Practical: Competing for Resources (SQA National 5 Biology): Revision Note

Investigating interspecific competition

• It is possible to investigate the effect of interspecific competition by determining whether growing two plant species together slows their growth in comparison with growing them alone

Apparatus

• Seeds of two fast-growing species with similar requirements, e.g.:

  • radish and lettuce

  • cress and mustard

• Small pots or trays, e.g. 9 cm pots

• Standard compost

• Plant labels

• Tray to catch water

• Light source, e.g. bright windowsill or growth lights

• Measuring cylinder

• Watering can or spray bottle

• Ruler

• Balance

Method

1. Prepare pots: fill each pot with the same mass / volume of compost

2. Sow seeds: sow seeds into 15 pots or compartments as follows:

   • monoculture A: sow five pots, each containing 10 seeds of species A

   • monoculture B: sow five pots, each containing 10 seeds of species B

   • mixed: sow five pots, each containing 5 seeds of A + 5 seeds of B

3. Place pots on a tray and put tray in a well-lit location

4. Water using a set volume of water, e.g. 50 mL per pot, on the same schedule

   • Avoid overwatering, e.g. you may decide to water only when the soil feels dry

5. Thin the seedlings so that each pot contains equal numbers

   • This is only necessary if some seeds fail to germinate

6. Measure growth weekly for 2–3 weeks; record:

   • height (mm): measure the height of all the seedlings in each pot

   • leaf number, e.g. count the number of leaves on each plant and calculate a total for each pot

7. Harvest and record the final mass of plant material in each pot

   • remove seedlings from soil and gently wash roots;

   • blot dry to remove excess water

   • measure fresh mass of each pot by species (separate A and B in the mixture)

8. Analysis:

   • Calculate mean height and mean leaf number per plant

   • Compare mean height, mean leaf number and final mass of the mixture vs its two monocultures for each species

Expected results

• If interspecific competition is important, each species will grow less in the mixture than in its own monoculture, so will have lower mean height / leaf number, and lower final mass

• One species may be the stronger competitor and may be affected less by being grown in a mixture, while the weaker species may show a larger reduction in growth

• If resources are not limiting, growth may be similar across treatments

Limitations

Investigating intraspecific competition

• It is possible to investigate the effect of intraspecific competition by determining whether growing many individuals of the same species at high density slows their growth in comparison with growing the same species at low density

Apparatus

• 12–20 shallow trays or plant pots of identical size

• Seed compost

• Marker pens and labels

• Rapid-germinating seeds, e.g. mustard seeds, weighed out into packs of different mass, e.g.

  • High density = 10 g

  • Medium density = 5 g

  • Low density = 2 g

• Metal spoon

• Measuring cylinder

• Spray bottle / watering can

• Transparent lids or cling film

• Light source or bright windowsill

• Ruler

• Digital balance

Method

1. Prepare trays:

   • Fill each tray to the same depth and level the surface

   • Label trays clearly with density and replicate number

2. Sow seeds:

   • Collect three pre-weighed seed packs of the same species labelled low, medium and high density

   • For each tray, sprinkle the whole pack evenly over the soil; if provided, mix the seeds with a small spoon of dry sand first to aid distribution

   • Lightly press the seeds onto the soil so they make good contact, e.g. with the back of a dry spoon

3. Initial watering:

   • Use a measuring cylinder to measure out a set volume of water

   • Add water to each tray using a watering can or spray bottle until evenly moist

   • Cover with lids/cling film until germination starts (24–48 h), then remove covers

4. Growth conditions:

   • Position: keep all trays together in the same bright place; set them in a simple grid and swap positions once a day

   • Light & temperature: ensure that all trays receive the same number of hours of light per day, and that they are kept at the same temperature

   • Watering: check tray moisture levels daily using touch, and if the soil feels dry, or almost dry, add the same volume of water to each tray

5. Duration:

   • Grow seedlings for 7–14 days

   • Growth should be measurable, but severe crowding or collapse of plants should be avoided

6. Measurements:

   • Survival: count seedlings per tray

   • Morphology: select a random sample of, e.g. 10 seedlings per tray, and measure shoot length and leaf number

   • Biomass:

     • harvest the seedlings in each tray

     • wash off excess soil and blot dry with a paper towel

     • measure the total fresh mass of plant material per tray

7. Calculations & analysis: possible analysis could include:

   • calculate:

     • density = surviving seedling number ÷ tray area

     • mean biomass per plant = total biomass ÷ number of surviving plants

   • plot on a graph:

     • density (x axis) vs mean mass per plant (y axis)

     • density (x axis) vs total biomass per tray (y axis)

Expected results

• If intraspecific competition is important, we may expect to see that:

  • average seedling size / mass decreases as density increases

  • seedling survival rates decrease as density increases

  • total biomass per tray increases from low to medium density, then levels off at high density as resources become limiting, e.g.

    • seedlings shade each other

    • water and minerals are shared between more roots

• If resources are not limiting, growth may be similar across densities with little difference between treatments

Limitations