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A In the United States, the average family size continues to decrease. At the same time, the average home size has grown. B According to the U.S. Census, the average home size in the United States was 2,479 square feet in 2007, up from 983 square feet in 1950. In communities across the United States, it is possible to see homes that are larger than 10,000 square feet. For their owners, these vast homes represent a piece of the American dream. However, a growing number of people, including artists, architects, city planners, and environmentalists, have started to question the need for such large domestic spaces. C They point out that small homes offer a number of benefits, not just to their owners, but to the world.

“I have been living in houses smaller than some people’s closets,” says California architect Jay Shafer. D Shafer currently lives with his wife in a 70-square-foot home that he built himself outside San Francisco. The narrow wooden home has a small working fireplace, a kitchen, and a table that lifts up to reveal a computer. Beyond the main room is a bathroom with a shower; upstairs is a tiny loft with a bed and storage for clothing.

D Shafer is one of the original members of the “Small House Movement,” a group of artists and architects that promote small living spaces. The group’s goal is to help people create a balanced and enjoyable life by living simply. The group has become more popular as more and more people have discovered the benefi ts of small living spaces.

D One obvious reason to live in smaller homes is their low price. Whereas prices in downtown New York City easily reach $1,200 per square foot or more, costs of construction company Jot House’s small homes are as little as $100 per square foot, depending on location. Jot House founder Bryant Yeh says he wanted to create low-cost homes with modern designs. The Los Angeles company’s one-story homes have a simple layout, with a kitchen and bath. The simple design of the houses means that they can be built in just three weeks, compared to the typical year-long building process for many homes.

D The low price of smaller homes makes them especially useful in difficult times. After Hurricane Katrina flooded much of the city of New Orleans in 2005, thousands of people were left homeless. Among them were Julie Martin and her family. “Prior to Katrina we specialized in restoring beautiful historic homes,” she says. Since then, Martin has worked with Shafer to create the Martin House Company, which sells small homes. Many hurricane victims like Martin have been able to rebuild their lives by moving to small homes. The U.S. government has spent millions to assist in their construction. The low price of the houses makes them a better option than keeping hurricane victims in hotels or government buildings.

D For Small House Movement founding member Susan Susanka, owning a small house is not just about saving money. It is also about saving the environment. In her book The Not So Big House, the North-Carolina-based architect points out that half of the energy use in the United States is for buildings. Reducing the size of homes is an easy way to limit energy use and pollution. By focusing on detail and making maximum use of available space, Susanka says residents can have a better living experience, without wasting natural resources.

D Perhaps the greatest advantage of small houses is the flexibility they offer the people who live in them. The small size and weight of many small homes means they can be moved by truck. This allows people to live in a variety of places without having to pack and move their possessions. The small homes are also easy to take care of. This gives their owners more time to do the things they truly love. “The reason I’ve [lived in small homes] is mostly because I don’t like … taking care of a lot of stuff that I’m not really using,” says Shafer.

E In many ways, the Small House Movement represents a return to the past and a time of simplicity. People who choose a small house point to a past when people had fewer possessions and children played outside. F For these homeowners, bigger is not better after all.

A Definition Symbiosis is an important concept in biology. Biologists define symbiosis as the close, interactive association (living together) of members of two or more species over a period of time.

B Classification of Symbiotic Relationship Symbiosis may have a good, bad, or neutral effect on one or more of the species. One way of understanding symbiosis is to consider the benefit (positive effect) or harm (negative effect) to each species in the relationship, as shown in Figure 1. However, it is important to note that many associations between species do not fit neatly into one category. (See One Final Note on Classification Systems below.)

D Commensalism (+, 0) Commensalism refers to an association in which one or more species benefits. The other species is not affected. One example of commensalism is the association between the marine sponge and a variety of small marine organisms, including shrimp and certain species of worms. The marine sponge serves as a “living hotel” for these marine organisms. There are many benefits to the organisms living in the sponge. These include shelter, protection from predators, and easy access to food that is washed into the sponge by the ocean. The sponge does not appear to be affected by the association.

D Mutualism (+, +) Mutualism refers to an association that is beneficial to both species. One example of mutualism is the association between the shrimp goby (a fish) and certain varieties of shrimp. The shrimp goby and the shrimp live together in a small hole in the ocean floor. The shrimp digs and maintains the hole. It also cleans the goby by eating parasites that live on its body. In exchange, the goby guards the entrance to the hole. It warns the shrimp when a predator is near. This is essential to the shrimp, which is nearly blind.

C When the shrimp emerges from the hole, it always keeps one of its antennae on the goby. If the goby goes back into the hole, the shrimp feels the movement and quickly follows. If the shrimp is inside the hole and the goby is outside, the shrimp reaches out with one of its antennae and touches the fish’s tail. If it is safe for the shrimp to emerge, the goby will move its tail from side to side. Otherwise the shrimp will remain inside, protected from potential predators by its constant companion.

D Parasitism (+, –) Parasitism refers to an association in which one species, the parasite, lives on or in a second species, the host, for a significant period of its life. It uses the host for food, shelter, and/or protection. In the process, it harms the host in some way. A parasite does not usually kill its host. However, the harmful effects of the parasite’s actions can lead to the death of the host.

C One example of parasitism is the association between a tape worm and a dog. The tape worm lives and reproduces inside the intestine of the dog. It has no mouth, so it feeds by absorbing the contents of the dog’s intestines through its skin. If the tape worm is not removed, the dog’s health will be harmed.

E One Final Note on Classification Systems Clearly, manmade classification systems with distinct categories help us understand the natural world. However, in nature such distinct categories do not exist. Rather, you might think of relationships among species as existing on a continuum. F For example, symbiotic relationships exist on a continuum from beneficial (+) to harmful (-). Very few relationships fit neatly into just one category.