Habitats and Niches
|VIDEO 7-1: Ecological Niche|
Where does a species live? At first that seems a simple question. It is easy to list the geographical regions where a plant or animal species can be found. Texas horned lizards are found from Kansas to Texas and west to southeast Arizona. Isolated populations also exist in Louisiana. The geographical locations and environmental conditions where a plant or animal lives are called a species’ habitat.
But the concept of habitat does not fully answer the question of where an organism lives. Why does the Texas horned lizard live in Kansas and Texas, and why are there isolated populations in Louisiana? To answer this question we need to understand the concept of a niche, which includes the totality of a species’ environmental requirements. These requirements include how and where an organism gets its energy and nutrients and the ways it interacts with other species. For example, the Texas horned lizard lives only in areas where there are large ant populations. Being an ectotherm, it also must live in climates where it can maintain its active body temperature for a significant portion of the year.
But a species’ niche extends beyond its direct requirements. Plants and animals also change the environment in ways that affect the habitat of other species. The American alligator digs holes in the banks of lakes and rivers (Figure 7-3). In addition to providing refuge for the alligator, these holes store water during the dry season, providing habitat for smaller animals such as turtles. Thus the alligator’s niche includes the function of making water available for other species.
|Figure 7-3 Niche of American Alligator An alligator near its burrow in the Big Cypress National Preserve, Florida. Photo: Dave Griswold|
The concept of niche is the basis for understanding where species live. The competitive exclusion principle states that no two species can share the same exact niche indefinitely unless other factors limit the density of the better competitor. Over time, natural selection favors changes that reduce the degree to which species share the same niche. For example, plant communities in Alaska known as tussocks usually are limited by nitrogen, yet many species coexist within a small area (Figure 7-4).
|Figure 7-4. Alaskan Tussocks A tussocks community in Wulik River Valley, AK. Photo: Ryan Jordan|
Shouldn’t the species that is best able to obtain nitrogen succeed while the others disappear? In Alaska natural selection has generated a different solution: Species have evolved different strategies for obtaining nitrogen. Each species gets its nitrogen in a different form, during a different month, or from a different soil depth (Figure 7-5). For example, the Bigelow sedge gets most of its nitrogen as nitrate during June, whereas the dwarf birch gets most of its nitrogen as ammonium during August. These strategies allow the species to split the niche and thereby coexist.
|FIGURE 7-5 Splitting the Niche The source of nitrogen for five plant species in the Alaskan tussock. Species differ in (a) the form of nitrogen uptake (b), the month of uptake, and (c) the soil depth. Data: R.B. McKane et al., “Resource-Based Niches Provide a Basis for Plant Species Diversity and Dominance in Arctic Tundra.” Nature 415: 68–71; Figure: Kaufmann, Robert K. and Cleveland, Cutler J. 2007. Environmental Science (McGraw-Hill, Dubuque, IA).|
Adaptations are characteristics that affect an individual’s chance to survive and produce offspring. Some adaptations are well suited for a fairly wide geographic habitat; others work best in a relatively narrow range of conditions. Such adaptations are specializations. For a description of human adaptations to our environment, see Policy in Action: Climatological and Ecological Determinants of Human Land Use.
Niche and Adaptation
The living world is filled with adaptations. Things you might overlook are often a response to some aspect of the environment. Many of you probably have not thought about the shape of a leaf. Myriad shapes are possible: long or short, thick or thin, smooth or rough, and so on. Why do leaves look as they do? A leaf’s shape is no accident. Natural selection favors a shape that allows each plant to thrive in its environment. Plants that live in areas with lots of water have thin leaves with lots of surface area. A large surface area allows the leaf to capture significant amounts of sunlight, which is needed to power photosynthesis. But a thin leaf transpires much water, so the plant must have access to sufficient water. The banana tree, which lives in tropical rain forests, has giant leaves (Figure 7-6). On the other hand, plants that live in areas with little water often have thick stubby leaves with relatively little surface area. This shape slows the rate at which water is lost from the leaf. The creosote bush, which lives in the southwestern U.S. deserts, has very small leaves.
|FIGURE 7-6 Leaves That Fit the Environment The banana tree (left) and the creosote bush (right). Photos: Wikimedia Commons (left) and Flickr contributor Al_HikesAZ (right).|
Animals also have adaptations. Most frogs rapidly lose water across their skin, so they live near water. Some frogs, such as the waxy tree frog, extend their niche to include relatively dry areas. The waxy tree frog prospers in these areas by secreting a waxy substance that it spreads constantly across its body to slow the loss of water.