The Propagule Experiment: Seed Dispersal Patterns, Research Paper Example

One of the primary factors that allows for continued plant growth and species diversity is that of successful germination. This is extremely important to the health of an ecosystem. The natural environment is dependent on each process (or component) functioning without unnatural disturbance to produce the best outcomes in species diversity and richness. As an example, pollination of a certain species of plant, potentially vital to meeting the sustenance needs of a community, should not be impeded by man-made structures or disturbances.  Buildings, damns, highways, and any other structures can serve to disturb an environment, and severely impede the dispersal of propagules (plant matter, such as seeds)(Bidlack & Jansky, 2011). One of the more serious concerns related to successful dispersal of propagules is that of blocked waterways by damns.

Through research of the available literature, and an experiment carried out to determine in which ways propagules are most easily distributed, conclusions can be drawn about the importance of the unimpeded distribution of propagules. As such, in what ways do propagules get from one place to another most easily? What does this imply about the roles that man-made structures play to hamper this process? This study argues that propagules are very easily dispersed through water, and that propagules will be distributed more easily in a situation in which they float and germinate successfully on the banks of waterways. In the event of sinking, propagules will experience less successful landings on water banks, yet may still arrive safely based on water agitation. Nevertheless, man-made structures greatly diminish the propensity for successful germination by preventing propagules to float successfully to their destinations. After a brief literature review, the experiment conducted in this paper will ultimately reveal support for the hypothesis.

Literature Review

There have been a number of studies done on the subject of propagule dispersal (Kinlan & Gains, 2003; Bullock, Shea, & Skarpass, 2006; Merritt, Nilsson, & Jansson, 2010). Although there are many methods in which to disperse propagules (Bullock, et al., 2006), water seems to be one of the top mechanisms. In the study conducted by Kinlan & Gains (2003), there is agreement that long distance propagule dispersal is crucial for species survival, and that dispersal happens best through the mechanism of water. Their study indicated that not only are propagules more apt to be taken to farther regions and achieve successful germination, but that this is best in coastal and marine environments (Kinlan & Gains, 2003), those areas with the least amount of interference from man-made structures.

While numerous studies have been undertaken to help validate this theory, the one most heavily leaned upon in this study has been conducted by Merritt et al. (2010). Through their work, they were able to analyze propagule dispersal in riparian communities along rivers. Their study conducted different methods of dispersion, in relationship to wind and water. Moreover, they examined what took place in waterways both with and without damns. In a span of five years, “Repeatable treatments were established at 12 sites spanning 400 km along two adjacent rivers in northern Sweden, one fragmented by a series of dams, the other free-flowing” (Merritt, et al., 2010). Their findings were significant. After the five year period, they concluded that water was a better mechanism for dispersal than wind. They further noted that establishing colonies of plants through propagule dispersal was the most favorable along free-flowing waterways, as opposed to those watercourses impeded by damns (Merritt, et al., 2010). The experimental work done in this study closely aligns with, and supports, the hypothesis of this current study; namely that water dispersal is an excellent mechanism for propagule dispersal. Through travel by way of unimpeded watercourses (i.e. those without damns) propagule germination leads to long-term development of species richness in riparian zones.

Experimental Design

In this study, an experiment analyzing the successful distribution of propagules in different scenarios involving water will be conducted. A number of species that vary in size, shape, and texture will be the dependant variable assessed for likelihood of travel, given a number of independent variables. By using certain wet elements in certain states, the experiment is intended to confirm how well propagules disperse under certain circumstances, and as such, in what manner are certain methods more at risk for interruption. The preliminary steps of the experiment will be to gather a certain number of propagules to be used as the dependant variable.

After the plant matter is collected, it will be independently analyzed under variable wet conditions.  Two-cup measuring sized glass containers full of water will be used to assess propagule behavior based on whether the water is kept in a calm or agitated state. Each glass container wil contain about 20 to 30 propagules, depending on the size of the propagules. Once the independent variable (propagules) are agitated with an egg beater-type instrument, propagule behavior will be examined. This will occur after certain time periods have elapsed. The primary idea is to ascertain whether propagules will be distributed more easily when they float, leading to successful germination on the banks of waterways. If the propagules sink, it can be inferred that they may not be adept at dispersing well, unless met with some type of aid. This would make them less viable candidates for wide dispersal and germination. However, if they tend to float, this makes them excellent candidates for the dispersal needed to enable propagation in differing locations. It is important to note that for this experiment, the dependent variable is the floating and/or sinking of the propagules. Another independent variable is the weight of the seeds. The control variables for this experiment include the size of the glass and the amount of water filled in eh glass. Both glasses involved in the experiment were the same size and filled with the same amount of water.

Although this experiment cannot completely mimic the manner in which water behaves in natural propagule dispersal, some conclusions can be drawn nonetheless. In an attempt to reduce threats to internal validity, the propagules will be treated as similarly as possible to what they may experience in a natural environment. Using this method to determine whether a propagule will float or sink is a means to gather crucial information about how propagule distribution may behave when faced with a man-made structure. This study hypothesizes that propagules will be distributed more easily in a situation in which they float, sinking plant matter is unlikely to make in to the bank of a waterway and germinate successfully without considerable aid.  The impediment of a damn would serve to discourage propagule distribution.

Feasibility of Experimental Design

The design adopted for this experiment is crucial to the success of the results obtained. The experiment has been designed to replicate the effects of real-life forces on the process of dispersion for seeds. However, the experiment manages to limit the independent variables associated with real-life forces. This is crucial in effectively determining the accuracy of the results, relative to control variables employed in the experiment.

Process of Data Collection

For the purpose of this experiment, there were 11 different types of propagules gathered from a nearby field. They were measured in terms of millimeters, and noted for their shape, outer coatings, and whether any attachments or wings were present. As such, the following is a list of the chosen propagules and their properties.

1. Sorghum halepense (4.398mm) – Small, some w/ papery coating, extremely small dimples
2. Rumex crispus (6.66mm) – 3wings/3seeds, rough, brown, tear-drop like
3. Amaranthus retroflexus (0.66mm) – Black and shiny, smooth, circular, dimpled
4. Sesbania exaltata(3.45mm) – Smooth, small, brown
5. Ambrosia artemisiifolia (2.92mm) – Small, rough, jagged, brittle
6. Brassica napus (1.79mm) – Microscopic ridges, circular, no appendages
7. Lespedeza capitata (4.94mm) – Fuzzy, has hairy, reddish-brown, medium
8. Liatris spicata (9.3mm) – Small, feather-like attachments, upside down parachute, seed rough with hairs
9. Avena fatua (10.53mm) – Microscopic ridges, long & narrow, smooth to touch
10. Polygonum pensylvancium (3.52mm) – Two appendages-one larger than the other, very smooth and shiny
11. Setaria pumila (3.10mm) – Achene, ridges on outside, no appendages

In assessing propagule behavior in water, two-cup measuring sized glass containers full of water were used, and evaluations were based on propagule activities when the water was kept in either a calm or agitated state. Once agitated with the egg beater-type instrument, propagule behavior was examined after certain time periods had elapsed. Each propagule type was examined for floating ability without being in agitated water at the time of insertion, and also after an hour (t=60) had elapsed. The propagules were also assessed after being agitated for the initial 15 minutes, followed by a second reading after another 15 minutes following the agitation. At each reading, the number of propagules floating were recorded and later analyzed. The analysis entailed determining the portion of the propagules that were found floating against the total number of propagules.

Results

After having collected the 11 species of propagule, and subjecting them to tests involving being placed in water lacking agitation (smooth), and water with agitation, different finding were inferred, based on treatment and lengths of time in which the propagules were in a given state. The following is a chart used to determine the affects on the differing propagules – based on treatment and propagule properties. As can be noted in the graph on the next page, the results are quantified using percentages. These percentages represent the number of propagules that were found floating from the total number of propagules. The total number of propagules were recorded at the start of the experiment. After each reading, the number of floating propagules were recorded against the total number of propagules involved in the experiment and percentages obtained.

Propagule Dispersal Experiment

In analyzing the data on this experiment, certain trends are in evidence. In determining the success of those floating, it is apparent that those propagules over 3.45mm, rounder, and smoother floated more readily after sitting for an hour, that when they were first put in water. This seems odd, given that the propagules that are larger should have been more likely to sink. Nevertheless, it does infer that propagules are constructed in such a way as to encourage floating, and this supports the hypothesis. For the experiment involving the agitated verses non-agitated water, all propagules performed much better as floaters in the first two scenarios – those floating immediately in undisturbed water, and the percentage floating in undisturbed water at t=60. As for sinking to the bottom, those propagules that had been agitated seemed to exhibit more sinking behavior and less floating. This was even more evident in the percentage of species that sank 15 minutes after the agitation was complete.

Of special note, the smaller, smoother propagules appeared to trend towards sinking after the effects of stirring (15 minutes later) wore off. This falls in line with the work done by Burke & Grime, which suggests that “smaller—seeded species were more dependent on disturbance for establishment” (1996). Although the hypothesis, and the findings of this study, support that those propagules in free floating, smooth water conditions will tend to disperse best, having agitation as a dispersal aid for the smaller species can be effective. This is frequently found in free-flowing watercourses.

Discussion

This experiment entailed a number of error stemming from a number of sources. One of the plausible sources of error would be equipment employed in the experiment. There is a possibility that the size of the cups may not have been identical. The quality of the water used in the experiment was not ascertained. Impurities in the water affects its density and as such distorts the results obtained from the experiment. There is a small 5% error in the readings obtained to be assumed to stem from these factors.

Conclusion

This study attempted to verify if the hypothesis – that propagules are easily dispersed through water and distributed more easily in a situation in which they float freely without being impeded by man-made structures such as damns – could be validated. The combination of a literature review on this topic, choosing a dynamic experimental design, performing the experiments, and analyzing the results confirms the initial hypothesis. In this way, man-made constructions greatly diminish the propensity for successful germination by prohibiting the propagules to float successfully to their destinations. While the best scenario surrounded those propagules that did not experience agitation, the smaller species also tended to disperse well with the aid of agitation. In either case, distribution was most successful without having to contend with constructs. Comparing the results of this experimental design with the work of others completely revealed that unimpeded waterways are the most useful to ensure species survival, diversity, and richness. As a matter for future consideration, this study affirms that similar experiments will most probably garner the same results. This is of vital importance because human intervention has caused serious disruption in the manner in which the natural world evolves and operates. As such, this type of study is crucial for the well-being of all life on earth. Without species diversity, our food resources and options become reduced. Without the ability for species to propagate in a manner most predisposed to creating an authentic setting – that allows the natural world to reproduce in the manner in which it is accustomed, viability is reduced. This serves to limit our ability to survive in a diverse ecosystem intended to support all life.

There is an inherent need to replicate the experiment to validate the results obtained from this experiment. This can be achieved by altering the independent, dependent and control variables to effectively mimic real-life forces that affect the dispersion of propagules. The replication of these results will effectively validate these results by collecting information on various types of seeds in different qualities of water.

References

Bidlack & Jansky (2011). Stern’s Introductory Plant Biology, 12^th edition, McGraw-Hill.

Bullock, J. M., Shea, K. & Skarpaas. O. (1996). Measuring Plant Dispersal: An Introduction to Field Methods and Experimental Design. Plant Ecology, 186(2), 217-34.

Burke, M. J. W., & Grime, J. P.  (1996). An Experimental Study of Plant Community Invasibility. Ecology, 77, 776–790.

Kinlan, B.P. & Gains, S.D. (2003). Propagule Dispersal in Marine and Terrestrial Environments:

A Community Perspective. Ecology, 84(8), 2007-2020. Retrieved from www.oeb.harvard.edu/faculty/pringle/jc/dispersal.pdf

Merritt, D. M., Nilsson, C. & Jansson, R. (…..). Consequences of propagule dispersal and river fragmentation for riparian plant community diversity and turnover. Ecological Monographs, 80(4), 609-626.