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  Cacti and plant species found at the field site. The significance of my study is supported by other scientific research. There are many long-term benefits to having diverse flora and fauna. One benefit is to enhance the stability of the ecosystem. By increasing its diversity, a system increases the range of environmental challenges with which it is capable of dealing (Abrahamson, p. 270). Identifying specific plant-animal relationships may indicate which plants must be preserved so that their dependent animal species will not become extinct (Abrahamson, p. 2). Plants make up much of the physical and biological environment for the everyday necessities of animals. This is why plant diversity greatly affects patterns of animal distribution and abundance (Abrahamson, p. 21). Examples in which animals benefit from plants include the use of the physical habitat provided by plants, i.e., for nesting and camouflage. While owls, woodpeckers, and doves all use the saguaro cactus as a nesting structure, lizards and rabbits use desert brush for hiding (Abrahamson, p. 19). Horvitz found that mature fruit split open and drop their seeds onto the desert floor near the plant, thereby attracting various ant species (Horvitz, p. 321). Likewise, Bertin found numerous interactions between flowers and their pollinators. These included hummingbirds and ocotillo plants, and various bee species and palo verde trees (Bertin, p. 127). Bird populations have been shown to be directly affected by plant diversity (Anderson, p. 216). Hundreds of different Sonoran desert animals rely on plant life for their survival, and are found where desert plants are preserved (Moore, p. 36). Roots, stems, leaves, flowers, and seeds form the basic diet of most desert animals. For example, most cacti are flowering plants that birds visit to extract sugar, or to search for insects. In addition, most desert animals get all the water they need from the food they eat (Baker, p. 10). The link between plant and animal life in the Sonoran desert is strong in view of the fact that rodents, desert tortoises, and wild pigs all consume cacti as a source of food and water. These animals also break open barrel cacti as an important source of water (Robichaux, p. 153). Century plants, yuccas, and mesquite trees are known to attract hummingbirds, bats, hawkmoths, sweet bees, and solitary (Perdita) bees (Buchman, p. 206). The spectacular and unusual vegetation of the Sonoran desert is home to many other animals. Tarantulas, flies, grasshoppers, and solifugids are common. The Sonoran desert also has more that 60 species of lizards and many kinds of scorpions. More than 25 species of birds live here. The saguaro cactus alone is home to woodpeckers, elf owls, cactus wrens, centipedes, beetles, and scorpions (Moore, p. 35). From these examples it is easy to see the importance of desert plants to the animals that rely on them.  Master grid map showing surveyed plant locations. (Click to enlarge) An acre-plus field site was selected in an undeveloped Sonoran desert area located in suburban Phoenix, Arizona (left). The study site is bordered by one-acre-plus-size residential lots with undeveloped desert landscaping. A large arroyo borders the northeast side of the site. The site is a native Sonoran desert mixed-plant community. |
 Sample of animal locations sited during a survey period. (Click to enlarge) November 2, 2002: I subdivided the study plot into 100 sections, each section measuring 441 square feet. I placed orange flags every 21 feet along the grid system. Each flag was marked for location identification. This made each section measure 21 feet by 21 feet. Once my study site was marked, all the plant species in each section were identified and their locations recorded on a master grid map. Tomorrow I will begin surveying the field site for fauna. November 3, 2002: I set out at 4:30 p.m. into the desert. I walked along each transect and surveyed each grid section for approximately one minute (for an approximate total survey time of 1.5 hours per day). I recorded all observed fauna and noted each animal's location. I conducted my surveys between 4:30 p.m. and 6:30 p.m. daily. Dusk was the time chosen for the survey because animal activity increases then.  The table shows the relationship between plant and animal densities in a Sonoran Desert ecosystem. Note that as the number of plants increase, so does the number of animals. (Click to enlarge) After collecting all my field data, I calculated plant and animal diversities and densities. Diversity and density were calculated for each animal and plant species in each subsection. In addition, both the total plant and animal diversity, and the total plant and animal density, were determined in each subsection for all species present. Although there are various methods available to calculate plant and animal diversities and densities, I used the direct counting method. Given an open habitat such as this one, with sparse vegetation, and the small size of the study site, the plants and animals can be counted directly (Dajoz, p. 169). This was accomplished by recording the density (number of individuals present per unit area of volume) of each plant and animal species per subsection (441 square feet). The same procedure was utilized for determining the total number of plant and animal densities and diversities per grid subsection. This method not only shows how many animals and plants are in a subsection, but also shows if certain animal species are clustered within subsections having specific plant species. Data analysis showed a relationship between plant and animal densities, plant density and animal diversity, and plant and animal diversities. An average plant and animal density was obtained by finding the average number of plants and animals per 441 square feet (the size of one grid subsection). This was accomplished by averaging together the daily plant and animal counts per grid subsection over the study period. The number of plants recorded per grid ranged from 0 plants in some grids to 14 plants in others. I then looked at the average number of animals found in the grids with 0, 1, 2, 3, etc. plants. I looked at each day's observations and noted the number of animals on each day that I found in grids with said number of plants. I then averaged together all days for each plant density number to get an average number of animals for that specific number of plants. The same method was used to find the relationship of plant diversity to animal density. Here, however, the plant diversity was found by counting the number of different plant species in each grid. The number of different plant species ranged from 0 to 5. Finally, the relationship of plant diversity to animal diversity was looked at. When looking at each grid over the study time, the animal diversities for each grid on each study day were averaged together to get an average number of different species per 441 square feet for plant diversities of 0, 1, 2, 3, 4, and 5. |
