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The Impact of Plant Density and Diversity on Animal Populations in a Sonoran Desert Environment

Laurenbg

Ever since I can remember, I have explored the undeveloped Sonoran desert environment right out my back door in suburban Phoenix, Arizona. Adjacent to my home is an acre of undisturbed desert flora and fauna. It is home to abundant cacti and desert plant species as well as the desert animals that inhabit them. The rocky property is slightly sloped, with a deep ravine bordering the northeast side. It is the wild nature of this desert habitat that attracted my family to the area, as it has so many others. The smell of the creosote bushes after a rain and the sounds of coyotes howling in the evening are intoxicating.

Because of the abundant wildfires that have plagued Arizona during the last two years, our neighborhood association required that all dry vegetation be removed from residential properties. Following this decision, a neighborhood newsletter article was submitted by a property owner opposing the removal of native Sonoran desert plant species. The resident pointed out that although many types of desert vegetation appear to be dead in summer, they are only dormant during low rainfall periods. This individual pointed out that it is this same type of vegetation that attracts the diverse desert fauna that are enjoyed by homeowners in the area. Most people intentionally chose to live in this area because of its undeveloped nature. Because of an increase in urban sprawl, few intact Sonoran desert areas exist within the Phoenix metropolitan area.

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A map showing the field site and grid transects.


It was this article that gave me the idea for my expedition project. The purpose of my study was to determine whether Sonoran desert flora densities and diversities would have any impact on the densities and diversities of desert fauna. The results of the study might be used to manage the development of undisturbed deserts for human use in such a manner as to preserve native animal species. If any impact was found, the study might suggest guidelines for the native plant densities required per acre in order to preserve the natural Sonoran desert fauna. It is my hypothesis that Sonoran desert plant density and diversity will affect the density and diversity of the desert-dwelling fauna in an undeveloped desert ecosystem. I believe that it will be found that the greater the plant density and diversity, the greater that of the fauna. This study documents the effect of plant diversity on animal populations associated with the Sonoran desert. Plant-animal interactions are used to suggest guidelines for the development of Sonoran desert environments in order to preserve the natural environment and encourage the survival of native animals. This study will show the importance of preserving native Sonoran desert plant species.

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Cacti and plant species found at the field site.


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Cacti and plant species found at the field site.


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.

An acre-plus field site was selected in an undeveloped Sonoran desert area located in suburban Phoenix, Arizona (below). 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.

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Master grid map showing surveyed plant locations.


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. 

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Sample of animal locations sited during a survey period.


 

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.

This table shows the relationship between plant diversity and animal densities in a Sonoran Desert ecosystem. An increase in plant diversity is associated with an increase in animal density. (Click to enlarge.)


The results of this study show three main patterns. First, the results show that as the density of native plants increases, so does the density of animals. Plant density varied from 0 to 14 plants per 441 square feet, while animal density increased from .05 to 2.78 animals. Second, the results show that as the diversity of plants increases in a Sonoran desert ecosystem, the density of animals also increases. While plant diversity varied from 0 to 5 plants, animal density varied from .05 to 2.23 animals per 441 square feet. Finally, as the diversity of plants increases, the diversity of animals in the study ecosystem increases. Plant diversity ranged from 0 to 5 different plant species per 441 square feet, while animal diversity ranged from .04 to 2.17 different animal species.

In view of this analysis, the data clearly supports my hypothesis that plant density and diversity directly affect animal densities and diversities in this Sonoran desert ecosystem. The greater the density and diversity of Sonoran desert plants, the greater the density and diversity of animals that live in these ecosystems. Animals and their different kinds were more numerous where plant numbers were greater. In addition, the greater the plant species index, the greater the habitat variation, which allowed higher animal densities and diversities.

Future exploration could look at other variables. Future research could look at factors which could have had a stronger influence on animal densities and diversities than did the presence of plants. Weather, water availability, soil conditions, and non-plant-related food sources are factors that may affect the results. Further research could include the measurement of these factors to determine if they, instead of plant density and diversity, affect animal density and diversity. Finally, future studies could look at which species of plants attract specific animal species.

The present research has important applications. It could be used as a management tool to attract animal species by suggesting minimum plant densities per 441 square feet in the development of natural Sonoran deserts. This management tool could minimize the unwanted effects of development on the animal populations in Sonoran deserts. Today, developers often bulldoze all natural vegetation during construction in desert ecosystems. Cities could pass laws requiring developers to keep a minimum number of native desert plants per acre when clearing these areas. In addition, studies that focus on which species of plants attract specific animal species could help landscape architects create natural desert environments that attract desired animal species. Although the current study is just a beginning, these future research suggestions could help to further protect and maintain natural desert ecosystems.

 

References

Books

Abrahamson, Warren. Plant-Animal Interactions. New York: McGraw-Hill, 1989.

Anderson, S.H., and H.H. Schugart. "Habitat Selection of Breeding Birds." Ecology 55 (1974).

Ashworth, William.  Encyclopedia of Environmental Studies. New York: Facts on File, 1991.

Bertin, R.I. "Floral Biology, Hummingbird Pollination and Fruit Production of Trumpet Creeper (Campsis radicans, Bignoniaceae)." Am. J. Bot. 69 (1982).

Buchman, Stephen and Gary Nabhan. The Forgotten Pollinators. Washington, D.C.: Island Press, 1996.

Dajoz, R. Introduction to Ecology. New York: Crane, 1976.

Kendeigh, Charles S.  Ecology. New Jersey: Prentice Hall, 1974.

Moore, Randy, and Darrell Vodopich. The Living Desert. New Jersey: Enslow Inc., 1991.

Quick, Horris. Population Ecology. Indianapolis: Pegasus, 1974.

Robichaux, Robert H. Ecology of Sonoran Desert Plants and Plant Communities. Tucson: University of Arizona Press, 1999.

 

Journal Articles

Horvitz, C.C., and A.J. Beattie. "Ant dispersal of (Calathea marantaceae) seeds by carnivorous ponerines (Formicidae) in a tropical rain forest."  Am. J. Bot. 67 (1980).

 

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