Niche differentiation

The term niche differentiation (synonymous with niche segregation, niche separation and niche partitioning), as it applies to the field of ecology, refers to the process by which natural selection drives competing species into different patterns of resource use or different niches. This process allows two species to partition certain resources so that one species does not out-compete the other as dictated by the competitive exclusion principle; thus, coexistence is obtained through the differentiation of their realized ecological niches.

Niche differentiation is a process which occurs through several different modes and on multiple temporal and spatial scales. In most cases, niche differentiation has created a relationship between two species where current competition is small or non-existent. Because of this, the presence of niche differentiation can be methodologically difficult to prove or disprove. The lack of evidence for current or past competition can blur the line between 1) two competitive species differentiating their niches to allow coexistence as opposed to 2) two non-competing species which occupy similar niches. It is important to keep in mind that niche differentiation and inter-specific competition cannot always be considered linked.

As an example of resource partitioning, seven Anolis lizards in tropical rainforest share common food needs — mainly insects. They avoid competition by occupying different sections of the rainforest. Some live on the leaf litter floor while others live on shady branches, thereby avoiding competition over food in those sections of the forest. All resources are subject to partitioning, for example; space, food, nesting sites. This minimizes competition between similar species.

Niche differentiation: detection and quantification
The Lotka-Volterra equation states that two competing species can coexist when intra-specific (within species) competition is greater than inter-specific (between species) competition (Armstrong and McGehee 1980). Since niche differentiation concentrates competition within-species, due to a decrease in between-species competition, the Lotka-Volterra model predicts that niche differentiation of any degree will result in coexistence.

In reality, this still leaves the question of how much differentiation is needed for coexistence (Hutchinson 1959). A vague answer to this question is that the more similar two species are, the more finely balanced the suitability of their environment must be in order to allow coexistence. There are limits to the amount of niche differentiation required for coexistence, and this can vary with the type of resource, the nature of the environment, and the amount of variation both within and between the species. To answer questions about niche differentiation, it is necessary for ecologists to be able to detect, measure, and quantify the niches of different coexisting and competing species. This is often done through a combination of detailed [natural history] studies, controlled experiments (to determine the strength of competition), and mathematical models (Strong 1982, Leibold 1995). To understand the mechanisms of niche differentiation and competition, much data must be gathered on how the two species interact, how they use their resources, and the type of ecosystem in which they exist, among other factors. In addition, several mathematical models exist to quantify niche breadth, competition, and coexistence (Bastolla et al. 2005). However, regardless of methods used, niches and competition can be distinctly difficult to measure quantitatively, and this makes detection and demonstration of niche differentiation difficult and complex.

Development of niche differentiation
Over time, two competing species can either coexist, through niche differentiation or other means, or compete until one species becomes locally extinct. Several theories exist for how niche differentiation arises or evolves given these two possible outcomes.

Current competition
Niche differentiation can arise from current competition. For instance, species X has a fundamental niche of the entire slope of a hillside, but its realized niche is only the top portion of the slope because species Y, which is a better competitor but cannot survive on the top portion of the slope, has excluded it from the lower portion of the slope. With this scenario, competition will continue indefinitely in the middle of the slope between these two species. Because of this, detection of the presence of niche differentiation (through competition) will be relatively easy. It is also important to remember that there is no evolutionary change of the individual species in this case; rather this is an ecological effect of species Y out-competing species X within the bounds of species Y’s fundamental niche.

Via past extinctions
Another way by which niche differentiation can arise is via the previous elimination of species without realized niches. This asserts that at some point in the past, several species inhabited an area, and all of these species had overlapping fundamental niches. However, through competitive exclusion, the less competitive species were eliminated, leaving only the species that were able to coexist (i.e. the most competitive species whose realized niches did not overlap). Again, this process does not include any evolutionary change of individual species, but it is merely the product of the competitive exclusion principle. Also, because no species is out-competing any other species in the final community, the presence of niche differentiation will be difficult or impossible to detect.

Evolving differences
Finally, niche differentiation can arise as an evolutionary effect of competition. In this case, two competing species will evolve different patterns of resource use so as to avoid competition. Here too, current competition is absent or low, and therefore detection of niche differentiation is difficult or impossible.

Resource partitioning
When two species partition [divide] a resource based on behavioral or morphological variation, it is termed differential resource utilization or resource partitioning. There are three types of differential resource utilization.

Temporal partitioning
Temporal resource partitioning is when two species eliminate direct competition by utilizing the same resource at different times. This can be on a daily scale (e.g. one species of spiny mouse feeds on insects during the day while a second species of spiny mouse feeds on the same insects at night, Kronfeld-Schor and Dayan 1999) or on a longer, seasonal scale. An instance of the latter would be reproductive asynchrony, or the division of resources by the separation of breeding periods. An example of reproductive asynchrony would be two competing species of frog offsetting their breeding periods. By doing this the first species’ tadpoles will have graduated to a different food resource by the time the tadpoles of the second species are hatching (Lawler and Morin 1993).

Spatial partitioning
Spatial resource partitioning occurs when two competing species use the same resource by occupying different areas or habitats within the range of occurrence of the resource. Spatial partitioning can occur at small scales (microhabitat differentiation) or at large scales (geographical differentiation). Microhabitat differentiation occurs when two competing species with overlapping home ranges partition a resource. Two examples would be different species of fish feeding at different depths in a lake or different species of monkey feeding at different heights in a tree. Geographical differentiation is when two competing species have non-overlapping home ranges and thus partition resources. An example might be given with monkeys again; two competing species of monkey using the same species of fruit trees, but in different areas of the forest.

Morphological differentiation
The final type of differential resource utilization is morphological differentiation or niche complementarity. Morphological differentiation happens when two competing species evolve differing morphologies to allow them to use a resource in different ways. A classic example of this is a study detailing the link between bumblebee proboscis lengths and flower corolla lengths (Pyke 1982). In this study, the long-proboscis bee species would preferentially feed on the long-corolla plants, the medium-proboscis bee species would feed on the medium-corolla plants, and so on. By evolving different proboscis lengths, several competing bee species are able to partition the available resources and coexist.

Conditional differentiation
The second form of niche differentiation is conditional differentiation, which occurs when two competing species differ in their abilities to use a resource based on varying environmental conditions. One species may be more competitive in one set of environmental conditions, but another species is more competitive in another set of conditions. Therefore, in a varying environment, each species is sometimes a better competitor and they can coexist. Differentiation based on environmental conditions is often difficult to separate from resource differentiation, and often conditional differentiation includes one or more types of resource partitioning.

Utilization of two resources (Tilman’s R*)
The final type of niche differentiation is based on Tilman’s (1990) notion that if two species are competing for the same exact resource then the ultimate winner will be the species which can deplete the resource the lowest, surviving on the lowest amount of the resource. This alone does not allow coexistence. However, if two species rely on two resources differentially, then coexistence is possible when each species can tolerate a lower amount of only one resource compared to its competitor. An example would be if grass species 1 is more limited by nutrient B than A and grass species 2 is more limited by nutrient A than B. Then, if species 1 uses more of nutrient B than A and species 2 uses more of nutrient A than B, species 1 can out-compete species 2 for nutrient A and species 2 can out-compete species 1 for nutrient B.  Additionally, the starting point for each nutrient’s availability must be roughly an equal distance from each species limits (i.e. no species can quickly lower one resource while the other is still in abundance). Coexistence is now possible because each species uses more of the nutrient that limits its own growth, but each still requires both nutrients to survive. This subtle form of niche differentiation is dependent on two conditions: 1) the habitat needs to be such that one species is more limited by one resource, and the other species is more limited by the other resource, and 2) each species must consume more of the resource that strongly limits its own growth.

Coexistence without niche differentiation: exceptions to the rule
Some competing species have been shown to coexist on the same resource with no observable evidence of niche differentiation and in “violation” of the competitive exclusion principle. One instance is in a group of hispine beetle species (Strong 1982). These beetle species, which eat the same food and occupy the same habitat, coexist without any evidence of segregation or exclusion. The beetles show no aggression either intra- or inter-specifically. Coexistence may be possible through a combination of non-limiting food and habitat resources and high rates of predation and parasitism, though this has not been demonstrated. This example illustrates that the evidence for niche differentiation is by no means universal. Niche differentiation is also not the only means by which coexistence is possible between two competing species (see Shmida and Ellner 1984). However, niche differentiation is a critically important ecological idea which explains species coexistence, thus promoting the high biodiversity often seen in many of the world’s biomes.