INTRASPECIFIC COMPETION: THE RESPONSE OF THE SUNFLOWER (genus HELIANTHUS) TO INCREASING DENSITY.
All species, including plants, are impacted by density. Plants, of course, cannot leave their habitat as animals can, so they tend to respond in different ways to density. As populations grow more dense, they compete for resources such as food and space and are more prone to disease. Less dense populations are more susceptible to predation pressure.
It is hypothesized that as plants in small spaces compete for space, the plants compensate by reducing individual stem weight and frequency of bud formation as density increases. This would be intraspecific competition. A factor is density-dependent when it kills more of a population at higher densities and less at lower densities (Stilling 2002). The factor of competition between individual plants of the same species would be considered density dependent.
The experiment was designed to test the hypothesis by planting an increasing number of genus Helianthus (sunflower) seeds in pots to see how they respond to increased density in limited space. Two replicates each of 2, 4, 8, 16, 32, 64, and 128 seeds were planted in similar size pots containing an equal mix of potting soil and perlite. All plants were kept in greenhouse conditions exposed to similar light and temperatures settings and were watered at equivalent intervals. At the end of the time period each pot was evaluated for number of seeds that had germinated as well as number of stems with blooms. Stems and blooms were cut and weighed.
Table 1 shows averages of each replicate for each series of seeds relating to mortality rate/germination rate, budding rate, and mean plant weight (total, with buds, and without buds).
Figure 1 indicates an increasing mortality rate (positive slope) in plants as density increases; Figure 2 shows the corresponding germination rate reflecting the exact opposite trend. Mortality...
References: Pilson, D. 2005. Evolution of response to herbivory in wild sunflower Helianthus annuus. Ongoing research project. University of Nebraska. Accessed from www.unl.edu/dpilson/interests.html May 9, 2005.
Raven, P., Evert, R., and Eichhorn, S. 1999. Biology of Plants. Freeman & Co.: New York. 6th ed.
Stilling, P. 2002. Ecology. Prentice Hall: New Jersey. 4th ed.
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