INTRA SPECIFIC COMPETI ON: THE RESPONSE OF THE SUNFLOWER (genus HELIANTHUS) TO INCREASING DENSITY. INTRODUCTION 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 intra specific 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. METHODS 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.
RESULTS 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 remains zero until 8 seeds are planted (6% mortality), then is reduced to zero at 16 seeds then increases to 14 percent (32 seeds), 32 percent (64 seeds), and more than half (57 % -- 128 seeds). Figure 3 shows a dramatic decrease in bud formation of approximately 20 percent from four to eight seed plantings, followed by slight decreases of 3 percent of less in bud formation as significantly greater quantities of seeds are planted. Figure 4 reflects a decreasing mean weight per plant as density of planted seeds increases -- from a high of 11.
73 grams (2 seeds) decreasing gradually from between 3 to. 5 percent intervals to a low of 1. 88 grams (128 seeds). Figure 5 breaks the weight down by plants with buds and plants without buds, continuing to show a relationship between decreasing weight as more seeds are planted. In addition, a trend is noted that average weight of plants with buds are always higher than average weight of plants without buds. The most significant variance was seen when two seeds were planted and all plants had buds while the least variance was when 64 seeds were planted (avg.
weight with buds-2. 24 grams, avg. weight without bud-2. 05 grams). DISCUSSION An increase in density appears to show an overall corresponding decrease in average plant yield by weight as well as percentage of germination of seeds of the sunflower species planted. The decrease is most significant at the lowest densities (between 2 to 16 seeds) and less so at greater densities (above 32 seeds).
It is less clear that percentage of budding is affected by density. It appears to decrease, though less significantly, as density increases, even seeming to level after 32 seeds are planted. Mortality, on the other hand, shows a definite correlation to increasing density, with survival rate cut by almost half for every doubling of number of seeds planted after 32 seeds are planted. The sunflower would seem to be better able to tolerate less crowded conditions and able to germinate more successfully, achieve bud formation of the majority of plants, and maintain prime weight of each plant. As conditions become more dense, up to half of the seeds do not germinate, and more energy seems to be focused on budding plants that have germinated and maintaining weight possible for the space and nutrients available.
The plant may be expending more energy to produce a higher quality plant as less seeds are germinated so that more seed can be produced for future generations. Thus plant mortality of the sunflower is seen in this experiment to be density dependent, while plant weight yield and bud formation is most influenced by density at seed plantings of 32 or more. Flowers of the family Asteraceae-sunflowers, black-eyed Susan's, and daisies-are successful in part due to their flower formation, which appears as one large flower but is composed of individual flowers which mature over several days; the ovules of each flower head are able to be fertilized by multiple pollen donors. (Raven, Evert and Eichhorn, 1999). This could explain why the sunflower commits its energy to bud formation as more seeds are planted without additional space being provided. Diana Pils on (2005) describes the evolutionary trade off of plants like the sunflower which bloom later when experiencing high levels of intra specific competition as a plant response to herb ivory.
Being an annual flower, the plant needs to expend its energy only when there is a certainty of survival. This is the ultimate link to maintaining fitness in the plant world. REFERENCESPilson, D. 2005. Evolution of response to herb ivory in wild sunflower Helianthus annulus. Ongoing research project.
University of Nebraska. Accessed from web May 9, 2005. Raven, P. , Evert, R. , and Eichhorn, S.
1999. Biology of Plants. Freeman & Co. : New York.
6 th ed. Stilling, P. 2002. Ecology.
Prentice Hall: New Jersey. 4 th ed.