A population is a group of organisms of the same species that share a gene pool, meaning they are able to breed with one another and therefore share genes. Not all members of a single species are part of the same population because barriers may exist that prevent gene flow. For example, the bison of Custer State Park comprise a different population than the bison herd at Blue Mounds State Park (Luverne, MN) because they are not able to interbreed.

Certain aspects of a population are of particular interest. Population size is the number of individuals comprising a population. Age structure is an analysis of what percentage of a population is pre-reproductive, reproductive and post-reproductive age. Because we often need to predict the future size of a population, age structure provides vital information for such predictions.

The simplest equation to predict population growth is G = rN. In the equation, G is population growth per unit time, r is net population growth rate per individual per unit time and N is the number of individuals in the population. To determine r, one must account for additions to the population (through birth and/or immigration) and reductions in population (deaths and/or emigration). Any r-value greater than zero means the population is growing. Consider the hamsters I had in my room a couple of years ago: I started with seven Siberian Dwarf Hamsters and, due to the complaints of some BVHS students, I was prevented from feeding any of the young to snakes at the Great Plains Zoo. Hamsters, like most rodents, have an incredibly high biotic potential, meaning the maximum number of offspring they can produce under ideal conditions. A female can have a litter of 4-10 young about every three weeks. In the course of about three months, the original population of seven hamsters did what hamsters in a cage do, and produced approximately 60 offspring. During that same time, three hamsters died. To figure out r for the population, you must consider the number added to the population (60) and subtract the number lost to the population (3), giving a value of 57 individuals per three months. Because r is expressed per individual, we divide this number by 7 (the original population size) and get an r-value of 8.1 offspring per individual per three months.

If you were to graph the growth of the hamster population over time, it would be a J-shaped curve. This is called exponential growth.

Of course, population growth cannot continue forever. Numerous limiting factors exist, including death from disease, predation and starvation. As a population grows, the risks of death also increase as starvation and disease become more likely. Over time, all populations will begin to exhibit logistic growth, characterized by a S-shaped curve. It is not possible for any population to continue logistic growth indefinitely because resources are limited. When a population reaches its maximum size for a given environment, it is said to have reached carrying capacity. Page 818 in the text diagrams an ideal curve illustrating carrying capacity. Realize that few populations will exhibit such perfect curves in nature. The diagrams of wolves on Isle Royal provided in your packet are more accurate diagrams of carrying capacity.

We often treat humans as separate from the rest of the natural world, falsely believing that the limits of ecology don’t apply to our species. In fact, the history of the human race illustrates that human populations are influenced by the limits of ecology. Figure 46.13 (page 825) is a frightening graph of human population growth through time. It may be tempting to view the graph as an indication that humans have been able to ‘outsmart’ nature and avoid any limiting factors or the carrying capacity of Earth. In fact, humans have managed to avoid many limiting factors (think of medicine and all the lives that have been saved) and through agriculture and industry we have greatly increased the carrying capacity for humans (converting land to fields to supply our food needs).

Can we forever sidestep the limits of growth? Look at the dip in the graph just after 1000 AD/CE. This represents the impact of a single bacterium, Yersinia pestis, the cause of the Great Plague. Certain limiting factors are what we call, density-dependent factors, meaning they are increasingly important as population density increases. The Plague/Black Death likely started with a single ship, which brought with its cargo an infected rat. Rats can carry various fleas capable of hosting the bacterium responsible for the deadly plague. In Europe in the mid-1300s, cities were increasing in size and sanitation was terrible as sewage and garbage were commonly discarded on city streets. The combination of many people living in close proximity and the poor sanitation, which lead to huge rodent populations, set the stage for the Plague. Humans learned to improve sanitation and now antibiotics cure most cases of the bubonic plague. This should not lure us into believing that this can continue forever. AIDS/HIV can be viewed as a similar situation in today’s world. Certain countries face perennial food shortages, as their populations are too large for the nation’s agricultural base to support. It is a certainty that at some point in the future, the human population will be limited by disease and/or starvation. Consider the Plague (which struck when Earth’s population was at about 250,000,000) in modern times (when Earth’s population is over 6,000,000,000)!