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Discussion Assignment 7: Ecological Niches

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OVERVIEW

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This week in lecture we learned about different species interact for space and food in an ecosystem and biodiversity. In this discussion, you’ll use scientific data and videos to explore different examples of niche partitioning in the African savanna. In ecology, the term “niche” describes the role an organism plays in a community. A species’ niche encompasses both the physical and environmental conditions it requires (like temperature or terrain) and the interactions it has with other species (like predation or competition). The niche partitioning theory is central to our understanding of biodiversity. The term niche partitioning refers to the process by which natural selection drives competing species into different patterns of resource use or different niches. Perhaps the most obvious way that species can partition resources is in terms of what they consume.

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For example, in the African savanna ecosystem, many species of large herbivores share similar habitats. How do all these species coexist, or live together, without some species outcompeting the others? 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.)

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One example of niche partitioning is the resource partitioned of African 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.

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Figure 1 shows three types of grazers — zebra, wildebeest, and Thomson’s 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 Thomson’s 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), they take up more nutrients and proteins than when zebras digest food (via fermentation in the hindgut). So, a ruminant can extract more energy from a smaller amount of food if that food is more nutritious. Smaller ruminants, such as Thomson’s gazelles, need less energy than larger ruminants, such as wildebeests.

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Answer the following questions based on Figure 1, video and the information above.

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youtube.com/watch?v=J9HMnyZd2cU

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1. Describe how the relative zebra density changes over time. What characteristics of zebras could explain why zebra densities are greatest when the P. maximum grass is tallest and most abundant?

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2. Describe how the relative wildebeest density changes over time.

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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.)

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4. Describe how the relative Thomson’s 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?

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5. Would you describe the interactions between zebras, wildebeests, and Thomson’s gazelles as competition or facilitation among species? Support your answer with data from Figure 1.

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6. Is Resource Partitioning a Solution for Coexistence?

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7. Describe how both biotic and abiotic factors can affect the population size of deer in a forest.

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8. Match each term to a statement listed below.

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• Mutualism
• B. commensalism
• Exploitation
• competition

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1. an elk eating an aspen sapling

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2. a bird building a nest within a tree

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3. bears and wolves both consuming carcasses within the same area

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4. a fish that consumes parasites found on a shark