Darwin observed that all populations ability to

Info icon This preview shows pages 9–11. Sign up to view the full content.

View Full Document Right Arrow Icon
Darwin observed that “all populations’ ability to increase is greater than required,” which is very nearly a restatement of one of Malthus’s major points. After Darwin, population studies, particularly of animals, flourished. In the 20th cen- tury,there have been many studies of invertebrates,especially insects (because of their eco- nomic importance),farm animals (for the same reason),and game animals (because of an intense need to manage and predict changes in their populations).These studies continue today. Only recently have ecologists begun to study population dynamics in plants. The Size of a Population Is Determined by Natality, Mortality, Immigration, and Emigration The size of any population is in large part determined by a balance between several factors, some obvious, some less so. One obvious factor is the number of new individuals being born into the population. This is usually expressed as its birth rate , or natality . 1 Another factor, working in the opposite direction, is the population’s death rate , or mortality .Theoretically, any population is stable—it neither grows nor shrinks—if these rates are balanced. If not, the population changes.That is, the population’s growth rate is determined by its birth and death rates. Said more succinctly, where r is r = 1 b - d 2 , Growth Rate: Determine the growth rate of two populations.
Image of page 9

Info icon This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
496 C HAPTER 15 Population Ecology: How Do Organisms Interact to Form Populations? Alaska CANADA Anchorage Forty-Mile Herd Nelchina Herd Fairbanks Bering Sea Figure 15-6 The Nelchina caribou herd ranges over a large area of central Alaska. In the early 1980s, segments of the herd joined the Forty-mile herd to the northeast in a large emigration that significantly reduced the size of the Nelchina herd. (Map adapted from J. E. Hemming, 1971.The distribution movement patterns of caribou in Alaska. Wild Tech Bul 1 ,APFG p2.) the population’s growth rate (also referred to as “little r”), b is its birth rate, and d is its death rate. (What happens to a population if its ? In each case, what are the values of its “little r”?) Simple, right? Let’s see how things work out in real life. In south central Alaska, about equally distant from its two largest cities,Anchorage and Fairbanks,lives the Nelchi- na caribou herd (Figure 15-6). During its fall migration, the bulk of the herd comes close to a well-traveled highway, and sport hunters kill many of the animals each year. In the early 1960s, game managers, charged with the responsibility of managing the herd, wor- ried about how this hunting pressure might affect the herd’s size. They launched an ex- tensive study of the herd’s population dynamics. They determined the birth rate by monitoring the calving grounds in early summer.They already knew a great deal about the natural mortality rate of the population, caused mainly by wolves, bears, wolverines, and severe winter weather.Their results revealed that the population’s “little r”was in fact quite large, indicating that the herd’s numbers would probably increase.A large harvest
Image of page 10
Image of page 11
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern