BACKGROUND TO APPLIED POPULATION BIOLOGY
The Big Picture: Human Population Dynamics Meet Applied Population Biology
The metabolic rate of history is too fast for us to observe it. It’s as if, attending to the day-long life cycle of a single mayfly, we lose sight of the species and its fate. At the same time, the metabolic rate of geology is too slow for us to perceive it, so that, from birth to death, it seems to us who are caught in the beat of our own individual human hearts that everything happening on this planet is what happens to us, personally, privately, secretly. We can stand at night on a high, cold plain and look out toward the scrabbled, snow-covered mountains in the west, the same in a suburb of Denver as outside a village in Baluchistan in Pakistan, and even though beneath our feet continent-sized chunks of earth grind inexorably against one another, go on driving one or the other continent down so as to rise up and over it, as if desiring to replace it on the map, we poke with our tongue for a piece of meat caught between two back teeth and think of sarcastic remarks we should have made to our brother-in-law at dinner.
Russell Banks (1985:36–7), Continental Drift
Experience with game has shown, however, that a determination to conserve, even when supported by public sentiment, protective legislation, and a few public reservations or parks, is an insufficient conservation program. Notwithstanding these safeguards, non-game wild life is year by year being decimated in numbers and restricted in distribution by the identical economic trends – such as clean farming, close grazing, and drainage – which are decimating and restricting game. The fact that game is legally shot while other wild life is only illegally shot in no [way] alters the deadly truth of the principle that it cannot nest in a cornstalk.
Aldo Leopold (1933:404), Game Management
Should Texas panthers be brought in to breed with Florida panthers? What factors are most likely to explain global amphibian declines, and what is the most efficient path to reverse the decline? Do wolves reduce the numbers of elk available for hunters? What factors affect harvest regulations for waterfowl? Was the introduction of foxes to Australia likely to have driven native prey species to extinction, and how best to decrease the numbers of the exotic predator? These are just a few samples of the sort of real-world questions that can be informed by knowledge of applied population biology.
To set the stage for this book, consider some of the key words in the title. Population has many meanings, but for now let us consider the term in a broad sense, referring to a collection of individuals of a species in a defined area; the individuals in a population may or may not breed with other groups of that species in other places. A similar definition traces back to Cole (1957:2) who defined a population as “a biological unit at the level of ecological integration where it is meaningful to speak of a birth rate, a death rate, a sex ratio, and an age structure in describing the properties of the unit.” The advantage of such vague yet practical definitions is that they allow discussion of both single and multiple populations, with and without gene flow and demographic influence from other populations.
Next, some thoughts are provided on the term wildlife. Although about 1.5 million species have been described on Earth, vertebrates comprise only about 3% of the total and terrestrial vertebrates less than 2%. Yet policy, public opinion, and ecological research still deal disproportionately with vertebrates, particularly birds and mammals (Leader-Williams & Dublin 2000, Clark & May 2002). Certainly, harvest management outside of fisheries and forestry centers mostly on terrestrial vertebrates.
However, the term wildlife means considerably more than merely terrestrial game (harvested) species. Even Aldo Leopold’s classic book Game Management (Leopold 1933) made clear that harvested species should be considered a narrow segment of “wild life” (two words). Recognition of “The little things that run the world” (Wilson 1987) has emphasized the importance of small creatures – especially insects – to ecosystem structure and function, and, of course, Leopold (1953) reminded us more than 50 years ago that “To keep every cog and wheel is the first precaution of intelligent tinkering,” an admonition that our focus should be on all the parts. Happily, it seems that now, more than ever, people value the conservation of all species (Czech et al. 1998). Reflecting these philosophies, US federal wildlife law in its broadest sense recognizes all nonhuman and nondomesticated animals (plants occupy a different conceptual status in law; Bean & Rowland 1997). Recent texts with wildlife in the title have considered all free-ranging undomesticated animals, and in some cases plants (e.g. Moulton & Sanderson 1997, Krausman 2002, Bolen & Robinson 2003). This perspective has historical precedent: the first issue of the Journal of Wildlife Management (1937) stated that wildlife management actions “…along sound biological lines are also part of the greater movement for conservation of our entire native fauna and flora.”
This book will embrace a broad view of wildlife, because most concepts in population biology can be applied to all taxa. However, several core ecological, genetic, and life-history phenomena are idiosyncratic to plants, insects, or fish (e.g. seed banks, larval instars, anadromous breeding, self-fertilization, etc.), and so would require detailed treatment to understand population biology in detail for those taxa. For one book to effectively convey applications for species that are – at this point in human civilization – most prominent in the public eye, the majority of examples and case studies in this book will focus on the subset of wildlife consisting of amphibians, reptiles, birds, mammals (and fish to a much lesser extent).
Finally, some thoughts on the word management. This term is a pejorative in the minds of some, conjuring up images of manipulation and arrogance. It is certainly true that, in most cases, humans and human actions are ultimately what is managed, not the animals themselves. For others, the inclusion of management in the same book title with conservation is repetitive. Nevertheless, I have included management in the title because it is convenient shorthand for applied outcomes of population biology, ranging from measuring and interpreting trends to setting harvest limits, to evaluating viability of endangered species, and to determining the effects of predation on prey populations.
The overall influence of a species – any species – on its community and ecosystem is a function of its local density, its geographic range, and the per-capita impact of each member of the population. Virtually every problem related to wildlife conservation can be traced at least in part to human population growth – in terms of absolute numbers and distribution – as well as the per-capita impact of humans as strong interactors on the global stage (Channell & Lomolino 2000, Pletscher & Schwartz 2000). In the spirit of acknowledging that managing wildlife populations is really a matter of managing anthropogenic factors, the following section considers human population ecology, both emphasizing the role that humans play in affecting other species and conveying several principles to be elaborated on throughout the book.
POPULATION ECOLOGY OF HUMANS
Human Population Growth
Humans have experienced remarkably positive, often exponential growth (see Chapter 5) for thousands of years, resulting in enormous abundances. However, human population growth has not been constant. Let us start about 12,000 years ago, some 30,000 years after the evolution of indisputably modern humans and just after the last major ice age had ended. Humans were beginning village life in some parts of the world and had recently spread into and through the Americas. Plant and animal domestication would begin in one or two thousand years (Diamond 1999). At this point, somewhere between 1 and 10 million humans existed worldwide. It took about 10,000 years – until roughly 1 AD – to increase to about a quarter of a billion (Fig. 1.1
). Thus, our population growth has historically been low, with increases of a tiny fraction of a percent per year.1
This relatively low growth rate continued over the next 1600 years, with some noticeable setbacks such as the outbreaks of Black Death (bubonic plague) that killed one-quarter of the people in Europe between 1346 and 1352.
Human population growth from 10,000 BC to the present day. The dip in the 14th century represents deaths due to bubonic plague.
Data from the US Census Bureau, International Database.
Between 1650 and 1850, growth of human numbers began to rocket (Fig. 1.1
), following development of global agriculture, the initiation of the Industrial Revolution in western Europe, and improved nutrition and hygiene across much of the world. By the late 1960s, the Earth held about 3.6 billion humans. At that point the rate of increase of our species had just passed its peak of 2.2% per year (Fig. 1.2
). Think about it: it took 10,000 years to increase by a quarter of a billion, but by 1968 our numbers were increasing by that much every 4 years.
Global human population growth rate (presented as the percentage change per year) since 1950.
Data from the US Census Bureau, International Database (http://www.census.gov/ipc/www/idb/worldpopinfo.php
). The dip in the global population growth rate 1959–61 was due to the Great Leap Forward in China, which resulted in over 20 million premature deaths from famine in a 2-year period (Becker 1996).
What about now? The global population growth rate has declined since the late 1960s (Fig. 1.2
). However, current growth is still positive, and multiplying this growth by the ever-larger numbers of our current population size results in enormous increases in abundance. In 2012 human numbers passed the 7 billion mark (Box 1.1
). At current rates of growth and population size (2012) we are adding about 75 million people per year to the planet. That is a little over 200,000 additional people per day, or about 8500 net new people – subtracting deaths from births – added to Earth during a 1-hour lecture.
Grasping the meaning of billions of people
A billion is a hard number to fathom. First, count the zeros. A billion is 1000 million, otherwise written as 1,000,000,000, or in scientific notation as 109. (In some European countries this number is the milliard, with billion referring not to a thousand million but rather to a million million, adding three more zeros; Cohen 1995.)
So how much is 7 billion ...