Plants, Bugs and Bacteria
Plants have friends that can help them survive, but they also have enemies that can make it harder to persist. Today we’ll focus both on plant friends and enemies and how these other organisms influence how well plants grow in different environments.
Background Information
Plant interactions with friends and enemies:
Plants interact with a variety of other organisms. Some of these may have positive impacts on plants. An interaction where both species benefit from the interaction is called a mutualism. The mutualism that we’ll look at today is the mutualism between legumes (plants in the pea family) and rhizobia (a certain type of soil bacteria). In this mutualism, plants provide the bacteria with a home (protection from predators) and carbon (fixed from photosynthesis), and in return, the rhizobia provide plants with nitrogen (rhizobia are a special type of bacteria that have the ability to fix atmospheric nitrogen into forms usable by plants). Rhizobia may benefit plants and increase their growth and survival, particularly in poor soils.
While plants are aided by rhizobia, they are negatively affected by herbivores—insects and mammals that consume plants. By consuming leaf tissue (or even entire plants), herbivores can decrease plant growth and reproduction. And all of these interactions occur in the presence of other plants that may be competing for resources. Plant friends (mutualists) may help an individual win or outcompete other plants. Plant enemies (e.g., herbivores) may cause plants to lose in competition with other plants.
Friends aren’t always good friends:
While mutualisms should result in increased growth and fitness for both partners, they don’t always. For example, sometimes the bacterial partner can be a drain on a plant’s resources. What do you think happens to the legume-rhizobium mutualism when there’s lots of nitrogen already available in the soil? The same thing that happens when you’ve agreed to pay someone $100 to plow your driveway all winter and there’s no snow—the benefit is gone, but the cost remains. The plants don’t need nitrogen from their rhizobia, yet they’re still paying the rhizobia carbon. As a result, the interaction with rhizobia may shift from a mutualism to a parasitism.
Similarly, what if plants with rhizobia have more nitrogen in their tissues and herbivores prefer to eat more nitrogen-rich plants? How do you think herbivores might affect the legume-rhizobium mutualism? Do you think the cost to plants of having rhizobia when nitrogen-loving herbivores are present might be higher than the benefit?
Another factor that can limit how much benefit a plant receives from a mutualism is cheating. Some friends are simply more helpful than others. In the legume-rhizobium mutualism, some rhizobia give plants a lot of nitrogen in exchange for very little carbon, whereas other rhizobia give plants very little nitrogen while still taking lots of carbon.
Putting it all together—How biotic interactions affect distribution and abundance:
It’s not just the abiotic environment (water, nutrients, temperature) that affects plants and determines where they can live—the biotic environment (i.e., the living part of the environment, interactions with other organisms) also influences the distribution and abundance of plants. The term niche describes the environmental conditions under which a species can survive and reproduce. We can divide the niche into the fundamental niche and the realized niche. The fundamental niche of a species describes the abiotic conditions that affect survival and the distribution of a species in the absence of other interacting species. The realized niche describes the actual species distribution because it includes biotic environmental variables that can restrict the distribution (e.g., competitors, predators) and that can expand the distribution (e.g., mutualists). For example, competition may limit a plant’s distribution to a subset of the area it could otherwise occupy, or the presence of mutualists (e.g., rhizobia) may allow plants to persist in extremely low nutrient environments that they could not otherwise survive in.
In class activity
In this workshop, we’ll look at the plant-rhizobium mutualism and consider how the benefits to both partners vary depending on the individuals involved (i.e., are there some rhizobia that are better mutualists and some that are almost cheaters)? We will look at soybean plants that have been inoculated with 9 different strains of rhizobia (and a non-inoculated control). We will each take one plant and will count the # of nodules and weigh the plant. We will then calculate the average nodule # and plant biomass of plants inoculated with each strain to see if there is variation in the degree to which rhizobia strains are better mutualists vs. cheaters.
Rhizobia Strain / # of nodules (ave) / Mass of nodule (g, ave) / Mass of plant (g, ave)1
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Control
Take home activity
Each teacher/school will receive three pots each containing two soybean plants. One plant is a normal soybean plant; the other (marked with a green toothpick) is a mutant that cannot interact with rhizobia. One pot will just contain the two soybean plants. One pot will contain the two soybean plants + rhizobia. And one pot will contain the two soybean plants + rhizobia + fertilizer.
Use the following questions to make predictions about what you think you will see in your three pots.
1. Which plant (normal or mutant) do you think will win (grow more) in the absence of rhizobia?
Why do you think that?
2. Which plant (normal or mutant) do you think will win (grow more) in the presence of rhizobia?
Why do you think that?
3. Which plant (normal or mutant) do you think will win (grow more) in the presence of rhizobia + fertilizer?
Why do you think that?
4. If you placed these plants outside, which plant (normal or mutant) will receive more herbivore damage?
Why do you think that?
The plants are seedlings now, but should grow quickly in full sunlight. You can take the plants to your classrooms, watch them grow and then measure each plant in a few weeks (the first week of May). Measurements could include plant height (in centimeters) and the number of leaves. If you are growing your plants outside (bring them in if there is a frost warning), you could also count the number of insects on each plant or estimate the proportion of each leaf chewed by herbivores. If you email me the data (), I will compile the data from all schools and post the entire dataset and an analysis of the results on my webpage (https://www.msu.edu/~jenlau/). You are welcome to use the dataset in anyway you want (e.g. to discuss the nature of variation, to practice calculating averages, to practice making graphs, etc.).