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Homeostasis and Environmental Stress
In order to live, you must maintain your internal body temperature within a certain range, whether the outside temperature is freezing or 117 degrees Fahrenheit. In order to have your nerves and muscles working properly, you must maintain your electrolyte balance within a certain range, whether you are hiking across the desert feeling really thirsty, or sitting in front of the TV with a beer. These are examples of HOMEOSTASIS, which means maintaining constant internal conditions regardless of changes in external conditions. To stay alive, organisms must maintain homeostasis. The result (as the glossary in your textbook explains) is "a dynamic steady state in a living system." In order to maintain homeostasis, organisms need to live under a certain set of environmental conditions with all the necessary critical environmental factors. This is shown in Figure 3.2 in your textbook, on page 48.
What does figure 3.2 in your textbook really mean? Each species of animal, plant, or any living thing needs a certain set of environmental conditions if that species is going to survive. For example, the population of butterflies in Figure 3.2 needs to have the temperature of the environment within a certain range of tolerance in order to survive. If the temperature gets too hot or too cold, all of the butterflies die. This area outside the range of tolerance is called the zone of intolerance. But, just like people, not every butterfly in the population likes to be at exactly the same temperature. And just like people, not every butterfly can survive extreme temperatures at the edge of the range of tolerance. That is why this range of tolerance describes the conditions needed for the population, not the individual. In the middle of the range of tolerance is the range where most of the butterflies are the most comfortable. This is the optimum range. As long as the temperature remains in this range, the population of butterflies will be healthy and reproductively successful. But look at what happens near the edges of the range of tolerance. If the temperature starts to get just a little too hot or just a little too cold, some of the butterflies in the population die. Even the ones that survive will probably be under some physiological stress. So this is called the zone of physiological stress. You would feel the same way if your environment became too hot or too cold. For instance, if the temperature inside the biology lab dropped to 40 degrees Fahrenheit or climbed to 105 degrees Fahrenheit, very few students in lab would die, but most of the students would be feeling stressed! How well the butterflies can survive if the temperature changes depends partly on how rapidly the change takes place. Think of it like sitting in a bathtub of hot water. You can tolerate the water a lot hotter if you gradually add hotter water. This is called acclimation. Once you become acclimated to the hotter water, you can gradually add a little more hot water, until it almost feels scalding. Through acclimation, you can tolerate near-scalding temperatures, much better than you can tolerate jumping right into a scalding hot bathtub. But acclimation has its limits. As the water gets hotter, you find yourself entering your zone of physiological stress. At some point, you will cross into your zone of intolerance. You suddenly reach a point where the next drop of hot water is more than you can tolerate. That point is called a threshold effect, where you cannot tolerate the water that hot. You then either die or get out of the bathtub. If the population of butterflies cannot get out of the heat, they just die. This threshold effect is essentially "the straw that broke the camel's back." It can happen suddenly, without warning. In fact, the threshold effect is why many environmental problems seem to appear suddenly. We do not always see when a population of butterflies, or a forest full of trees, or an entire ecosystem is in the zone of physiological stress. But then there is a threshold effect and suddenly butterflies, trees, or entire ecosystems begin dying. That is when we know there is a problem! Adapting to Environmental Stress It is a basic law of the natural world that there will always be environmental stress. Remember the Red Queen in "Through the Looking Glass" by Lewis Carroll? The queen told Alice that "around here you have to run as fast as you can, just to stay in the same place." Scientists use this same idea, called the Red Queen Hypothesis, to describe the effect of environmental stress on living things. Living things must be able to constantly adapt to changes in the environment just to stay alive. Environmental stress doesn't affect just individual organisms. Changes in environmental conditions can put stress on populations, communities, or even entire ecosystems. Examples of environmental stress that could affect an entire ecosystem include fire, flood, drought, volcanic eruption, hurricanes, disease, predation, competition, erosion, overgrazing, deforestation, overhunting, habitat destruction, pollution, and pesticides. Some of these are natural, while some are caused by humans. Some occur gradually, while some are sudden and catastrophic. Whatever the cause, these types of stress can bring about changes in an individual organism, a population, a community, or an ecosystem.
So how do organisms, populations, communities, and ecosystems adapt to environmental stress? Each has its own way:
Individual Organisms Survive Through Acclimation Acclimation occurs when individual organisms make physiological modifications to adapt to changes in the environment. This is much easier if the changes are occurring slowly. Think back to the earlier example of a bathtub of hot water. You can tolerate really hot water if you add hotter water gradually. You make the water a little hotter, let yourself become acclimated, then make the water a little hotter yet. By becoming acclimated you are able to tolerate really hot water much better than you can tolerate jumping right into a scalding hot bathtub. The same holds true for a variety of environmental stresses. If an individual organism can become acclimated to changing conditions, that organism will continue to live. If not, it will die. Populations Survive Through Evolution Genetic mutations occur randomly during reproduction, and are transmitted to the offspring. These changes in the DNA can be seen as changes in anatomy, physiology, or behavior of that offspring. Some mutations are harmful, and that organism dies. Some mutations aren't really noticeable, and have little effect on survival. But some mutations are helpful, and may give that organism an increased ability to survive changes in environmental conditions. These beneficial mutations lead to evolution in the population. We need to stop for a moment to clear up any misconceptions you may have about evolution, and the difference between acclimation and evolution. Students quite often make this mistake, so I want to make sure you understand. Only a population of organisms can undergo evolution! An individual organism can only become acclimated to changing conditions, and does not change it's DNA! Here is an example: Suppose you have a population of insect pests in your garden. You can spray a pesticide, and kill most of them. But if you keep spraying the pesticide on a regular basis, the number of individual insects in the population climbs right back up to where it was before you sprayed the first time. People say "the bugs are becoming resistant." Does that mean that INDIVIDUAL insects evolve and become resistant? NO! Only an insect born with a genetic mutation that allows it to survive the pesticide will survive. Most of the insects in the population do not have that mutation, so most of the population dies. Then the few survivors reproduce, lay eggs, and give rise to another generation of pests in your garden. The next time when you spray pesticides, more of the pests will survive, because more of them carry that mutation. Each time you spray, you are selecting for insects that have beneficial mutations, and they reproduce and give rise to the next generation of pests. So over time, the POPULATION (not the individual organism) adapts to the pesticide and becomes resistant. The population is evolving! Communities and Ecosystems Survive Through Ecological Succession Succession is the gradual process of change in the composition and function of the various populations within a community or ecosystem. One common example of succession is what happens in an old-growth forest after a fire burns down all the trees. At first the only things that grow are grasses and weeds. Eventually small shrubs take over, and soon a few species of trees can be found. Finally, after several years, or several dozen years, the forest reaches the mature state it was in before the fire.
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