1. We estimate the size of a population of small mice in a particular field by the mark-recapture method. Our estimate is 350. Later, we learn from experiments on the behavior of these mice that we can locate a baited trap faster if they have already been rewarded with food by visiting that trap once before. Does this mean that our original of 350 individuals was too low or too high? Explain your answer in terms of the principle that underlie the mark-recapture method.


2. An ecologist studying desert plants performed the following experiment. She staked out two identical plots that included a few sagebrush plants and numerous small annual wildflowers. She found the same five wildflower species in similar numbers in both plots. Then she enclosed one of the plots with a fence to keep out kangaroo rats, the most common herbivores in the area. After two years, four species of wildflowers were no longer present in the fenced plot, but one wildflower species increased dramatically. The unfenced control plot had not changed significantly in species composition. Using the concepts discussed in the chapter, what do you think happened?


3. Imagine that you have been chosen as the biologist for the design team implementing a self-contained space station to be assembled in orbit. It will be stocked with organisms you choose, creating an ecosystem that will support you and five other people for two years. Describe the main functions you expect the organisms to perform. List the types of organisms you would select, and explain why you chose them.


4. Biologists in the United States are concerned that populations of many migratory songbirds, such as warblers, are declining. Evidence suggest that some of these birds might be victims of pesticides. Most of the pesticides implicated in songbird mortality have not been used in the United States since the 1970's. Suggest a hypothesis to explain the current decline in Songbird numbers. Design an experiment that could test your hypothesis.



5. Birds and mammals have a four-chambered heart, with two ventricles and two atria, but other modern reptiles have a three-chambered heart, with just one ventricles. Paleontologists debate whether dinosaurs had a typical "reptile-like" heart or a "birdlike" heart. Long-necked sauropod dinosaurs could have had unusual circulatory demands because their head may have been raised far above their heart.The farther the head is above the heart, the greater the systolic pressure needs to be for the blood to reach the brain. For example, the long-necked dinosaur Brachiosaurus may have carried it's head as much as 6 m (20 feet)above its heart. It is estimated that such an anatomy demanded a systolic blood pressure of 500 mm of mercury for blood to reach the brain! Some paleontologists consider this to be evidence that dinosaurs must have had a four- chambered heart that supported a dual circulatory system similar to that of birds and mammals rather than the three chambered heart of nonbird reptiles. Can you explain why?



6. Most biologists believe that the immune system's defense against infections largely rests on it's ability to distinguish self molecules from nonself molecules. This concept seems central to our understanding of immune function. However, like all scientific ideas, it is not beyond question. Several immunologists have developed an alternative hypothesis: that the immune system's effectiveness rests mostly on it's ability to recognize damage to the body tissues caused by the invaders, not on the ability to recognize nonself. If you were going to test the "damage" hypothesis what might you look for? Which type of cell would you expect to be directly affected by damaged tissues? Why? Some proponents argue that the "damage" hypothesis makes more sense from an evolutionary perspective, claiming that it is more advantageous for an organism's defense system to respond to tissue damage than to the mere presence of a foreign microbe. Do you agree? Why or why not?