informed by data first gathered by population ecologists.

But this all sounds rather grand and theoretical. In the real world, knowledge of population ecology is absolutely necessary for conservation biologists, wildlife managers, and resource biologists. They are often faced with problems of preserving biodiversity or a wild living resource without adequate information. How can they best decide whether to limit or even shut down a fishery and for how long? Is it necessary or wise to allow wolf hunting in Alaska in order to increase the caribou herd? How do we control deer populations or is it foolish to attempt to control them in urban environments such as Rock Creek Park in Washington, D.C.? Has the introduction of wolves into Yellowstone actually decreased the elk herds? What are causes of reptile and amphibian declines throughout much of the world? What accounts for the proliferation of Lyme Disease in North America? Although an ecosystem approach may be helpful and necessary to answer many of these questions, basic population data are also necessary. Beyond this we must understand how populations with different life histories grow and/or are limited. We need a fundamental understand of the roles of competitors, parasites and predators and their potential effects on a given population.

When Audubon was in the state of Kentucky in 1813, he witnessed the passing of a great flock of passenger pigeons (Ectopistes migratoris). This flock blackened the sky for more than 3 days as they passed overhead. Later Audubon estimated their numbers at between 1.1 and 25 billion birds (Souder 2004). Yet the last passenger pigeon was shot in the wild in 1900, and the last individual in captivity died in 1914. How can a population decline to extinction so swiftly, even if one acknowledges the role of hunting and habitat destruction?

Red grouse go through population cycles every 4–5 years. The numbers oscillate over three orders of magnitude (Hudson et al. 1998), and these oscillations are synchronized over large geographical areas (Cattadori et al. 2005). Yet the population recovers regularly. On the other hand, when tawny owls (Strix aluco) were studied in Oxford, the number of mating pairs remained steady, at 17–30 pairs, even though their major rodent prey species oscillated from 10–150 per acre (Southern 1970). What are the differences between red grouse and tawny owls: reproductive parameters, developmental time, or survivorship? Or is it the fact that red grouse are primarily herbivores and owls primarily predators? Are there differences in their competitors, parasites, or predators? These are questions that only knowledge of population ecology allows us to answer.

When the moose population crashed in Isle Royale in Lake Superior, Michigan, in the late 1990s, was the cause wolf predation? Or was it parasites or over-browsing of the vegetation? By 2013 the wolf population was in decline and moose population was increasing, but fitting this interaction to a simple predator–prey model has proven problematic (Mlot 2013). Wildlife scientists have complained for many years that white-tailed deer are over-browsing their habitats and causing changes in the vegetation. If so, why don't these deer populations crash? Is the recent movement of coyotes into the eastern United States and puma into the mid-western United States the result of these large white-tailed deer populations? If not, what explains these dispersals from the “wild west” to the more urbanized areas of the United States, east of the Mississippi River (Bozarth et al. 2011)? One goal of this book is to give you the background and weapons that will allow you to address these questions.

In the twentieth century, the principles of population ecology, as we understood them, were applied to agriculture, forestry, wildlife management, fisheries, and conservation biology. Exploitation of populations in the name of “maximum sustainable yield” was based on the flawed logistic equation and/or inadequate data. Before the days of environmental impact statements, however, politicians and engineers largely ignored advice based on ecological science. While this situation has changed, ecologists, in order to remain credible, must work to develop better theoretical approaches. Applied ecologists must be able to recognize which of several possible theoretical approaches to use for the population or community of concern. The purpose of this book is to help guide undergraduates, graduate students, future wildlife refuge managers, EPA officials, or other applied ecologists through the workings of basic population principles and theory so that they make wise decisions in the future.

In part one of this book we will establish the fundamentals of population growth for single species populations. After determining these basic properties, we will examine how intraspecific competition affects population characteristics. We will also consider the evolution of different types of life histories and discuss whether a biological population is naturally “regulated.”

Once we have an understanding of how single populations grow and sustain themselves in particular environments, we can begin to examine how interactions with populations of other species affect their life histories. In part two we will progress to an examination of interspecific interactions such as competition, predation, parasitism, and mutualism. Finally, as we move through these interactions, we can evaluate their relative importance in population growth and regulation.