California Thrasher Essay

Midterm Exam

Question 3:

Niche theory is a very important concept in Ecology. However, the exact meaning of the concept has changed throughout the years. Joseph Grinnell introduced the idea of niche theory in his paper “The Niche-Relationships of the California Thrasher”. His idea was that a species is restricted to its niche because of the habitat it lives in and its behavioral patterns. In Grinnell’s paper, the California Thrasher stays within the chaparral habitat of California. He continues to say that the California Thrasher is restricted by humidity and temperature, but that is probably due to the fact that this is the best environment for the chaparral bushes. The California Thrasher is described as a “shy bird” so it uses the cover

of the chaparral bushes to hide from predators while it hunts for food (Grinnell, 1917). According to this model, a niche would be easy to define in nature because it relies heavily on the behavior patterns and environmental needs of the particular species.

In 1927, Charles Elton took niche theory a step further and included food. He defined the niche of an animal as its place in the biotic environment and its relations to food and enemies (Elton, 1927). Grinnell also wondered if food was a limiting factor to the California Thrasher’s niche, but since the bird is omnivorous, it could find food in other habitats such as a forest floor or a meadow. There had to be other factors that restricted the bird to the chaparral bushes (Grinnel, 1917). Elton believes that the niche of an animal can be defined by its size and its food habits. It is important to study niches because it enables ecologists to see how different animal communities may resemble each other in the essentials of organization (Elton, 1927). For example, in a forest there could be a niche of owls that feeds on small animals such as rats. This same carnivore niche is filled with kestrels in the open grasslands. This carnivore niche is then dependent on the small animals in the herbivore niche (Elton, 1927). Hutchinson also discusses niche theory in his “Concluding Remarks” paper. His theory seems to combine some of the ideas

from both Grinnell and Elton. Hutchinson’s niche uses the n-dimensional hypervolume model. The n-dimensional hypervolume is every point in which corresponds to a state of the environment which would permit the species to exist indefinitely. He calls this a species’ fundamental niche (Hutchinson, 1957).

In other words, a fundamental niche is where an organism can use all the resources (food, shelter, light, etc.) it needs to survive and reproduce without the interference from other species. This takes ideas from Elton because it states that food is an important part of a niche. However, Hutchinson also introduced the realized niche, the actual niche that an animal is restricted to because of outside forces such as predators and competition (Hutchinson, 1957). This is similar to Grinnell’s idea because the California Thrasher could live outside the chaparral zone because it has a diverse diet, but its realized niche is the chaparral zone because it is restricted there because of predators. Because of this, niches aren’t easy to define in nature because scientists can figure out what the ideal niche would be for an organism but multiple outside forces can make it difficult to determine its realized niche. In recent years, niche theory has not been restricted to ecology and in fact can be applied to cancer research. Studies have found that stem cells differentiate into cancer stem cells causing Cancer in

different parts of the body. Scientists have proposed that one of the differences between normal stem cells and cancer stem cells is their degree of dependence on the stem cell niche, a specialized microenvironment in which stem cells reside. Like ecological niches, stem cells are restricted to stem cell niches because this is the best area for stem cells to maintain themselves and proliferate. Even though cancer stem cells also originate from the stem cell niche, it isn’t certain if they are as dependent on the niche as regular stem cells (Li and Neaves, 2006). This could explain why cancer cells are able to spread throughout the body so aggressively. Further research on this could help scientists identify what type of environment is best suitable for cancer cells.
Question 1.
When community concepts were first being discussed in ecology, plants were used as the model and animals weren’t really discussed. The question that arises is that can the same ideas about plant communities be applied to animals? The first scientist to study community ecology was H.C. Cowles. He thought of the idea of ecological succession, the idea that vegetation develops overtime in a pattern of successional stages from one community to the next (Cowles, 1891). For example, the grasslands will transition into shrubbery and then into a forest. Animals didn’t seem very important in defining a community according to Cowles. In fact, he said that animals did not appear to have any dominant effect on the dune floras (Cowles, 1891). So in this case, animals are just accessory components to the community. Two very important men in community concepts are Gleason and Clements. Clements proposed the idea of a climax state for communities, which represented the final, or permanent, end-stage of succession. Climax states are the collection of characteristic plants that define an ecosystem, such as tall grasses in a prairie, or mature trees in a forest. Clements held that, after a disturbance, any given ecosystem would eventually return to its characteristic collection of species (Clements, 1936). Gleason on the other hand, had a different idea. He believed that communities were individualistic; communities were only the random grouping of species, and that there was no such thing as a climax state for ecosystems. Gleason recognized that the environment and species’ movements, had an important role in regulating species collection, and that community changes were not predictable (Gleason,1926). Concepts from both of these ideas can be applied to animal communities. I will use a forest fire as an example. According to Clements, after a forest fire which destroyed most of the trees, that forest system would eventually return to the climax community with the same original species of plants as before the fire. This wouldn’t necessarily happen in animal communities. A study by Foresman and Zwolak found that a forest that once had a large abundance of southern red-backed voles and red-toothed shrews before a forest fire, had shifted to a larger abundance of deer mice a couple of years after the fire (Zwolak and Foresman, 2007). This outcome coincides with Gleason’s idea of community changes not being predictable. Scientists can’t be absolutely sure of which species of animals will repopulate a forest. At the same time, you also can’t say that the red-backed voles and the red-toothed shrews won’t come back eventually, but it’s risky to say that they exact same species will come back to this particular community. I do agree with Clement’s idea of plants defining an ecosystem (grasslands, tropical forest, and temperate forests) and this idea can be applied to animals. There are animal communities that define ecosystems as well. Herded, grazing animals such as zebra and buffalo define a grassland ecosystem and animals such as deer, bears, and certain birds define the temperate forest ecosystem/biome.

Literature Cited
Clements, Frederic E. "Nature and Structure of the Climax." The Journal of Ecology 24.1 (1936): 252.
Cowles, Henry Chandler. "The Ecological Relations of the Vegetation on the Sand Dunes of Lake Michigan. Part I.-Geographical Relations of the Dune Floras." Botanical Gazette 27.2 (1899): 95.
Elton, Charles S. "The Animal Community." Animal Ecology. Chicago: U of Chicago, 2001. N. pag.
Gleason, H. A. "The Individualistic Concept of the Plant Association." American Midland Naturalist 21.1 (1926): 92.
Grinnell, Joseph. "The Niche-Relationships of the California Thrasher." The Auk 34.4 (1917): 427-33.
Hutchinson, G. E. "Concluding Remarks." Cold Spring Harbor Symposia on Quantitative Biology 22 (1957): n. pag.
Li, L., and W. B. Neaves. "Normal Stem Cells and Cancer Stem Cells: The Niche Matters." Cancer Research 66.9 (2006): 4553-557.
Zwolak, Rafał, and Kerry R. Foresman. "Effects of a Stand-replacing Fire on Small-mammal Communities in Montane Forest." Canadian Journal of Zoology 85.7 (2007): 815-22.