Resource Partitioning and Character Displacement
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You should understand the competitive exclusion principle and the concept of the ecological niche as a multidimensional hypervolume of resource use. Familiarity with basic population growth models and the definition of a limiting resource is assumed.
Why this matters
Resource partitioning explains how biodiversity is maintained in communities where competition is intense.
The competitive exclusion principle suggests that complete overlap in resource use leads to the extinction of one species. However, natural communities are often stable and diverse. This lecture explores the mechanisms that allow coexistence: resource partitioning, where species divide resources to reduce direct competition, and character displacement, where evolutionary divergence in traits further reduces overlap. These processes are fundamental to understanding community structure, speciation, and the maintenance of biodiversity in ecosystems ranging from tropical forests to marine reefs.
The Logic of Coexistence
Imagine two species of birds living on the same island. Both eat seeds. If they eat exactly the same seeds, from the same trees, at the same times, they are in direct competition. According to the competitive exclusion principle, one will eventually outcompete the other. But what if one bird prefers large seeds and the other prefers small seeds? Or if one forages in the canopy and the other on the ground? By dividing the available resources, they reduce the intensity of competition. This is resource partitioning. It is not just a passive outcome; it can be an active evolutionary response. If two species are too similar, natural selection may favour individuals that use slightly different resources, leading to character displacement. Over time, the species become more different in their traits, allowing them to coexist. This process is a key driver of evolutionary diversification. It explains why we see so many different species of finches, anoles, and cichlids in the same environment. Without partitioning, competition would be too fierce for all of them to survive. With partitioning, each species occupies a slightly different niche, reducing direct conflict. This is the essence of coexistence: not avoiding competition entirely, but managing it so that it is not lethal. The niche is not a fixed box; it is a flexible space that species can shift and reshape through evolution. This dynamic view of niches is crucial for modern ecology. It moves us beyond simple competition to a more nuanced understanding of how species interact and evolve together. The result is a stable community where multiple species can persist, each exploiting a different portion of the total resource pool. This is the foundation of biodiversity.
Resource Partitioning in Action
Resource partitioning can occur along several axes: spatial, temporal, and dietary. Spatial partitioning involves using different parts of the habitat. For example, in a forest, different bird species may forage at different heights in the trees. One species might feed on insects in the upper canopy, while another feeds on insects in the understory. This reduces direct competition for the same food items. Temporal partitioning involves using resources at different times. Nocturnal and diurnal species can coexist by feeding on the same prey but at different times of day. Dietary partitioning involves using different types of food. In the case of Darwin’s finches, different species have evolved different beak sizes, allowing them to eat different sizes of seeds. This is a classic example of dietary partitioning. The key is that the total resource pool is large enough to support multiple species, but each species uses a different portion. This reduces the per-capita competition rate. The strength of competition depends on the degree of niche overlap. If the overlap is high, competition is strong. If the overlap is low, competition is weak. Resource partitioning reduces the overlap, thereby reducing competition. This allows coexistence. In mathematical terms, the competition coefficient \(\alpha_{ij}\) measures the effect of species \(j\) on the growth rate of species \(i\). If \(\alpha_{ij}\) is low, competition is weak. Resource partitioning lowers \(\alpha_{ij}\) by reducing niche overlap. This is a crucial mechanism for maintaining biodiversity. In practice, partitioning is often incomplete. There is usually some overlap, but it is reduced enough to allow coexistence. This is why we see many species in the same habitat, each with slightly different traits. The result is a complex web of interactions, where each species plays a unique role. This is the essence of community ecology.
Character Displacement and Evolution
Character displacement is the evolutionary divergence of traits in sympatric species (species living in the same area) compared to allopatric species (species living in separate areas). This divergence is driven by competition. If two species are too similar, selection favours individuals that use different resources. Over time, the population shifts toward the new trait values. This is character displacement. A well-documented recent example comes from small islands in Florida: after the brown anole was introduced, the native green anole moved to higher perches and, within about 20 generations, evolved larger toe pads suited to them (Stuart and colleagues, 2014). Another example is the cichlid fish in African lakes. In lakes with many cichlid species, the fish have diverse mouth shapes, allowing them to eat different types of food. In lakes with fewer species, the mouth shapes are more similar. This suggests that competition has driven the evolution of diverse mouth shapes. Character displacement is a powerful force in evolution. It can lead to speciation, as populations diverge in traits and become reproductively isolated. It is a key mechanism for the maintenance of biodiversity. In mathematical terms, character displacement can be modelled as a shift in the mean trait value of a population. If the mean trait value of species \(i\) shifts away from the mean trait value of species \(j\), competition is reduced. This is a dynamic process, driven by natural selection. The result is a more diverse community, with species that are more different in their traits. This is the essence of evolutionary ecology.
Measuring Niche Overlap
To quantify resource partitioning, ecologists measure niche overlap. One common metric is the Schoener’s overlap index, \(D\), which ranges from 0 to 1. A value of 0 means no overlap, while a value of 1 means complete overlap. The index is calculated by comparing the resource use of two species. If species \(i\) uses resource \(k\) with probability \(p_{ik}\), and species \(j\) uses resource \(k\) with probability \(p_{jk}\), then the overlap is calculated as:
The concept
Competition and the niche
Two species cannot occupy the same niche indefinitely.
A common misconception is that resource partitioning always leads to complete separation of niches. In reality, partitioning is often incomplete, and some overlap remains. This residual overlap can still lead to competition, but it is usually weak enough to allow coexistence. Another misconception is that character displacement is always driven by competition. It can also be driven by other factors, such as predation or sexual selection. It is important to consider all possible drivers of trait divergence. C
Worked example
Problem. Two species of finches, Species A and Species B, feed on three sizes of seeds: small, medium, and large. The proportions of each seed size eaten by Species A are: small = 0.6, medium = 0.3, large = 0.1. The proportions for Species B are: small = 0.2, medium = 0.5, large = 0.3. Calculate the Schoener’s overlap index \(D\) to quantify the degree of resource partitioning.
- Identify the resource use probabilities for each species. For Species A: \(p_{A,\text{small}} = 0.6\), \(p_{A,\text{medium}} = 0.3\), \(p_{A,\text{large}} = 0.1\). For Species B: \(p_{B,\text{small}} = 0.2\), \(p_{B,\text{medium}} = 0.5\), \(p_{B,\text{large}} = 0.3\).
- Calculate the absolute difference in resource use for each seed size. For small seeds: \(|0.6 - 0.2| = 0.4\). For medium seeds: \(|0.3 - 0.5| = 0.2\). For large seeds: \(|0.1 - 0.3| = 0.2\).
- Sum the absolute differences: \(0.4 + 0.2 + 0.2 = 0.8\).
- Apply the Schoener’s overlap formula: \(D = 1 - \frac{1}{2} \times 0.8 = 1 - 0.4 = 0.6\).
- Interpret the result. A value of \(D = 0.6\) indicates moderate resource partitioning. There is some overlap, but it is not complete. This suggests that the two species can coexist, but competition may still be significant.
Units check. The index \(D\) is dimensionless, as it is a ratio of probabilities. The value is between 0 and 1, which is consistent with the definition of the index.
Three quick tests
- What is the main difference between resource partitioning and character displacement?
Answer
Resource partitioning is the behavioural or ecological division of resources, while character displacement is the evolutionary divergence of traits driven by competition. - How does the Schoener’s overlap index measure resource partitioning?
Answer
It calculates the degree of overlap in resource use between two species, with a value of 0 indicating no overlap and 1 indicating complete overlap. - Why is incomplete resource partitioning still sufficient for coexistence?
Answer
Because the residual overlap is usually weak enough to allow both species to persist, as long as the competition is not too intense.
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Lecture notes checked line by line by the editor; every number in the worked example was recomputed.