Niche Partitioning Activity

In the African savanna ecosystem, many species of large herbivores share similar habitats. How do all these species coexist, or live together, without having to compete for resources? These species can coexist due to a mechanism called niche partitioning, which is when species partition, or divide up, resources by using their environment in different ways. (A species’ niche is its place and role in an ecosystem, including where it lives and how it gets the resources it needs to survive.) In this activity, you’ll explore different examples of niche partitioning in the African savanna. The concepts you’ll learn can be applied to many other organisms and ecosystems, to help us better understand how species behave and interact. PART 1: Niche Partitioning by Time and Grass Height One type of niche partitioning in the savanna is shown in Figure 1. The resource partitioned in this example is a typical savanna grass called Panicum maximum. This grass’s growing season starts after the peak rain and continues for six months. When the grass is tall, it has lots of stems, which are relatively low-quality food for herbivores. The more nutritious parts of the grass are closer to the ground. If a grass-eating herbivore, or grazer, eats the top of the grass, the new parts of the grass that grow back are also more nutritious. Figure 1 shows three types of grazers — zebra, wildebeest, and gazelle — that graze, or eat, this grass over time. Zebras, the first grazers to use this resource, thrive when the grass is tall and abundant, even if it is less nutritious. The zebras have paired upper and lower teeth that help them bite off tall stems on the tops of the grass. Zebras can also digest food much more quickly than the other two grazers. This is because wildebeests and gazelles are ruminants, mammals with four-chambered stomachs that take longer to digest food. Sometimes ruminants must also regurgitate and rechew partly digested food before they can fully digest it. However, when the ruminants digest their food (via fermentation in the foregut or stomach), they take up more nutrients and proteins than when zebras digest food (via fermentation in the hindgut or large intestine). So, a ruminant can extract more energy from a smaller amount of food if that food is more nutritious. Smaller ruminants, such as gazelles, need less energy than larger ruminants, such as wildebeests. Answer the following questions based on Figure 1 and the information above. 1. Describe how the relative zebra , wildebeest and gazelle density changes over time, in relation with grass size The density fluctuates over time as the grass changes lengths. 2. What characteristics of zebras could explain why zebra densities are greatest when the P. maximum grass is tallest and most abundant? Zebras are herbivores meaning they need to eat a lot of plants, so they eat a lot of the less nutritious grass and then they need to supplement for that with the more nutritious grasses. 3. Propose a reason or reasons why the relative wildebeest density spikes when it does. Support your idea with evidence from what you know about wildebeests and P. maximum grass. (Hint: Remember that the more nutritious parts of the grass are closer to the ground. The grasses continue to grow after being grazed, and the parts that grow back are also more nutritious.) The wildebeest density spikes because they eat a moderately nutritious diet. The grass at the top is eaten by the Zebra and the Gazelle, so then the next part is eaten by the wildebeest 4. Describe how the relative gazelle density changes over time, in relation to the changes in the relative wildebeest density and in the grass height. Why do you think this is so? The gazelle reaches its peak density much quicker than the other animals. In relation to the wildebeest density, they raise and drop quicker. In relation to the grass, they change as the grass gets shorter. This is probably because of the rain and grass availability. 5. Would you describe the interactions between zebras, wildebeests, and gazelles as competition or facilitation among species? Support your answer with data from Figure 1. A facilitation because the grazer density for each species peaks at a different time. The zebra go after rainfall, then wildebeests, and then gazelles. PART 2: Investigating Dietary Niche Partitioning with Metabarcoding To investigate dietary niche partitioning, we first need to determine the diets of different animals. This can be done using a technique called DNA metabarcoding, which uses DNA to accurately identify what plant species an animal ate. Watch the video clip “Metabarcoding” to learn more about this technique. The results of DNA metabarcoding can be displayed as shown in Table 1, which has individual animals in rows and plant species in columns. If an animal ate a specific plant, the corresponding table cell is marked “Yes.” Table 1. An example of data that could be determined through DNA metabarcoding. Stargrass (Cynodon plectostachyus) Brihati (Solanum indicum) Burr grass (Tragus berteronianus) Elephant #1 Yes Yes No Dik-Dik #1 No No Yes Data like these can be analyzed using a statistical technique called nonmetric multidimensional scaling (NMDS), which calculates similarities in animals’ diets across many different plant species. NMDS simplifies information from these many dimensions into two-dimensional scatterplots that are easier to interpret. Figure 2 shows an NMDS plot where each data point represents the overall variety of plant species found in an individual animal’s diet. Animals whose points are closer together have a more similar diet than animals whose points are farther apart. Figure 2. An NMDS plot comparing herbivore diets during a single wet season. Each point represents the diet of an individual animal, and they are color-coded by species. In NMDS plots like these, the axes (NMDS1 and NMDS2) do not correspond to specific variables. Instead, they represent a combination of multiple variables. Use Figure 2 to answer the following questions. 6. In general, how does the diet of the plains zebra compare to that of the Grevy’s zebra? Are they eating the same species of plants? In general, they are eating the same species of plants. 7. In general, how does the diet of the plains zebra compare to that of the impala? The plains zebra have more variety in their diet and the impala eats more similar things. 8. The data in Figure 2 are from a single wet (rainy) season. Why would it be important to run the experiment again during other seasons? It would be important to experiment in other seasons because their diet can change based on what resources may be available at the time. 9. Which of the following statements best describes niche partitioning? a. Varying prey species maintain biodiversity. b. Superior species enjoy success because of competitive exclusion. c. Coevolution between two species means they can always share the same niche. d. Similar species can coexist because of slight differences in each one’s niche. 10. How can niche partitioning increase biodiversity? Because it allows for more than one species to access a limited resource.