Final PDF to printer SECTION III Population Ecology 9 Population Distribution and Abundance A population of saguaro cactus, Carnegiea gigantea, at Saguaro National Park near Tucson, Arizona. The natural distribution of the 9.4 Population density declines with saguaro cactus is restricted to the Sonoran Desert, extending from central Arizona through northwestern Mexico. Because saguaro cactus increasing organism size. 212 are very sensitive to freezing, the northern limit and highest elevations Concept 9.4 Review 213 occupied by the species appear limited by low temperatures. Applications: Rarity and Vulnerability to Extinction 214 Summary 216 CHAPTER CONCEPTS Key Terms 217 Review Questions 217 9.1 Environment limits the geographic distribution of species. 200 Concept 9.1 Review 204 LEARNING OUTCOMES After studying this section you should be able to do the following: 9.2 On small scales, individuals within populations are distributed in patterns 9.1 Define population, density, and abundance. that may be random, regular, or 9.2 Compare the characteristics of gray whale, monarch clumped. 204 butterfly, and Monterey pine populations. Concept 9.2 Review 208 9.3 Outline some of the reasons ecologists study populations. 9.3 On large scales, individuals within a population are clumped. 208 he distributions and dynamics of populations vary Investigating the Evidence 9: widely among species. While some populations are Clumped, Random, and Regular T small and have highly restricted distributions, other Distributions 209 populations number in the millions and may range over vast areas of the planet. Standing on a headland in central California Concept 9.3 Review 211 overlooking the Pacific Ocean, a small class of students spots a group of gray whales, Eschrichtius robustus, rising to the 198 moL37282_ch09_198-217.indd 198 27/08/14 10:59 pm Final PDF to printer Chapter 9 Population Distribution and Abundance 199 from as far away as the Rocky Mountains of southern Canada. As the students watch the whales, the male monarch butterflies pursue and mate with the female monarch butterflies. After mating, the males die, while the females begin a migration that leads inland and north. The females stop to lay eggs on milkweeds they encounter along the way and eventually die; however, their offspring continue the migration. Monarch caterpillars grow quickly on their diet of milkweed and then transform to a pupa or chrysalis. The monarch butterflies that emerge from the chrysalises (or chrysalides) mate and, like the previous generation, fly northward and inland. By moving far- ther north and inland each generation, some of the monarch butterflies eventually reach the Rocky Mountains of southern Canada, far from where their ancestors fluttered around the (a) group of students on the pine-covered coastal headland. Then, as the autumn days grow shorter, the monarch butterflies begin their long flight back to the coastal grove of pines. This autumn generation, which numbers in the millions, flies southwest to its wintering grounds on the coast of central and southern California. Some of the monarchs might fly over 3,000 km. The monarch butterflies that survive the trip to the pine grove overwinter, hanging from particular roost trees in the thousands. They mate in the following spring and start the cycle all over again. Gray whales and monarch butterflies, as different as they may appear, lead parallel lives. The Monterey pines, Pinus radi- ata, covering the headland where the monarch butterflies over- winter and by which the gray whales pass twice each year are quite different. The Monterey pine population does not migrate (b) each generation and has a highly restricted distribution. The cur- rent natural range of the Monterey pine is limited to a few sites Figure 9.1 (a) A young gray whale, Eschrichtius robustus. During on the coast of central and northern California and to two islands their annual migration, gray whales migrate from subtropical waters off off the coast of western Mexico. These scattered populations are Baja California to the Arctic and back again, passing along the coast of California as they do so. (b) Along that same coast, monarch but- the remnants of a large, continuous population that extended for terflies, Danaus plexippus, winter in huge numbers, some having flown over 800 km along the California coast during the cooler cli- thousands of kilometers from the Rocky Mountains to reach the trees mate of the last glacial period. This history of the Monterey pine where they roost in winter. In contrast, the entire natural population of underscores a very important fact about species distributions: the Monterey pine, Pinus radiata, is restricted to five small areas along they are highly dynamic, especially over long periods of time. the California coast. With these three examples, we begin to consider the ecol- ogy of populations. Ecologists usually define a population surface and spouting water as they swim northward ( fig. 9.1a ). as a group of individuals of a single species inhabiting a spe- The whales are rounding the point of land on their way to cific area. A population of plants or animals might occupy a feeding grounds off the coasts of Alaska and Siberia. This mountaintop, a river basin, a coastal marsh, or an island, all particular group is made up of females and calves. The calves areas defined by natural boundaries. Just as often, the popu- were born during the previous winter along the coast of Baja lations studied by biologists occupy artificially defined areas California, the gray whale’s wintering grounds. Over the course such as a country, county, or national park. The areas inhab- of the spring, the entire population of over 20,000 gray whales ited by populations range in size from the few cubic centime- will round this same headland on their way to the Bering and ters occupied by the bacteria in a rotting apple to the millions Chukchi seas. Gray whales travel from one end of their range to of square kilometers occupied by a population of migratory the other twice each year, a distance of about 18,000 km. Home whales. A population studied by ecologists may consist of a to the gray whale encompasses a swath of seacoast extending highly localized group of individuals representing a fraction from southern Baja California to the coast of northeast Asia. of the total population of a species, or it may consist of all the The grove of pine trees on the headland where the individuals of a species across its entire range. students stand gazing at the whales is winter home to another Ecologists study populations for many reasons. Popu- long-distance traveler: monarch butterflies, Danaus plexippus lation studies hold the key to saving endangered species, ( fig. 9.1 b ). The lazy flying of the bright orange and black mon- controlling pest populations, and managing fish and game arch butterflies gives no hint of their capacity to migrate. Some populations. They also offer clues to understanding and con- of the butterflies flew to the grove of pines the previous autumn trolling disease epidemics. Finally, the greatest environmental moL37282_ch09_198-217.indd 199 27/08/14 10:59 pm Final PDF to printer 200 Section III Population Ecology challenge to biological diversity and the integrity of the entire writings, Grinnell’s ideas of the niche centered around the biosphere is at its heart a population problem—the growth of influences of the physical environment, while Elton’s earliest the human population. concept included biological interactions as well as abiotic fac- All populations share several characteristics. The first is tors. However their thinking and emphasis may have differed, their d i s t r i b u t i o n . The distribution of a population includes the it is clear that the views of these two researchers had much size, shape, and location of the area it occupies. A population in common and that our present concept of the niche rests also has a characteristic pattern of spacing of the individuals squarely on their pioneering work. within it. It is also characterized by the number of individuals We can now point to a single paper authored by G. Evelyn within it and their d e n s i t y , which is the number of individuals Hutchinson (1957) as the agent that crystallized the niche con- per unit area. Additional characteristics of populations—their cept. In this seminal paper titled simply “Concluding Remarks,” age distributions, birth and death rates, immigration and emi- Hutchinson defined the niche as an n-dimensional hypervolume, gration rates, and rates of growth—are the subject of chapters where n equals the number of environmental factors important 10 and 11. In chapter 9 we focus on two population character- to the survival and reproduction by a species. Hutchinson called istics: distribution and a b u n d a n c e , the total number of indi- this hypervolume, which specifies the values of the n e n v i r o n - viduals, or biomass, of a species in a specified area. mental factors permitting a species to survive and reproduce, as the f u n d a m e n t a l n i c h e of the species. The fundamental niche defines the physical conditions under which a species might 9.1 Distribution Limits live, in the absence of interactions with other species. However, LEARNING OUTCOMES Hutchinson recognized that interactions such as competition may After studying this section you should be able to do the following: restrict the environments in which a species may live. He referred to these more restricted conditions as the r e a l i z e d n i c h e , which is 9.4 Define niche and distinguish between the fundamen- the actual niche of a species whose distribution is limited by biotic tal niche and the realized niche.