Ecological Interactions - Definition, Types

Ecological Interactions

You might have already learnt about the ecosystem in the previous units. Ecosystem is defined as, a system where biotic and abiotic components interact to function as a single unit. Since, all organisms depend on resources (food, pace, mates and so) or the services provided by ecosystems for their survival, growth and reproduction, no single species can exist in a complete isolation. This tendency of organisms has resulted in a relationship between the species and the ecosystems.

Ecological Interactions

In order to grow, survive and reproduce, every individual on earth requires some sets of environmental conditions. These set of favorable conditions are essential to maximize the role of species in its habitat. Since resources are not uniformly distributed across the landscapes, species cannot utilize the entire available resources. They occupy a part of it which is often competitively dominant region of entire niche. In this unit, you will learn about the above mentioned categories of interactions with their examples along with their applications in the real world. Besides the niche concepts and it significance.

Ecological Interactions and its applications

In everyday life, we use the term symbiosis which means “living-together”. However, in ecology, symbiosis is a close ecological association occurring between the individuals of two or more different species for the long period of time. This term can be used to describe all types of biotic interactions. Like most ecological interactions, symbiosis is recognized as one of the essential forces behind evolution.

Fig. 12.1. Types of ecological interactions existing in various ecosystems

We can define ecological Interaction as a direct or indirect association between organisms in an ecosystem. These interactions can between plants-plants, plants-animals or animals-animals. These interactions influence the population of interacting species. Sometimes, the effect could benefit mutually or can have detrimental impact on the interacting individuals. When it neither benefits nor harms the individuals, known as neutral interactions.

Depending upon the nature of effects on the interacting organisms, these interactions can be classified into three broad categories- positive, negative and neutral as illustrated in the fig 12.1.

Types of Interactions

As mentioned in the figure 12.1. the three broadly classified categories of ecological interactions can further be divided into several types of biotic interactions. These may include mutualism, commensalism, amensalism, competition and contramensalism (herbivory, parasitism and predation). An overview and nature of these interactions can be understood from the table 12.1. Obtained from Odum (1959) and Haskell (1947).

Table 12.1 Concise description ecological interactions existing between species.

We will discuss each of these interaction in coming sections in more detail.

Positive Interactions

When two organisms living in a defined geographic area, they interact with each other. During the interaction, either individuals of one species benefits the individuals of the interacting species or both species benefit in the partnership. Such interactions are known to be positive in nature. There are two kinds of such interactions exist in ecosystems. These may include mutualism and commensalism.

Mutualism

You may have learned about the nitrogen cycle in previous units. As illustrated in the fig. 12.2. nitrogen-fixing bacteria, rhizobium dwells in the root nodules of leguminous plants such as beans, lentils, peas, etc. These microorganisms convert atmospheric nitrogen into an ammonia-like substance to be utilized by the leguminous plants. In exchange, legumes supply bacteria safe environment and sufficient nutrition. This illustrates a beneficial mutualistic beneficial relationship between legumes and nitrogen-fixing bacteria.

Fig. 12.2. Symbiotic relationship between legumes and nitrogen-fixing bacteria.

From the above example, we can define mutualism as a symbiotic relationship occurring between the individuals of two different species, in which both the individuals of association benefit. Individuals of each interacting species grow, survive and reproduce more effectively in the presence of the other interacting species. Since, both interacting species benefit from the interaction, it is a win-win (+ +) condition, neither of species is harmed by the other. Benefits may include essential resources such as food, space, defense against predators, pollination, dispersal and so on.

The number of species involved in the interaction is determined by how each species benefits from it. It can be species- specific or different species. When interaction is exclusive between the two species, it is species specific, like formation of mycorrhiza which is formed due to a close association between fungi and plant species. But, sometimes interactions may involve several species as in case of pollination, where several pollinators interact with several species of plants to derive their food. The coexisting species and their interaction become so significant in mutualism that at least one species it become completely dependent on the other. In other words, sometimes, interactions become so important for some species that they completely dependent on the interaction for their existence. Based on this dependency of one species on the other, mutualism can be classified as obligatory and facultative.

Obligate Mutualism

You might have studied about the classic example of lichens, formed due to a close association between photobiont algae and mycobiont fungi to form a thallus (lichen), an entirely different species from the interacting species. During the association, fungi obtain nutrients from the photosynthetic algae, and in return, fungi provide protection and stability to algae as shown in fig. 12.3.

Fig. 12.3. Obligatory mutualism

Such interactions where species are forced or compelled to depend on each other for their survival are known as obligatory mutualism. These interactions show very high degree of interdependency as the absence of either of species can result in the death of other species or both. Hence, these interactions are exclusively species-specific in nature as well. 

A similar kind of relationship can be observed between coral polyps and zooxanthellae too. As we know coral polyps are small, soft-bodied organisms. They produce carbon dioxide and water during the cellular respiration. And, zooxanthellae are algae living in the tissues of coral polyps. While interacting, zooxanthellae uses this carbon dioxide and water to carry out photosynthesis. During the process, zooxanthellae produces sugars, lipids and oxygen as its byproducts. Coral polyps, then utilize these byproducts to grow. This continuous recycling of the byproducts between the cells of coral polyp and zooxanthellae force them to remain in a close association. Since, in the absence of zooxanthellae for long period of time will result in the death of coral polyps. Other such examples may include the mutualistic relationship between fungi and roots of higher plants; between termites and protozoans and so on.

Facultative Mutualism

Facultative mutualism, on other hand, is a relationship between two or more species in which the species benefit from the interaction but may also survive without it, independently. Such interactions are also known as proto cooperation. Unlike obligate mutualism, facultative mutualism can exist between multiples of species, but benefits obtained from it are less than that of obligate one.

Fig. 12.4. Facultative Mutualism

A well-documented example of facultative mutualism is between hermit crab and sea anemone as shown in fig. 12.4. During their lifespan, a young hermit often carries a young sea anemone on its back. Even when a hermit crab outgrows its shell, it transports or takes sea anemones to fresh feeding sites with plenty of food. In exchange, hermit crabs are protected from predators by sea anemone. Since, sea anemone attached to the shell, crabs become undetectable by their predators. This is because predators avoid attacking the crab due to the presence of cnidoblasts (cells that are often toxic, tubule to capture prey) in sea anemones.

Facultative mutualism can be described in three ways based on the benefits as,

• Resource-resource mutualism: both species provide a resource to the other species. e.g., association between coral polyps and zooxanthellae. 
• Service-resource mutualism: one species provides service, while the other provides a resource. E.g., a relationship between honeybees and flower. 
• Service-service mutualism: both species provide service to the other species. E.g., an interaction between an anemone and a clownfish.

Since, species growth and survival are not interdependent, facultative mutualism is nonobligatory in nature.

Applications of Mutualism

1. Transfer of nutrients. Mutualism plays a significant role in obtaining nutrients from plants and animals during the interaction. As can be seen in,

• Nitrogen-fixing bacteria derive its nutrients from the legumes.
• Coral polyps uses byproducts of zooxanthellae for cellular respiration
• Fungi and algae provides essential nutrients to each other.
• In mycorrhizae, fungi provide nutrients to the host plant.

2. Defense. Mutualistic relationships often provide protection against the parasites and predators of the interacting species.

• A wasp-like ants provide protection too acacia trees from the herbivores.
• Cleaner fishes feed on ecto-parasites of their host fish.

3. Ecosystem services. Mutualism also provide regulating ecosystem services to the interacting species.

• Honey bees and flowers during pollination
• Dispersal of seed by animals

4. Impacts population dynamics. Some symbiotic mutualistic relationships, especially where obligatory mutualism came to its role, it can influence the population density of interacting species, since they are interdependent for their existence. And, as removing one species may change the dynamics of the other species or both.

Commensalism

Commensalism term was coined by Pierre-Joseph van Beneden in 1876, which means “sharing a table”. Commensalism is a positive interaction, but it is not the same as mutualism. It can be defined as an ecological interaction between two or more species in which one of the species benefits while the other remains unaffected. i.e., neither suffers nor benefits. It often exists between a larger host (an organism that provide the benefit but are unaffected) and a smaller commensal (an organism which is benefited from the interaction). Such interactions may exist between the individuals of the same species or different species.

Fig. 12.5. commensalism in Epiphytes

For instance, you might have seen epiphyte plants that grow upon another plant or object for physical support. Since, they do not possess any attachment to the ground, they obtain their food from the host plants. It is often that small epiphytes grow on the surfaces of large trees and attach themselves on the branches of trees. From there, they derive nutrition that flow down the branches without harming the host tree. A similar kind of association is common in cattle egret and cattle.Commensalism is a long-term interaction where some species remain in the relationship for their entire life. And, depending on the relationship, the commensal may exhibit various morphological and functional adaptations. Since these adaptions accumulate over time, commensalism also plays a crucial role in the evolution, like other ecological interactions.

Negative Interactions

Symbiotic associations are not always beneficial to the interacting species; in certain circumstances they can be harmful or detrimental too. Negative interactions are the ones that have a direct or indirect negative impact on the interacting species. Amensalism, parasitism, predation and competition are examples of antagonistic interactions. Let us learn about the various kinds of such antagonistic ecological interactions that exist in ecosystems.

Amensalism

Amensalism is a symbiotic relationship between individuals of different species in which one species gets harmed or destroyed while the other either benefits or remains unaffected. 

Generally, it is considered to be exist between the individuals of two different species, however it can be seen within the same species as well. Amensalism is often confused with the term antagonism and used as its synonym which is ecologically incorrect. This is due to the fact that in antagonistic interaction, one species actually benefits at the expense of the other. While, during amensalism one of the species either harmed, and the other species either benefits or remains unaffected. Since, sometimes, neither of the species profits from the interaction, amensalism also referred as asymmetrical competitive interaction. It is one of the processes of evolution, which involves defensive strategies including as chemical or physical barriers to protect themselves or to derive nutrients from the environment. One such phenomenon occurring in the ecosystem is allelopathy. It is a process of inhibiting the growth of the other interacting species by releasing toxic chemical compound into the environment.

Allelopathy

Allelopathy term was coined by Hans Molisch in 1937, deriving from two Greek words, “allelon” means of each other and “pathos” means to harm. It can be defined as the biological phenomenon in which species interacts to influence the growth, survival, and reproduction of the other interacting species by secreting certain chemical compounds”.

The synthesis or the production of such chemicals do not occurs as in response to the interaction, but as part of the species usual functioning. It is a form of chemical competition in which one species inhibits the growth of another in order to take advantage of all available resources. Since, it limits the competition for resources among the species, it is considered to be most critical factor in determining abundance and distribution in the plant communities.

For instance, as shown in the fig. 12.6.a black walnut produces a non-toxic, colorless chemical substance known as hydro juglone. It is secreted by leaves, stems, fruit hulls, inner barks and roosts of black walnut tree.

Fig.12.6. Black walnut (Juglone nigra)

The chemical hydro-juglone on exposing to the environment gets oxidized into a highly toxic allelochemicals, called juglone. Juglone inhibits the growth of nearby vegetation including grasses, shrubs and trees.

Applications of Amensalism

1. Sustainable weed management. Extracts of allelopathy often utilize in organic weed management.
2. Straw mulching. Agrochemicals from decomposed straws are used to suppress the growth of unwanted plants including weeds and invasive species. It also helps in reducing pest and diseases.
3. Crop allelopathy. Intercropping of field crops to control weeds in the fields.
4. In medicine. Penicillium spp. secretion, penicillin is extensively used as antibacterial drugs to kill wide range of bacteria.

Contramensalism

One of the approaches to classify interaction is, on the basis of the net effect of one species on the other interacting species. With the combinations of positive (+), negative (-), and neutral (0) effects, this classification provides six hypothetical outcomes as illustrated in the table 12.2.

However, there is issue with this classification, (+, -), because predators and herbivores do not have a significant negative effect on the dynamics of prey population. Furthermore, the trophic mechanisms do not describe accurately all possible ways in which their results can be achieved. Therefore, in 1986, Arthur on the basis of their non-trophic mechanism to represent predation, parasitism and herbivory under contramensalism.

Table 12.2. Interactions on the basis of interspecific effects between two species

Continue reading to learn more about contramensalism through various examples from existing ecosystems.

Herbivory

In nature, plants and animals have evolved together. During the process of co-evolution, they had built a close relationship between them. Plant-pollinator interactions have been beneficial to both plants and dispersers. However, this is not the case in plant-herbivore relationships, as it is antagonistic in nature. That means herbivores benefits at the expense of plant species.

As you know, herbivore are the animals which feed on plant species. Hence, it can be defined as an interaction between plants and herbivores species in which herbivorous animals consume plants to derive nutrition and energy. Since, plant species either loses part of itself or dies during the interaction, it is considered to be an antagonistic type of relationship.

Fig. 12.7. an antagonistic relationship: herbivory

Previously, the herbivory was thought to be unequal, favoring herbivores and against the plants. However, in recent studies reveals that herbivory is beneficial to plants as well. For instance, a gypsy moths grazing on the canopy of trees. This allows penetration of more sunlight to reach ground, which benefit plants with more diversified and productive ground layer.

Besides developing a strong relationship with herbivore, plants as well as herbivores have also adapted defense to balance their relationship. In plants, development of thorns, production of allelochemicals, etc. While in herbivores, they have evolved to recognize plants with high nutritional values and fewer defensive chemical compounds.

Parasitism. The term parasite is a Greek word “parasitos” means one that eats at the table of another. It can be defined as the relationship between species in which one species benefit from the interaction at the expense of the other, often without killing the host species. An individual that derives the benefit from the interaction is termed as a parasite, while an individual that is harmed during the interactions is termed as the host. Generally, parasites are smaller in size than their host with high fecundity to ensure their survival. But, unlike predators, parasites do not kill their host and usually live in (tapeworm and hookworm) or on (ticks and mites) them for a period of time. Parasites live in association with the host to use it as habitat and a source of nourishment. Since, some of these interactions are highly specialized, a parasite of one species may not be parasitic on another. E.g. humans are primary host for trypanosomes.

Despite the fact that most parasites feed on their host for nutrition, some parasites may use some species as their secondary hosts for the transmission of one primary host to another. 

Parasites, often do not impact their primary host as much as they do in secondary hosts. E.g. Humans are secondary host of tsetse-flies.

Fig. 12.8. Brood-parasitism in common cuckoo

Brood Parasitism. Brood parasitism is a kind of social parasitism in which, parasites rely on hosts to raise their offspring and conserve their energy for locating food and producing more offspring. It can often be seen in certain species of birds, insects and fishes where parasitic species manipulate the host to provide parental care to their young ones on their own. The true parents place their young ones under the foster care of host species.

They do so by replacing host eggs and killing them. In order to reduce the chances of rejections, females of parasitic species usually select a species as the potential host whose eggs resembles their eggs. For instance, brood parasitism existing in cuckoo and a great reed warbler.

Predation. Predation is an antagonistic relationship as predators gain energy, while the prey suffers. It can be defined as an act of capturing, killing and eating of one organism by another for obtaining nourishment. Predators are the organism that kill and feed on the other organism, while the organism that gets killed is called the prey. Predators are highly specialized organisms with keen senses that allow them to capture their prey. As, these animals actively search for their prey, once identified, they attack it. The most common type of predation can be seen between two species from different trophic levels in the food chain. A competition between predators may develop in a complex ecosystem where multiple predators feed on the same prey. However, there are some exceptions. Predation is not as same as scavenging for dead organisms. Predators do often scavenge, but as a part of their feeding strategies. Similarly, it cannot be categorized under symbiosis as it results from the short-term interaction.

Being at the top of a food chains, they play a vital role in ecosystems. It helps in maintaining ecosystem at equilibrium by regulating the prey population, while promoting biodiversity.

Since, the population of prey-predator are not constant over time. It changes with time in a cyclic manner, and thus influences the dynamics of population of each other. Prey population is directly proportional to the predator population. That means as the population density of prey increases, food availability for the predators also increases. In such environment, the population of predators expands in an exponential manner. With the decrease in prey population, predators’ numbers also start reducing due to shortage of the food. Again, with the increase in the availability of food, prey population flourishes in the absence of predators. Hence, a new prey-predator cycle starts like in case of lynx snowshoe interaction.

In order to provide defense against the predators, the prey unlike predators has develop various anti-predatory adaptations such as, mechanical and chemical defense, camouflage, and mimicry. Let us examine the various types of defense mechanisms evolved by prey and predators in detail.

Mechanical Defense. It is a physical defense against predators to avoid being attacked/eaten. It includes development of the thorns on plant species like Acacia spp. trees, the hard shell on turtles

Chemical Defense. It is the process of releasing or injecting toxic chemical compounds to protect themselves from predation. E.g., allelochemicals juglone secreted by black walnut to reduce competition by inhibiting the growth of nearby plant species. 

Camouflage. It is the phenomenon in which individuals of prey change their color or pattern of the body to their surrounding environment to avoid being detected by their predators. For instance, chameleon. There are five various kinds of camouflage exist in nature. These are:

Cryptic Coloration. This is the most basic way of animal camouflage. In cryptic coloration, animals change their body shape, color, and patterns according to their surroundings to avoid being detected by their predators. E.g., a flat fish living in the ocean blend so well into a variety of surroundings (imitates the colors and textures of the seafloor), that makes difficult to spot prey and thus, allows it to flee or hide from their predators.

 

Descriptive Coloration. In this, animals disguise themselves by using more than one color or patterns. They often conceal themselves with the spots, stripes and shapes on their bodies in order to keep the outline of their body shape consistent. Leopards, as predators use this type of camouflage technique. They hunt using their coloration and marks as a hunting tool.

Self- Coloration. Sometimes, animals make use of the opportunities available in their environment. For instance, some turtle species that live in shallow water hide by growing algae on their hard shells to protect themselves from being attacked by their enemies.

Aposematic Coloration This is often known as warning coloration. Aside from disguising, some animals usually do the exact opposite. They change their skin color tone to a brought and vibrant that is easily visible, along with a foul odor or taste, a toxic chemical, and the ability to sting or bite, and so on. Such coloration allows animals to warn predators and reduce the chance of being eaten. If predators eat them despite the coloration, they will have a terrible taste or be exposed to a toxic chemical and will learn to avoid such organisms in future. E.g., insectivorous birds can remember the unpleasant experience and likely to avoid eating them in future.

Mimicry. Not all organism in nature do have the defense mechanisms. Some species confuse their predators by imitating its surroundings or something else. Such animals may imitate leaves, twigs, and other objects that predators would avoid. There are two types of organisms that mimic others. These are:

Batesian mimicry. A harmless species imitates the warning coloration of a poisonous one in this type of mimicry. E.g., Monarch butterflies consume toxic chemicals from milkweed plants and store them in their bodies. When an insectivores bird tries to eat them, it gets sick. Thereafter, the bird not only avoid eating monarchs, but also similar-looking butterflies.

Mullerian mimicry. In such mimicry, two or more harmful or unpalatable aposematically colored organism that resembles each other. It often found in mimetic species that are unpleasant or obnoxious. It is important that all of the species be identical in appearance and color. E.g., wasps and bees. 

Competition 

Competition is an ecological relationship between two or more organisms of same or different species, where one species competes with each other for different resources. Majority of competition arises due to increased demand of food supply among the individuals. However, individual may nevertheless, be in competition for other resources such as space, light, mates and so on.

In nature, when two individuals of the same or different species compete for the same resources, there is often a winner (+) and a loser (-). The weaker of the two competing species must either adapt or die out, whereas the stronger species acquires the resources.

However, if the competitors struggle to the death and kill each other, the interaction becomes antagonistic in nature and affects both individuals adversely (-, -). Even though it is commonly believed that competition favors the stronger species, since weaker species tend to become extinct due to unavailability of resources. However, there are instances, when even the number of the stronger species has declined. There are several factors influencing the intensity of competition between the individuals.

Some of these are:

• Population density. When population expands rapidly, it increases the demand of natural resources which are limited in the environment. As the available resources decline or deplete, competition among the individuals will occur.
• Resource sharing or partitioning. the degree of competition can also be determined by how individuals share resources. When two different species compete for the same resources, the competition becomes more intense.

Classification of Competition 

Based on the different classifications, competition can be grouped into three major categories. These are:

On the basis of taxonomical relationship

Intraspecific. This competition involves individuals of similar size competing for the same resources. It can be defined as, the form of competition in which individuals of the same species competes for the same limited resources, known as intraspecific competition. It becomes intense in rapidly growing population, where resources availability decline as the number of individuals increases due to increase in the demand of resources. Intraspecific competition regulates the growth of population and also act as selective force during the evolution, and often, results in both individuals reducing fitness. For instance, some plant species produces various kinds of chemical compounds that inhibits other plants of the same species from growing around them.

Interspecific. It involves individuals of different sizes with significant differences in their abilities. That means that the competition occurs between the individuals of two different species for the same resources. Thus, can be defined as the form of competition in which two individuals of two or more different species competes for same limited resources, called as interspecific competition. It can also be intense with the increase in the size of population, like intraspecific competition. Since, two different species usually differ in fitness which influences the intensity of completion, interspecific competitions are more intense than intraspecific ones. It plays a vital role in regulating ecological communities and also serves as an agent of natural selection during evolution. For example, coral covers large surfaces of the oceans to obtain enough sunlight for autotrophic bacteria, along with other aquatic organism for the availability of nutrients and sunlight.

On the basis of mechanism

Interference A form of competition in which competing individuals have a direct control over the process of hunting, growth, or reproduction of a species in a particular environment. It may include attacking, grabbing and killing while competing for resources. It often occurs when two species existing in the same habitat come in contact with others to defend their territory or utilize the available resources. 

For example, in case of allelopathy, one plant releases a toxic allelochemicals to poison the soil for others, so that it would prevent other plants from deriving nutrients from that soil. 

Exploitative Sometimes, one species exploits resources such as food, space, etc., in common with another species without coming in its direct contact. Exploitative competition is a kind of indirect competition in which two or more species are interacted through a limiting resources such as space, water, sunlight, nutrients or other similar resources that acts as a mediator. As, the individual’s consumption and use of limited resources makes it challenging for others to access them.

Apparent Like exploitative competition, apparent competition is one of the form of indirect competition, in which both the individuals of competing species are preyed upon by the same predator. For instance, imagine Species A and B are preyed upon by Species C, the predator, any change in the population of species A may affect the population of species B. Thus, suggesting that as predator feeds on species A and become more viable and fit at the expense of the population of species B.

On the basis of influence 

Direct It occurs in the ecosystem directly influence each other by affecting the availability of resources in the ecosystem. Generally, it can be seen where the two species share the same niche or community. E.g., deer inhabiting and competing for same limited resources, i.e., grasses and leaves in the same ecological niche.

Indirect It influences the resource availability indirectly. It often occurs where the two species occupy different niche or communities. E.g. presence of deer near to the pond may affect other deer nearby, by decreasing the amount of water in the pond.

Direct and indirect competition can be observed in both interspecific and intraspecific competitions. Such forms of interactions enable us to study the behavior of the species along with their life styles.

Like other ecological interactions, competition is conserved to be one of the driving forces in evolution. It often results in change in the fitness of interacting species. Along with the change in the community structure, it also exerts evolutionary pressures on the development of adaptations at a community level.

Neutral Interactions

In ecology, an ecosystem, a relationship exists between two species that interact but do not affect one another, called as Neutralism. It is a form of an interaction in which one species’ fitness does not impacts/ influence the fitness of the other. That means having no or very insignificant effect on both populations. It differs from predation and mutualism, since one or more species gain from their interactions. Similarly, neutralism is not same as commensalism, in which one of the species gains profit without affecting the other interacting species. For instance, cacti, a plant species and tarantulas, species of spider live together in the desert. Another such example can be some species of bacteria, lactobacillus and streptococcus can coexist without affecting each other.

As we know, all life is interconnected to some extent, i.e. a complex networks of interactions exist in an ecosystem, it is impossible to affirm that there is no competition or benefit to either species. Since, true neutralism is uncommon or non-existent, it often used to circumstances where interactions are either insignificant of negligible. Hence, it is almost impossible to demonstrate true neutralism.

Ecological Niche

The term habitat is derived from a Latin word “habitare”, meaning to inhabit. We can define a habitat as a natural environment where a particular organisms live and utilize the resources of that place for its survival, such as food, shelter, defense and mates. It is determined by its physical (soil, sunlight, temperature and climatic conditions) and biological (food and predators) characteristics, and comprises of four components including space, food, water and shelter. It may change over a period of time due to environmental changes (like volcano, climate change and so on) and anthropogenic activities (like deforestation, urbanization, etc.). 

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, there are certain essential sets of environmental conditions required for its existence. It includes the sum total of all resources and physical environmental conditions required by an individual 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.

Niche 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 in it. Each component exhibits the tolerance ranges of certain essential environmental conditions that are critical for the survival of the species.

Fig. 12.9. Hutchinson’s niche

These components include: 

• a habitat. a physical space that a species occupies within its habitat, where all essential conditions are present for an organism to grow, survive and reproduce infinitely.
• a trophic position. a fundamental position of an organism in its community or the food chain.
• a multidimensional hypervolume. It comprises of 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 a long period of time that can be filled by another species. Some species, on other hand, create a special niche for themselves. Having a specialized unique niche is always advantageous for the species. It always minimizes the chances of two more species competing for the same limiting resources. 

The number and types of factors that affects an ecological niche varies with species. And, the relative significance of these factors may also vary depending upon the geographical and biotic conditions. Since it is important for a species to be able to adapt to the changing ecosystem to avoid extinction, they evolve to expand in a wide range of environmental conditions. 

Classification of Niche 

On the basis of occupancy of habitat, niches can be classified as: 

Fig. 12.10. Types of Niches

Fundamental and Realized Niche 

Fundamental niche can be defined as a hypothetical set of physical (abiotic) and biological (biotic) conditions which is occupied by a species or an organismas shown in the fig. 12.11.

It is actual entire space that is occupied by an organism when they are not in competition for the same resources. In other words, fundamental niche is the total range of the environmental variations that are suitable for the survival of an organism, 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 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, biogeography. Since, they cannot utilize their entire niche due to 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 fundamental niche occupied by species is known as their realized niche.Since the resources are unevenly distributed across the habitat, presence of competitors and predators may vary. Hence, realized niche varies with the populations of same species. Subsequently, each species possesses both fundamental and realized niche. And since realized niche are part of a fundamental niche, they are smaller than fundamental niche. 

Alpha and Beta Niche

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

• Alpha Niche. These are resources specialized niches occurs in the habitat, in which several species co-exist. Presence of two different alpha niches means the two species are using different resources. In other words, they are using similar resources in such a way that overlap in their used is reduced. 
• Beta Niche. Due to variations in the environmental tolerances, some species prefer to inhabit in separate habitats. Such species rarely get together to interact physically. This is due to the fact that these species are exposed to a different set of environmental variations including physical, chemical and biological factors.

Niche Width

Niche breadth as illustrated in the fig 12.12. is also known as niche width. It is an environment that encompasses all of the appropriate sets of conditions including physical, chemical, and biological. To be more specific, it is the habitable range between the maximum and minimum values of each parameter that constitute of tolerance limits for an individual life.

Fig. 12.11. Niche Breadth and Niche Overlap

It includes suitable physical (temperature, precipitation, pressure, humidity), chemical (salinity, pH), and biological (Inter and intraspecific interactions) for the existence of an individual. 

Niche Overlap

When two or more species compete for the same niche space, it leads to the overlapping of niches. Generally, it occurs when the resources used by the several species are identical or similar in nature. 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.

Fig. 12.12. Types of Niche Overlapping

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 as depicted in the fig. 12.13. These are: 

• i. Partial Overlapping. When the curves of two competing species partially cover each other. It occurs when the intensity of competition is low, the stronger or the dominant species acquires the niche from other species, thus reducing the niche width of weaker species. 
• ii. 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 niche where the degree of competition is more as compared to the fundamental niche. 

Competition and the Niche 

As you have learnt, the resources are finite in the constant environment. When population expands at an exponential rate, demand for the limited resources also increases. In an unavailability or shortage of resources required by species for their survival, species competes with other species. Furthermore, situation gets worsen when the resources required by two different species are similar or identical in nature. Consequently, several impacts can be observed in an ecosystem. These impacts can broadly be categorized as, 

• Short-term Effects. It operates on the ecological time scale. It eliminates or removes one of the competitive species from the niche through Competition Exclusion Principle. However, there are several ecological processes exiting in nature to minimize the impact of competition exclusion or to avoid it. This can be done by competitive species through Resource Partitioning, Competitive Release.
• Long-term Effects. It operates on the evolutionary time scale. It splits or divide closely related species into multiple by changing their characteristics through character displacement.

Gauss Competitive Exclusion Principle 

Imagine populations of two species inhabiting in the same habitat with an identical niche. Initially, when the size of the populations of both species is very small, they may coexist in the same niche for some period of time. But, as soon as the population starts expanding, demand for the identical resources also increases and due to unavailability or insufficient of limited resources, competition prevails. 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 comes out of that competition. In 1934, a Russian ecologist conducted an experiment in the laboratory on two species of paramaecium competing for the same resources. In experiment, he grew the two species of paramaecium, Paramaecium aurelia and Paramecium caudatum separatey (different habitats or niche) in the lab, both species thrive. But, when grown in the same test tube (same habitat or niche) with the same amount of nutrients (food resources), both species grew poorly. And eventually, P.aurelia outcompetes P.caudatum for ood resources, resulting in the elimination or extinction of P.caudatum species, as shown in the fig. 12.4.

From this experiment, he concluded, “closely related species cannot coexist for long one single limiting resources; the competitively superior or stronger species survives, while the weaker ones geteliminated”.

Fig. 12.13. Example of Competitive Exclusion Principle

In nature, it is difficult to observe that two species occupy exactly the same niche. However, larger the overlap between the species niche, more intense the competition between them tends to be. In order to reduce the overlapping of niche, it becomes necessary to minimize the degree of competition between the species. This can be done through resource partitioning, character displacement and competitive release. Let us discuss each of these in detail. 

Resource Partitioning

Although individuals compete for the limited resources, coexisting species must evolve themselves to become more specialized and narrow their focus in their use of those resources. This can be achieved by species through the utilization of different resources, occupying separate region of niche, or feeding during different times of the day to reduce the competition for the similar resources.

Figure 14. Resource Partitioning

This is known as resource partitioning, as illustrated in fig. 12.14. In other words, competing species for the similar resources can coexist by minimizing the competition intensity through the niche separation. Thus, resulting in the large non-overlapping and separation of the niche. Hence, we can define resource partitioning as a phenomenon of dividing or partitioning of the limited resources by coexisting species to avoid competition in an ecological niche. It is an evolutionary change in the species occurs when two species coexist despite competing for the same resources. 

For instance, an anole lizards found on the island of Puerto Rico. Natural Selection, an evolutionary force divided single species of anole lizard into 11 different species. Each species has its own preferred habitat, which is characterized by factors such as plant type and height; penetration of amount of sunlight and moisture. 

Character Displacement

When two similar species with an identical niche come in contact, they are likely to compete more aggressively. Different species under the influence of competition may develop certain traits to utilize available resource in their partition more efficiently. And individual’s adaptation to these traits in the species minimizes 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.

Fig. 12.15. Niche Shifting

Competitive 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, an expansion of the ecological niche in the absence of a competitors. It often occurs in a habitat where one of the two species competing for the similar resources, usually the stronger one, is removed from the habitat. This allow the remaining competitor species to utilize the resources more efficiently than they did when the first one was present. In the process, there occur a significant increase in the population of a less widely distributed species. 

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, acts as a mirror of character displacement.

Fig. 12.16. Competitive Release in Barnacles

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 balanus spp. was experimentally removed, Chthamalus colonized the intertidal zone and outgrew balanus, thus effectively preventing balanus from establishing itself again in intertidal zone. The removal of 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. 

Summary

• In this unit you have studied various aspects of ecological interactions. So far you have learnt that: 
• In ecology, symbiosis is a close ecological association occurring between the individuals of two or more different species for a long period of time. 
• A mutualism is a win-win situation in which both the individuals of two different species benefit from each other. Obligate mutualism shows very high degree of interdependency as neither of two populations, can survive or grow in the absence of the other organism. 
• Allelopathy is a form of chemical competition to inhibit the growth of other species in order to utilize all available resources by itself. 
• Brood parasitism is a strategy developed by one species to trick another species into raising their offspring. 
• Predator-predator populations are intrinsically linked and undergo cyclic changes throughout time.All organisms have their own defense mechanism against their enemies or predators to survive themselves 
• Competition is a relationship between organisms in which one species inhibits another from gaining access to resources, affecting fitness beyond the cost of resources. 
• Total range of environmental conditions, suitable for the existence of a species without being influenced by competition, predation and parasitism,called as fundamental niche. 
• Niche overlapping is an index of similarity between or among the resources utilization of different species. The intensity of competition between co-existing species is proportional to the degree of overlapping. 
• Two species with distinct ecologies cannot coexist in the same environment. 
• Competitive exclusion may be avoided if one or both species evolves to use a different resource, called as resource partitioning, it helps the species coexist more effectively.