A Study of Plant Population Density

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All living organisms require living space. When your little brother invades your room, you can quickly escape to another room or the backyard. Most animals, some bacteria and some protists have this same advantage and are able to move. When living conditions decline, these organisms can get up and move to a new environment. If food runs out, if the temperature changes, if there is a lack of water or sunlight, these organisms can pick up and move to an environment where the conditions are more favorable.

What would happen if your brother poked and prodded you and you were unable to move away—like on a long road trip in the car? Your living conditions would quickly decline and may affect your well being!

Most plants are rooted to the spot and cannot move to a new environment if living conditions decline. Unless the plant has adaptations that allow it to survive poor conditions like drought, lack of nutrients, or crowding, the plant may not be able to survive.

Using the experiment below, you can test the effects of crowding (or population density) on plants.

Terms, Concepts and Questions to Start Background Research


* population density
* biomass


* a plant's basic needs including water, air, sunlight and living space
* the process of germination


* What do plants need to survive?
* What does your particular plant need to survive? Does it have any special soil, water or temperature requirements?
* How close together do your plants typically grow?
* Which plants are most sensitive to overcrowding?
* What effects does overcrowding have on individual plants?

Materials and Equipment

* 6 6-inch plastic pots with holes punched in the bottom
* potting soil
* tap water
* seeds (small seeds like radish or mustard work well)
* measuring cup or beaker
* 8 plates or plastic lids for catching excess water under pots
* metric ruler
* metric scale (e.g., postal hand scale; alternatively, use the self-service scale at the Post Office)
* marker for labeling pots
* flat surface near a window, or a grow light

Experimental Procedure

1. Read the back of the seed packet for recommended growing conditions. Plant the seeds in the depth of soil recommended on the back of the packet and water seeds according to packet instructions. Make sure to record the following information in your lab notebook:
* amount of soil in each pot
* recommended growing conditions found on seed packet
* depth of each seed planted
* watering schedule
* amount of water added at each watering
2. For your control pots, plant the seeds at the recommended distance apart (found on the back of the seed packet).
3. For your experimental pots, increase the number of seeds and decrease the distance apart that each seed is planted. For any experiment, it is important to do multiple trials to assure that your results are consistent. Plant at least three separate pots for each # of seeds per pot, and record your results.
4. Make sure to record your initial measurements in your lab notebook:
* how many seeds you planted in each pot
* how far apart the seeds were planted
5. Check each pot at least once a week and record observations in lab notebook. For information on scientifically measuring plant growth see the Science Buddies How-to page, Measuring Plant Growth. Here are some suggestions on what types of information to record:
* the date on which each seed germinated
* how many seeds germinated in each pot
* the overall health of each plant (give each plant a score from 1 to 5, based on criteria that you develop)
* the height of each plant
* the number of leaves on each plant
* the final root length of each plant
* the final biomass of each plant
* the total biomass of each pot
6. Summarize your data.
* average the data for the plants in each pot
* calculate the population density for each pot by dividing the number of plants by the area of the growing surface
* then average the data for your control pots and your crowded pots
* collect your summary in a table, and make one or more graphs to illustrate your conclusions
7. Example Summary Data Table:

(name) # of Seeds Planted
(#) # of Seeds Germinated
(#) Population Density
(#/cm2) Average Height
(cm) Average # of Leaves
(#) Total Biomass
(g) Average Biomass
Pot #1 Control
Pot #2 Control
Pot #3 Control
Pot #4 Crowded
Pot #5 Crowded
Pot #6 Crowded
8. You can also summarize your results by condition by averaging your results for the control pots and the crowded pots.


* Do larger seeds need more room to grow than smaller seeds?
* Do the effects of overcrowding appear earlier/later with larger seeds than with smaller seeds?
* Are there plants (like grasses) that are not affected by crowding?
* What happens to the soil in areas of greater plant population density?

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