Ecological Niche

The term habitat is derived from the Latin word "habitare", meaning to inhabit. We can define a habitat as a natural environment where a particular organism lives and utilizes the resources of that place for its survival, such as food, shelter, defence and mates. Since every individual on the planet has a position and role to play in its environment, such as how it obtains food, survives, and reproduces, to name a few, certain essential sets of  environmental conditions are required for its existence. It includes all resources and physical environmental conditions an individual requires to survive, function and reproduce indefinitely in an ecosystem. This is the area within a habitat occupied by an organism and known as the ecological niche. 

1.0Niche Concept

• In 1957, a zoologist introduced the term "niche". 
• According to him, a niche is a model with a multidimensional hypothetical space with several components. 
• Each component exhibits the tolerance ranges of certain essential environmental conditions critical for the species' survival.  These components include: 
• Habitat - A species occupies a physical space within its habitat, where all essential conditions are present for an organism to grow, survive and reproduce infinitely. 
• Trophic Position -  A fundamental position of an organism in its community or the food chain 
• A multidimensional hypervolume - It comprises the basic concepts of niche along with the limiting factors. That means that all sets of environmental conditions (physical, chemical, biogeographical) required by an organism for its existence. 
• An interaction -  A relationship of an organism with other organisms and with its environment.

So, fundamentally, an ecological niche is a multidimensional representation of available resources, habitat requirements and environmental limitations of a species. 

• Sometimes, a niche remains unoccupied for an extended period, and another species can fill it. 
• On the other hand, some species create a special niche for themselves. 
• Having a specialized, unique niche is always advantageous for the species. It always minimizes competing for the same limited resources. 
• The number of factors affecting an ecological niche varies with species. 
• The relative significance of these factors may also vary depending on the geographical and biotic conditions. 
• Since species need to be able to adapt to the changing ecosystem to avoid extinction, they evolve to expand in a wide range of environmental conditions. 

2.0Classification of Niche 

• Based on occupancy of habitat, niches can be classified as: 
• Fundamental and RealizedRealized Niche
• A fundamental niche can be defined as a hypothetical set of physical (abiotic) and biological (biotic) conditions occupied by a species or an organism. 
• It is the entire space that is occupied by an organism when they are not in competition for the same resources. 
• In other words, the fundamental niche is the total range of the environmental variations suitable for an organism's survival without being influenced by antagonistic interactions such as competition, predation, and parasitism. 
• This allows an organism to expand, survive and reproduce more effectively. However, in nature, species do not occupy the entire niche due to the uneven distribution of resources across the landscape. 
• Resources are present in the form of patches that are occupied by species.
• Furthermore, species are often under biotic controls such as competition, predation, and biogeography. Since they cannot utilize their entire niche due to the presence of other species, they usually remain in competition. 
• In such circumstances, a species uses only a part of its niche and becomes relatively dominant. 
• This small subset of the fundamental niche occupied by species is known as their realized niche. Since the resources are unevenly distributed across the habitat, the presence of competitors and predators may vary. 
• Hence, the realized niche varies with the populations of the same species. Subsequently, each species possesses both a fundamental and realized niche. 
• Since realized niches are part of a fundamental niche, they are smaller than a fundamental niche.  

A niche can further be divided into two categories, namely, alpha and beta niches.  

Alpha Niche

• Alpha Niches are resource-specialized niches that occur in habitats where several species coexist. 
• The presence of two different alpha niches means the two species are using different resources. In other words, they use similar resources, reducing overlap in their use. 

Beta Niche

• Due to variations in the environmental tolerances, some species prefer to inhabit separate habitats. Such species rarely get together to interact physically. 
• This is because these species are exposed to different environmental variations, including physical, chemical, and biological factors. 

3.0Niche Width 

• Niche breadth, as illustrated, is also known as niche width. It is an environment that encompasses all appropriate conditions, including physical, chemical, and biological. 
• More specifically, the habitable range between the maximum and minimum values of each parameter constitutes tolerance limits for an individual's life. 
• It includes suitable physical (temperature, precipitation, pressure, humidity), chemical (salinity, pH), and biological (Inter- and intraspecific interactions) for an individual's existence. 

Niche Overlap 

• When two or more species compete for the same niche space, it leads to overlapping niches. 
• Generally, it occurs when the resources used by several species are identical or similar. 
• The degree or the intensity of competition between coexisting species determines the width of the niche. It is an index of similarity between the resource utilization by several species. 
• Niche overlap increases when the two peaks of species distribution move closer to one another. Based on the intensity of competition occurring between or among the species, overlapping.
• Partial Overlapping occurs when the curves of two competing species partially cover each other. 
• It occurs when the intensity of competition is low, and the stronger or dominant species acquire the niche from different species, thus reducing the niche width of weaker species.  

Significant Overlapping

• When the curves of two species completely cover each other. 
• It occurs when one of two competing species for the same resources eliminates other species from the niche. 
• It often occurs in realized niches where competition is higher than in the fundamental niche. 

Competition and the Niche 

• The resources are finite in the constant environment. 
• When the population expands exponentially, the demand for limited resources also increases. 
• In an unavailability or shortage of resources required by species for their survival, species compete with other species. 
• Furthermore, the situation worsens when the resources required by two species are similar or identical. 
• Consequently, several impacts can be observed in an ecosystem. These impacts can broadly be categorized 
• Short-term Effects - It operates on the ecological time scale. It eliminates or removes one competitive species from the niche through the Competition Exclusion Principle. However, several ecological processes exist in nature to minimize or avoid the impact of competition exclusion. Competitive species can do this through Resource Partitioning, Competitive Release.
• Long-term Effects-  It operates on the evolutionary time scale. It splits or divides closely related species into multiple by changing their characteristics through character displacement. 

4.0Gauss Competitive Exclusion Principle 

• Imagine populations of two species inhabiting the same habitat with an identical niche. Initially, when the populations of both species are very small, they may coexist in the same niche for some time. 
• But, as soon as the population starts expanding, demand for identical resources also increases, and competition prevails due to unavailability or insufficient limited resources. 
• During the competition, the weaker species will either go extinct or shift to other resources where they adapt and evolve. Sooner or later, both species come out of that competition. 
• In nature, it is difficult to observe that two species occupy the same niche. However, the larger the overlap between the species' niches, the more intense the competition between them tends to be. 
• To reduce the overlap of niches, it becomes necessary to minimize the degree of competition between the species. 
• This can be done through resource partitioning, character displacement, and competitive release.

5.0Resource Partitioning 

• Although individuals compete for limited resources, coexisting species must evolve to become more specialized and narrow their focus on using those resources. 
• Species can achieve this by utilizing different resources, occupying separate regions of niches, or feeding during different times of the day to reduce the competition for similar resources. 
• This is known as resource partitioning. In other words, competing species for similar resources can coexist by minimizing the competition intensity through niche separation. 
• This results in a large non-overlapping and separation of the niche. 
• Hence, we can define resource partitioning as dividing or partitioning the limited resources by coexisting species to avoid competition in an ecological niche. 

6.0Character Displacement

• When two similar species with identical niches come in contact, they will likely compete more aggressively. 
• Different species under the influence of competition may develop certain traits to utilise available resources in their partition more efficiently. 
• An individual's adaptation to these traits in the species minimises the competition in the other partition, causing the divergence of traits and, thereby, resulting in the shifting of the niche. 
• When such a shift is related to the morphological changes in the species, it is known as character displacement or niche shift. 

7.0Competitive Release

• Individuals often confine themselves to a small geographic region in the presence of competitively superior species. 
• However, when stronger competitors are experimentally removed from the ecological niche, the other species tend to expand their distributional ranges. 
• Thus, competitive release can be defined as a mechanism in which a species tends to expand its distributional range in the absence of competitively superior species. 
• In brief, it is an expansion of the ecological niche without competitors. It often occurs in a habitat where one of the two species competing for similar resources, usually the stronger one, is removed from the habitat. 
• This allows the remaining competitor species to utilize the resources more efficiently than when the first one was present. 
• A significant increase in the population of a less widely distributed species occurs in the process. When two closely related species coexist, they are quite different. 
• But, when one of the two species occurs alone, it converges towards the other species, appearing virtually similar in some characteristics. 
• Thus, it acts as a mirror of character displacement. For example, Balanus and Chthamalus. 
• According to Connell, Balanus spp. Often prefers the intertidal zone, whereas Chthamalus spp. Lies in the shoreline above the high tide. 
• When the balance spp. Once experimentally removed, Chthamalus colonized the intertidal zone and outgrew Balanus, thus effectively preventing Balanus from establishing itself again in the intertidal zone. 
• Removing the Balanus species allowed Chthamalus to undergo competitive release in the presence of Balanus spp. Chthamalus could occupy only its realized niche, while in its absence, Chthamalus can occupy its fundamental niche.