Niche Partitioning

Part 1: Niche Partitioning by Time and Grass Height

Overview

• Niche partitioning is a mechanism that allows multiple species to coexist in the same ecosystem by using resources in different ways or at different times.

• A species’ niche = its place and role in an ecosystem, including where it lives and how it gets resources to survive.

• In the African savanna, large herbivores (zebra, wildebeest, Thomson’s gazelle) share the same grass species, Panicum maximum, yet they persist together due to niche partitioning by time and by the height/part of the grass consumed.

• Panicum maximum (P. maximum) is the niche resource examined: its growing season starts after peak rain and lasts for 6 months. When grass is tall, stems are abundant but of relatively low-quality food; the more nutritious parts are closer to the ground. Grazers feeding on the top can influence later regrowth, which often yields more nutritious new growth nearer the ground.

• The grass’s height over time is shown in Figure 1, along with the relative population densities of zebra, wildebeest, and Thomson’s gazelle. Densities are normalized to each species’ maximum observed count.

• Key digestion differences:

  • Zebras are non-ruminant grazers with paired upper and lower teeth, enabling them to bite tall stems. They digest faster because they use hindgut fermentation.

  • Wildebeests and Thomson’s gazelles are ruminants with four-chambered stomachs and foregut fermentation, which allows greater extraction of energy and nutrients from the same plant material, especially when that material is nutritious.

  • Ruminants can regurgitate and rechew partly digested food (rumination) to improve digestion efficiency.

  • Because foregut fermentation extracts more energy from nutritious food, ruminants can extract more energy from smaller amounts of high-quality forage compared to hindgut fermenters.

  • Smaller ruminants (e.g., Thomson’s gazelle) require less energy than larger ruminants (e.g., wildebeest).

Part 1: Describing the system in Figure 1

• Grass resource: Panicum maximum

  • Growing season: after peak rain, lasts about 6 months.

  • Grass height over time: tall early on, with new growth closer to ground as it regrows after grazing.

• Grazers:

  • Zebra: first to use the resource; thrives when grass is tall and abundant, even if less nutritious.

  • Wildebeest: larger ruminant; depends on nutrient-rich regrowth and efficient foregut fermentation.

  • Thomson’s gazelle: smaller ruminant; also relies on nutritious regrowth but has lower energy needs than wildebeest.

• Mechanisms of niche partitioning observed in this system:

  • Temporal partitioning: different species exploit the same grass resource at different times of growth/availability.

  • Nutritional/height-based partitioning: the nutritious parts of the grass are closer to the ground; the top portions are initially consumed by tall-grass specialists, while regrowth near ground becomes more valuable for ruminants later.

• Conceptual outcome: Coexistence arises because species use the same grass resource in different ways (time and height-related changes in forage quality), limiting direct competition.

Part 1: Answer the following questions based on Figure 1 and the information above
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?

• Zebra density is highest early in the season when P. maximum is tall and abundant. This early peak corresponds to zebras exploiting the tall grass stems, which are easier to reach with their dental morphology (paired upper and lower teeth to bite tall stems). Zebras digest fast (hindgut fermentation), so they can capitalize on plentiful but lower-nutrition top growth even when overall forage quality is not at its highest.

• Why tall, abundant grass favors zebas: tall stature provides readily accessible forage across a broad area; their fast digestion allows rapid energy extraction from this structure, enabling them to accumulate biomass quickly when top growth is plentiful.

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

• Wildebeest density tends to increase after the initial zebra-dominated period, as the nutrient quality of regrowth improves and forage becomes more energy-dense due to foregut fermentation.

• As the grass regrows after grazing and the parts that grow back are more nutritious (especially near ground level), wildebeests, with foregut fermentation, can extract more energy per unit of food and support higher densities.

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.)

• Reason for wildebeest spike:

  • After initial grazing by zebras, P. maximum regrows with new, near-ground tissues that are more nutritious. This makes the forage more energy-dense for foregut fermenters, increasing their net energy gain per bite.

  • Wildebeests’ larger body size and higher energy requirements are better supported by the regrown, nutrient-rich basal parts of the grass, leading to higher densities when these parts are abundant.

  • Foregut fermentation allows more efficient extraction of nutrients from regrown, nutritious forage, enabling wildebeests to convert intake into population growth during this window.

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?

• Thomson’s gazelle density changes more gradually and often tracks the regrowth phase that follows the zebra and wildebeest activity. When grass height remains tall but nutrition is lower (top-growth dominance), gazelles can persist but may not peak as strongly as the larger ruminants.

• As regrowth near ground becomes more nutritious, gazelles can benefit from the improved forage but still have lower energy demands than wildebeests, so their density may rise later in the season, following the wildebeest peak but before grass height declines significantly.

• Overall, gazelles occupy a middle niche: they respond to the same resource but with different energy needs and digestive strategies, leading to staggered densities that reduce direct competition with both zebras and wildebeests.

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.

• Answer: Competition is the primary interaction for the shared resource (P. maximum). The species differ in when and how they use the grass (tall top-growth vs regrown basal parts), leading to temporal and height-based partitioning that reduces direct competition. Figure 1 shows distinct density peaks for each species across the grazing season, indicating niche partitioning that mitigates competition. There is no explicit evidence of facilitation (one species improving another’s resource), and the observed pattern is consistent with coexistence via niche differentiation rather than mutualistic facilitation.

Part 2: Types of Niche Partitioning

Watch the video on niche partitioning and complete the table (general descriptions with specific examples)

• Spatial niche partitioning

  • Description: Different species use different physical spaces or habitats within the same area to reduce overlap.

  • Example: In a savanna, zebras grazing in open grass plains while gazelles use shaded edges or denser low-lying grasses nearby water sources.

• Dietary niche partitioning

  • Description: Species select different sets of foods or feeding behaviors to minimize competition for the same plant resources.

  • Example: Grazers primarily feeding on grasses vs browsers feeding on shrubs and forbs.

• Niche partitioning by resource height

  • Description: Species exploit different parts of the same plant by height or plant structure (e.g., top vs base, leaves vs stems).

  • Example: Zebras feeding on tall grass stems at the top while ruminants focus on regrowth near the ground after grazing, which is more nutritious.

• Niche partitioning by time (temporal niche partitioning)

  • Description: Species feed or forage at different times of day or seasons to avoid direct competition.

  • Example: Some herbivores graze in the morning while others feed in the afternoon or evening, or different seasons favor different forage quality.

Part 3: Investigating Dietary Niche Partitioning with Metabarcoding

Overview of metabarcoding and NMDS

• DNA metabarcoding: A method to determine animal diets by identifying plant DNA in an animal’s feces or gut contents. It tells which plant species were eaten with high specificity.

• Data structure: Table 1 shows individuals (rows) and plant species (columns); cells contain Yes/No indicating whether a plant species was present in an individual’s diet.

• Example given: Elephant #1 fed on Stargrass (Yes), Brihati (Yes), Burr grass (No); Dik-Dik #1 fed on Stargrass (No), Brihati (No), Burr grass (Yes).

• Data analysis: Nonmetric Multidimensional Scaling (NMDS) is used to visualize diet similarities across individuals.

  • NMDS reduces high-dimensional dietary data (many plant species) into two dimensions (NMDS1 and NMDS2) for visualization.

  • In NMDS plots, the distance between points reflects diet similarity: points closer together indicate more similar diets; axes themselves do not have direct variable meaning.

  • The plot in Figure 2 shows color-coded points by species, illustrating how diets cluster or separate among species during a single wet season.

Part 3: Questions based on Figure 2 and NMDS
7) 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?

• General pattern: Plains zebras tend to have a broader, more generalized diet focused on a wide range of grasses, while Grevy’s zebras are more selective and may consume a narrower set of plant species; thus, they do not eat exactly the same species of plants. NMDS separation indicates some overlap but not complete identity in their diets.

8) In general, how does the diet of the plains zebra compare to that of the impala?

• Plains zebras are primarily grazers (grass-dominated diets), whereas impalas are mixed feeders (both grasses and forbs/browse). NMDS is likely to show clearer separation along a grazer-browser gradient, with plains zebras clustering toward grazer-dominated diets and impalas occupying a mixed region.

9) How might the data in Figure 2 provide a greater understanding of the grazer-browser spectrum?

• NMDS1 appears to align with the grazer-browser continuum: lower values correspond to grazers (grass specialists), higher values to browsers (feeding on non-grass plants). The plot shows where species lie on this spectrum and how much dietary overlap exists, clarifying where intermediate feeders fall on the gradient.

10) Figure 2 includes six wild species and one domesticated species: cattle. How might these data inform wildlife management near areas with farming and/or ranching of domesticated animals?

• The data can reveal potential dietary overlap between cattle and wild herbivores and identify which species share similar diets (high overlap) and thus may compete for resources.

• Management implications: maintain habitat heterogeneity and forage diversity to minimize competition; designate protected zones with natural plant communities; consider seasonal shifts in diet when planning grazing schedules and land use near wildlife areas.

11) 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?

• Diet composition and plant availability vary seasonally due to rainfall, growth cycles, and plant phenology.

• Repeating the NMDS analysis in other seasons would reveal how dietary niches shift with seasonal changes, improving our understanding of resilience and coexistence under different environmental conditions.

Part 4: Applications of Niche Partitioning

12) For each of the examples in the following table, identify the mechanism by which resources, and thus the niches, are divided. Use the niche partitioning mechanisms described in Question 6.

• Example 1: During the warm daylight hours, bees collect nectar from the flowers on a linden tree. In the evening, different types of moths are on the flowers.

  • Mechanism: Temporal niche partitioning (time-of-day activity differences) to avoid competition for floral resources.

• Example 2: Two types of birds, pileated woodpeckers and yellow-bellied sapsuckers, get food from the same tree. Sapsuckers drill rows of little holes to eat the tree’s sap and insects in the sap; pileated woodpeckers dig deep holes to find insects in the tree trunk.

  • Mechanism: Spatial niche partitioning within the same tree (different feeding locations/habitats within the tree).

• Example 3: Prairie grasses use their roots to get water and nutrients from the soil. Smartweed roots reach nearly 100 cm underground, Indian mallow roots reach 70 cm, and bristly foxtail roots reach only about 20 cm.

  • Mechanism: Niche partitioning by resource height (root depth in soil) to access different soil strata.

13) Which of the following statements best describes niche partitioning?

• d. Similar species can coexist because of slight differences in each one’s niche.

14) How can niche partitioning increase biodiversity?

• By allowing more species to coexist in the same area through differentiation of resource use (space, time, diet, height, etc.), niche partitioning reduces direct competition and supports higher species richness and stability in ecosystems.

Notes on key terms and concepts

• Niche: a species’ role and position in an ecosystem, including its habitat and resource use.

• Niche partitioning mechanisms: spatial, temporal, dietary, resource-height (vertical plant structure or root depth), and combinations thereof.

• Grazers vs browsers (grazer-browser spectrum): grazers focus on grasses, browsers feed on non-grass vegetation; some species are mixed feeders.

• Panicum maximum (P. maximum): the study grass with a growing season shortly after peak rain and lasting about six months; height and regrowth influence forage quality.

• Digestive strategies: hindgut fermentation (non-ruminants, like zebras) allow faster digestion; foregut fermentation (ruminants, like wildebeest and Thomson’s gazelle) allows greater nutrient extraction from the same forage.

• DNA metabarcoding: a method to identify plant components in an animal’s diet using DNA; results can be organized in a presence/absence table across many plant species.

• Nonmetric Multidimensional Scaling (NMDS): a multivariate technique that reduces complex diet data into two dimensions (NMDS1, NMDS2) to visualize similarities/differences among individuals or species; axes are abstract and not tied to single variables.

• Seasonal effects: dietary niches and plant availability vary with season; repeating studies across seasons strengthens understanding of niche partitioning and coexistence.

Key figures referenced

• Figure 1: Relative population densities of zebras, wildebeests, and Thomson’s gazelle across six months after peak rain, with grass height changes for Panicum maximum.

• Figure 2: NMDS plot comparing diets of herbivores (colors by species) during a single wet season; shows diet similarities/differences among individuals using multiple plant species.

• Table 1: Example DNA metabarcoding results showing whether an animal ate specific plant species (Yes/No).

Formulas and notation used

• Grass resource growth window: 6 ext{ months} after peak rain.

• NMDS interpretation: axes ext{NMDS1}, ext{NMDS2} do not correspond to specific variables; points closer together indicate more similar diets.

• Normalization in Figure 1: population counts are normalized to the maximum count for each species, i.e., ext{count}_{ ext{species}}
  ightarrow rac{ ext{count}}{ ext{max(count)} } for comparison across species.

• Grass height relationship: nutritious parts are closer to the ground; top growth tends to be less nutritious; regrowth after grazing becomes more nutritious at the base.

End of notes