Chapter 42 Appendix A: Ecology Topic

Background on This Topic

In any ecological community, resident plants and animals must interact with and adapt to each other; these biotic environmental factors originate from other organisms. Organisms also must adapt to abiotic factors coming from non-living sources such as wind, temperature, etc. These two sets of factors affect everything from energy capture to fitness and reproduction. Among animals, intra- and interspecific interactions (which are a subset of the biotic factors) often require some sort of movement. Plants interact with their neighbors in the community too, but unlike animals must do so where they have rooted. Plant interactions are less obvious than animal behaviors, but they shape the entire community, and in large part determine the number and types of organisms that are present.

Many intra- and inter-specific plant interactions revolve around obtaining essential resources like sunlight, nitrogen, or water. These essential resources are not unlimited. For example, it may seem as if there is an endless supply of sunlight, yet there is a fixed amount of photosynthetically active solar radiation that reaches any given point on the ground. Similarly, there are finite amounts of water and bioavailable nutrients in soil. This means there is a limit to the amount of usable resources.

Rather than just spend resources randomly, most species have evolved to allocate their limited resources in a particular pattern. Why is this important?

First, different species have evolved different allocation strategies. A species’ typical allocation pattern determines how and where it is most likely to grow. They may expend more resources on one stage of their life history, and less on others. Alternatively, a species may expend more energy and mass to grow a particular structure, and reduce resources spent elsewhere. For example, an abandoned field has abundant sunlight but fairly dry soil. Pine seedlings are drought-tolerant and grow well in full sun. So they establish quickly in an open field. After pine trees become established though, oak and other hardwood tree seedlings appear that grow up, shade out, and replace the pines. Most hardwood seedlings are shade tolerant but require moister soil than pines, so they cannot easily colonize an open field. In contrast, pines cannot tolerate the shade produced by maturing hardwoods, and are not replaced as they die out.

Second, the resource allocation patterns of a species are genetically determined, but not completely fixed; there is some room for modification in response to variations in the environment an individual organism experiences. Individuals can modify their typical allocation pattern somewhat to allow them to adapt to different abiotic conditions, to the presence of others in their own species, and to the presence of other species. We call this allocation plasticity.

Third, allocation plasticity differs between species. Some species have very little, and do not tolerate any change from optimum conditions. They often are called specialists. Other species have significant plasticity, so can adapt to a variety of conditions. They often are called generalists.

The form that allocation plasticity takes can differ too. For example, some plants respond to drought by shifting resources to rapid downward growth of existing roots, while others reallocate their resources to forming a waxy protective cuticle on their leaves.

Informal Starting Questions & Observations

  1. If a plant species gets more or less light, will that affect growth? If so, then how much does the light need to change to affect growth? (This is the question we use for the first part of our inquiry-based lab.)
  2. Some vegetables like tomatoes and peppers need lots of fertilizer to grow well, while others like okra and peas grow better if they are not fertilized. Does fertilizing change how vegetable crops allocate resources? (This is the focus of the training dataset.)
  3. Some plants grow thicker and bushier when their branch tips are pinched off. Why? Does it have anything to do with allocation plasticity? (The 3 example reports all look at the effects of herbivory on root-shoot allocation.)

Testable Research Question(s)

Initial observations:

  • Plants will allocate the nutrient resources they get from fertilizer into growth above ground (shoots) or below ground (roots).
  • Some plants need more fertilizer to grow well, while others need less.
    • Is this a fact or an opinion?
    • Is there a citable source for this information?
  • The main nutrients in fertilizer are nitrogen, phosphorus, and potassium.
    • Is a citable source needed for this information, or is it common knowledge?
  • Nitrogen is used to make proteins, and phosphorus to make nucleotides. Only potassium does not get used in macromolecules.
  • Potassium helps to regulate movement of water in and out of stomata, so regulates the rate of photosynthesis.
    • This is probably common knowledge.
  • Excess ions in the soil can interfere with water uptake by changing osmotic pressure.
    • Is a citable source needed for this information, or is it common knowledge?

Testable hypothesis:

  • If potassium helps regulate plant resource allocation for growth, then plants grown in normal soil should have a different allocation pattern than plants grown in:
    • Potassium-deficient soil .
    • Soil with excess potassium.
  • We predict:
    • Roots of plants in potassium-deficient soil will grow longer, trying to find more potassium.
    • Shoots of plants in soil with excess potassium will be shorter than in controls, because the roots are not taking up enough water to drive shoot growth.
    • These responses will occur in different, unrelated species of plants.

Experimental Methods

  1. Four-inch square nursery trays containing 60 to 200, 10 days post-germination seedling plants growing in vermiculite (a soil-less growing medium) without additional nutrient supplements were provided by the lab instructor.
  2. Six trays of plants were chosen: 3 trays of buckwheat (Fagopyrum esculentum) and 3 trays of mung beans (Vigna radiata). One tray each of buckwheat and mung beans was soaked for 10 minutes in 0.1X Miracle-Grow liquid plant food (controls.) One tray of each species was soaked similarly in 0.1X plant food with 10mM KCl added (excess potassium). The last two trays were soaked in 0.1X plant food made without potassium (potassium-deficient).
  3. ll 6 trays of plants were returned to a greenhouse bench and given 12 hours of sun per day. Trays were watered daily using overhead misting. After 7 days, the trays were treated a second time with the same fertilizer mix they got at the start of the experiment.
  4. After 14 days of growth, all containers were brought back to the lab for analysis. Plants were harvested by gently separating the vermiculite in each tray into ~10 pieces, then carefully pulling out 10 healthy-appearing seedlings. Remaining vermiculite was rinsed off in a beaker of water, and the seedlings patted dry with a paper towel.
  5. Weights (to nearest 0.001 g) and lengths (to nearest 1.0 mm) of intact plants were measured, then the roots and shoots were cut apart at the soil line. Weights and lengths of shoots only and roots only were recorded again for each seedling.
  6. Root:shoot ratios were calculated for both length and weight, and recorded in the data summary table. Results were summarized as means and standard deviations for each control or treatment group.

Sample Dataset

Species Treatment Replicate Wt. plant Wt. shoot Wt. root Wt. R:S Length shoot (cm) Length root (cm) Length R:S
B’wheat Normal K 1 0.43 0.41 0.02 0.049 12.7 5.5 0.433
- - 2 0.94 0.91 0.03 0.033 15.3 4.1 0.268
- - 3 0.48 0.47 0.01 0.021 22.1 3.0 0.136
- - 4 0.66 0.61 0.05 0.082 18.1 5.1 0.282
- - 5 0.46 0.44 0.02 0.045 7.6 3.5 0.461
- Excess K 1 0.28 0.26 0.02 0.077 15.5 5.8 0.374
- - 2 0.23 0.22 0.01 0.045 10.9 2.9 0.266
- - 3 0.35 0.33 0.02 0.061 16.0 4.0 0.250
- - 4 0.36 0.34 0.02 0.059 13.4 3.5 0.261
- - 5 0.22 0.21 0.01 0.048 18.1 1.4 0.077
- Deficient in K 1 0.58 0.56 0.02 0.036 9.9 5.2 0.525
- - 2 1.65 1.60 0.05 0.031 19.7 5.3 0.269
- - 3 0.61 0.61 0.01 0.016 28.2 2.0 0.071
- - 4 0.96 0.88 0.08 0.091 22.8 6.7 0.294
- - 5 0.70 0.67 0.03 0.045 -2.9 5.6 -1.931
Mung beans Normal K 1 0.44 0.20 0.24 1.200 14.2 10.4 0.732
- - 2 0.76 0.61 0.15 0.246 16.8 9.3 0.554
- - 3 0.46 0.32 0.14 0.438 10.6 6.8 0.643
- - 4 0.83 0.70 0.13 0.186 13.9 7.8 0.558
- - 5 0.47 0.28 0.19 0.679 11.9 9.4 0.784
- Excess K 1 0.27 0.17 0.10 0.574 9.0 7.1 0.780
- - 2 0.09 0.07 0.01 0.164 6.8 6.4 0.940
- - 3 0.25 0.18 0.06 0.341 6.6 2.4 0.362
- - 4 0.17 0.12 0.05 0.409 9.1 6.6 0.726
- - 5 0.14 0.10 0.04 0.456 9.3 6.4 0.688
- Deficient in K 1 0.56 0.37 0.19 0.514 12.9 7.1 0.546
- - 2 0.62 0.54 0.08 0.148 12.3 6.4 0.520
- - 3 0.43 0.34 0.09 0.265 11.6 2.4 0.205
- - 4 0.45 0.31 0.14 0.452 15.5 6.6 0.427
- - 5 0.53 0.39 0.14 0.359 13.7 6.4 0.467

Notes For Instructors

This experimental model is simple to set up but at the same time is very adaptable. We have used it successfully for the first experiment that students design, and as a mini-capstone at the end of our first year sequence for majors.

Results of individual students’ and shared class experiments can be summarized using:

  • Bar graphs
  • XY graphs, or
  • Box-and-whisker plots

Individual student experiments usually can be analyzed using a simple t-test. Aggregated data from a full class that tested multiple species or treatments could be used to introduce students to ANOVA and post-hoc tests.