Chapter 11

Population Distributions

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A male collared lizard. Males of this species have a bright orange throat that intimidates other males and attracts females.
Photo by Francois Gohier/Photo Researchers, Inc.

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CHAPTER CONCEPTS

  • The distribution of populations is limited to ecologically suitable habitats.
  • Population distributions have five important characteristics.
  • The distribution properties of populations can be estimated.
  • Population abundance and density are related to geographic range and adult body size.
  • Dispersal is essential to colonizing new areas.
  • Many populations live in distinct patches of habitat.

Bringing Back the Mountain Boomer

When Alan Templeton was a Boy Scout, he encountered his first collared lizards (Crotaphytus collaris), also known as mountain boomers, in the Ozark Mountains of Missouri. He was struck by the brightly colored males that ran around in the forest openings that dotted the mountains. Two decades later, as a biology professor at Washington University in St. Louis, he returned to the Ozark Mountains and was shocked to find that most of the lizards were gone. He began a course of research to identify the causes of the decline and to determine the steps that could restore the lizard population.

The collared lizard is a fascinating animal that feeds on insects such as grasshoppers. It prefers to live in dry, open areas and has a geographic range that spans from Kansas to Mexico; Missouri lies on the eastern edge of this range. Although much of the Ozark Mountains are forested, there are openings in the forest, known as glades, that contain exposed bedrock with the hot and dry conditions that provided suitable habitat for the lizards. These small, patchy habitats were once surrounded by savannas and forests with open understories. Over time these habitats began to change.

“Over time, lizard populations in the glades declined and the geographic range shrank; only a few glades continued to support lizard populations.”

In the Ozark Mountains, forest fires were historically common. These fires removed the small understory trees and accumulating leaves scattered across the forest floor. Forest fires were also important for maintaining the glades as open, sunny areas. However, beginning in the 1940s, a national campaign to suppress forest fires caused the glade habitats to be invaded by eastern red cedar trees (Juniperus virginiana). The cedar trees shaded the glades and made them cooler, which is not conducive to the growth of an ectotherm such as the collared lizard. The shade also reduced the number of insect prey, including grasshoppers. As a result, the glades became less suitable for the lizard. Over time, lizard populations in the glades declined and the geographic range shrank. Only a few glades continued to support lizard populations. Moreover, there appeared to be no movement of the lizards from these glades to other glades that were unoccupied.

Templeton and his colleagues began their research in the 1980s. They discovered that many glades once occupied by lizards no longer had any lizard population. They took several steps to restore those populations. First, they cut down the cedar trees in several glades to make the habitat more suitable. Next, they reintroduced lizards to three sites. They marked the animals to estimate the abundance and density of each population and to determine whether the lizards they introduced would disperse to colonize the neighboring glades. Dispersal events were rare and no neighboring glades were colonized, despite the fact that some were only 50 m away. Like the cedar trees that shaded the glades, the thick forest understory and accumulating leaf litter shaded much of the ground, which made the forest cooler and caused lower abundances of insect prey. At this point the researchers realized that they had two tasks. They needed to improve both the habitat of the glades where the lizards spent most of their time and the forested habitat through which the lizards traveled when dispersing from one glade to another.

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In cooperation with the Missouri Department of Conservation, the team used controlled fires to burn away the cedars in many glades as well as the small understory trees and leaves in the forest habitat. The lizards responded so well that 2 months after the burn, they began to colonize new glades. In 2011, Templeton and colleagues reported that from the original three glades where lizards were reintroduced, lizards eventually spread to more than 140 glades. Burning the forested habitat allowed more sunlight to come in and dramatically increased the number of grasshoppers for the lizards to eat when they traveled between glades.

The great success in restoring the collared lizard required researchers to understand its habitat needs and to recognize that the regional population was actually composed of many small populations interconnected over a large spatial area. In this chapter, we will explore how spatial distribution and the movement of individuals among habitats influence the long-term persistence of species.

SOURCES: A. R. Templeton, The transition from isolated patches to a metapopulation in the eastern collared lizard in response to prescribed fires, Ecology 92 (2011): 1736–1747.

Restoration as science: The case of the collared lizard, Science Daily, August 22, 2011, http://www.sciencedaily.com/releases/2011/08/110822091918.htm

Spatial structure The pattern of density and spacing of individuals in a population.

In previous chapters we have focused on ecology at the individual level. The story of the collared lizard demonstrates that studying ecology at the population level is also fascinating and can have real-world applications. In this chapter, we will focus on the spatial structure of populations, which is defined as the pattern of density and spacing of individuals. We will begin by examining how the distribution of suitable habitat affects the distribution of populations, including pest populations that are new to a region. We will then discuss the many properties of population distributions and examine how we estimate these properties. Next, we will investigate the importance of individuals moving between patches of habitats. Finally, we will learn how providing strips of favorable habitat can facilitate the movement of individuals between habitats to help ensure the persistence of populations over time.

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