 |  Environmental Science: A Global Concern, 7/e William P. Cunningham,
University of Minnesota
Mary Ann Cunningham,
Vassar College
Barbara Woodworth Saigo,
St. Cloud State University
Biological Communities and Species Interaction
BE ALERT FOR: Natural Selection
Natural selection is considered one of the most fundamental processes in all of biology. It is also often misunderstood. Your text presents an excellent explanation of this phenomenon.
Notice a couple of key ideas. Survival of individuals to adulthood is more than a matter of chance. Some individuals just happen to have traits that better enable them to get food, to avoid being eaten, or to survive cold weather or illness than others. Do you think two baby owl siblings—one with nearsightedness, the other with normal owl vision—are equally likely to live to adulthood?
But if a sharp-eyed owl is to influence the traits of the next generation, survival alone isn’t enough. It must reproduce, thus passing those genes for good vision on to the next generation.
So owls didn’t one day invent good night vision because it might come in handy. It developed over many generations because a lot of baby owls with less than adequate vision simply didn’t get their genes for such vision into succeeding generations. The owls who-o-o happened to be born with better vision each generation were more successful in passing on those genes. If that process continues relentlessly for thousands and thousands of generations, you’ll end up with some sharp-eyed descendant populations.
BE ALERT FOR: Niche Definition
Habitat refers to the physical space an organism occupies. Its niche is considerably more comprehensive. The niche includes not only the specific spaces the species utilizes in the community, but the specific foods and other resources it uses as well. A niche also includes a time dimension: Is the animal actively feeding at night? Only at dusk? During the middle of the day? Does it go to its breeding ponds in late March? Mid-April? In reality, niches have many dimensions.
BE ALERT FOR: Factors Driving Succession
Understanding why species composition changes over time in a community may puzzle you. Here’s the key: Primary succession typically begins on bleak terrain—no nurturing soil, low amounts of usable nitrogen and moisture, and so on. Most plant species simply can’t survive under those conditions. (Remember the concept of limiting factors.)
Because of unique attributes, however, a few species can eke out a living on such sites. For example, some leguminous plants (including peas, beans, and relatives) have a symbiotic relationship with nitrogen-fixing bacteria. The bacteria produce nitrogen wastes usable by the plants, allowing these plants to live in places where the readily available nitrogen supply is simply too low to support other plants. So a legume settlement becomes established. You might expect that, once the legumes get started, the site would belong to them forever, but it doesn’t work that way.
As generations of these plants die and decompose, organic matter and nitrogen are added to the soil. This increases nitrogen levels over time to where other plant species can take hold. Unfortunately for the legumes, these second-wave plants are typically better overall competitors and soon replace them. These plants in turn may create shade, changing the temperature or moisture regime, which favors still other plant types, and so on.
So the physical, chemical, and biological conditions on sites are constantly changing. Since different plant species have different tolerance limits, these changing conditions favor first some species, then others, and then still others over time.
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2003 McGraw-Hill Higher Education