Genes control the traits of every plant and animal, and each generation transfers its genes to the next through asexual or sexual reproduction. Sexual reproduction gives an advantage to organisms because it creates more diversity in their gene pool by combining the traits from two unrelated parents. A greater variety of potential parents and greater variety of pairings in a breeding season therefore increase genetic diversity in offspring. Over time an animal population acquires advantages for survival in two ways: variations in its gene pool and random gene mutations. Even a minute change in an individual’s genes might give an animal a better chance of adapting to environmental change. Within a few generations, the advantageous gene has been passed on to many of the group’s offspring. For example, peppered moth populations living in London, England, changed their coloration during the 1950s from light gray to sooty black. The reason? Each generation of moths had gained genes for dark color so that the moths could blend into a landscape marred by pollution, smoke, and dark soot. This process increased the moths’ fitness, a species’ ability to sustain health and reproduce in its environment. Small genetic changes that enhance fitness in any specific population—London moths compared with the same species in Liverpool—over a few generations is called microevolution.
Animals evolve to meet changes in their environment by acquiring adaptations, and these adaptations may also determine a species’ role in an ecosystem. This role of a species within an ecosystem is called an ecological niche, or simply niche. Students often assume that to “occupy a niche” means an animal occupies a specific location. This is actually the definition for habitat; a niche is a species’ lifestyle or role in that habitat.
To occupy a niche, a species depends on certain foods, plants, and physical and chemical conditions within its ecosystem. Fundamental niche refers to the combination of potential physical, chemical, and biological factors that animal species use for survival. The concept of potential is important in this definition, especially when many niches overlap. Earth’s species have evolved to occupy fundamental niches that eliminate competitions, but when a species occupies a niche that overlaps with another species’ niche, it has two choices for survival: compete or adapt.
Competing directly with another species may increase the number of deaths in both species’ populations, so for the benefit of both, species often adapt to avoid competition. When different animal species adapt in this way, they are said to occupy a realized niche, a specialized portion of the fundamental niche. For example, elks in North America digest woody plants that grow on high mountain slopes, as well as plants that grow on flat lowlands. Elks are therefore capable of occupying a fundamental niche as a general grazer. Wolves have evolved to hunt elk on flat terrain to make the best use of the pack’s ability to chase a herd and separate out a single individual. Elk therefore increase their survival chances by spending as much time as they can (other than to find water) on mountain slopes,
where wolves tend not to hunt. They therefore occupy a realized niche, that of high-slope grazer of woody plants. Wolves benefit too because they conserve the energy it would take to chase elk over mountainous terrain. The wolves can carry out more successful hunts by targeting elk herds that descend for water.
North American elk make a behavioral change to occupy a realized niche, but other animals undergo a physical adaptation to accomplish the same thing. For example, two lizard species may look and act similarly and both may prefer to feed on the same types of insects. In the same habitat these species would compete directly for the same food, but through microevolution one species becomes slightly larger than the other. The larger variety of lizard ingests larger insects and leaves the tiny meals for the smaller lizard. Rather than compete, each lizard conserves its energy by feeding differently instead of competing. This behavior is called niche differentiation, or niche splitting, and it occurs only between two similar species.
How do niches affect biodiversity? The more specialized the niche, the more vulnerable an animal is to change in its habitat. Conversely, the best survivors, called generalists, thrive in broad niches; that is, they survive on many different types of food and tolerate a wide range of environmental conditions: crows, coyotes, cockroaches, and humans live as generalists. The “Case Study: The March of the Argentine Ants” gives an example of what happens when a generalist invades a habitat. Specialists, by contrast, are not as versatile and occupy narrow niches. Specialists tend to live in only one type of habitat, on a single type of food, or in a narrow range of environmental conditions. The northern spotted owl, discussed earlier, occupies a narrow niche. Other examples of specialists are giant pandas, polar bears, tiger salamanders, and red-cockaded woodpeckers. The woodpecker illustrates the precarious lifestyle of some specialists. This bird nests by carving holes only in longleaf pines that are at least 70 years old. Old longleaf pines have become limited to the southeastern coastal plain of the United States from the Carolinas to Louisiana. If the remaining longleaf pine forests disappear, the woodpecker will disappear, too. Meanwhile birds of the family Corvidae, more commonly known as crows, are generalists with no such restrictions. Crows range to just about every landmass on Earth and eat—with only slight exaggeration—anything in sight!
Source of Information : Green Technology Biodiversity (2010)
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