Community Ecology

Community ecology is the study of the interactions between different species within an ecosystem and how these interactions shape the distribution and abundance of species. It is a subfield of ecology that focuses on the relationships between species and the ways in which they interact with each other and their environment.

Community ecologists may study a wide range of topics, including the ways in which species interact with each other through processes such as competition, predation, and mutualism, and the impacts of these interactions on the distribution and abundance of species. They may also study the ways in which species respond to and are affected by environmental factors such as climate, soil, and disturbance.

To study community dynamics, community ecologists may use a variety of methods, including field observations, experiments, and data analysis. They may also use mathematical models and computer simulations to understand and predict the impacts of different factors on community dynamics.

Community ecology is an important field for understanding the functioning of ecosystems and for informing conservation efforts to protect and preserve species and their habitats. It is also important for understanding the impacts of human activities on ecosystems and the ways in which these impacts can be managed or mitigated.

All the living components of an ecosystem form a single community or bio-community co-existing together. A community is mainly attributed to characteristics like the co-occurrence of species, reoccurrence of groups of the same species, and homeostasis or self-regulation. Krebs (1994) defined community as ‘an aggregation of the populations of living organisms in a specified habitat .’According to Clarke (1954), the definition of a community is a group of mutually adjusted plants and animals living together in a natural area. A community always comprises plants and animals, as both are very much essential for the survival and functioning of the community.

In a community, there are various interactions between the populations of two species, resulting in the combinations like neutral (0, 0), positive (+, +), and negative (-,-). The interactions are ecologically essential and are of nine types. Among them, neutralism, direct interference type of competition, resource use type of competition, amensalism, parasitism and predation are Negative Interactions whereas, commensalism, proto-cooperation and mutualism are Positive Interactions. 

1. Neutralism: Neither of the population is affected by the interaction with each other. Example: The two closely related species of aquatic birds, the shag (Phalacrocorax aristotelis) and the cormorant (Phalacrocorax carbo), found in England living together during the breeding season, feeding in different kinds of fishes with no competition. Similarly, the tide pool communities show neutralism type of interaction; even if they exist together, many neither benefit nor harm one another. Its symbol is (0, 0).

1. Interference competition (direct interference type): In this type of interaction, both populations hinder each other. An interspecific match occurs when two species come into direct contact with each other such as fighting or defending a territory. Its symbol is (-,-).

1. Exploitation competition (resource use type): In this type of interaction, both populations badly affect one another during the struggle for resources during scarcity. Its symbol is (-,-). One species exploits a resource such as food, space, or prey along with another without direct contact. 

The competition between the populations can involve space, nutrients, light, waste materials, susceptibility to carnivores, disease, and many other types of mutual interactions. The results of the competition have been studied by many evolutionary biologists as one of the mechanisms of natural selection. The development of interspecific competition can be equilibrium adjustment between the two species or, if severe, replacement of one species population by another, or forcing the other to occupy another space or to use another food. 

Competition exclusion principle or Gause principle

Here is a clear case of exploitation competition. The inhabitation of closely related organisms having similar habits or morphologies in the same place merely occurs. The Russian biologist Gause (1932) explained the ecological separation of closely related species as the ‘Gause principle’ or ‘complete exclusion principle .’He first observed such separation of the closely related species of ciliate protozoans Paramecium caudatum and Paramecium aurelia in experimental cultures. When both species were grown in separate cultures, they exhibited typical sigmoid population growth. They maintained a constant population level in the culture medium that was maintained with a fixed density of food items (bacteria that did not multiply themselves in the medium and thus could be added at frequent intervals to keep the food density constant). When both protozoans were placed together in the same culture, Paramecium aurelia alone survived after 16 days. Neither of them attacked the other or secreted harmful substances. Still, populations of Paramecium aurelia had a high rapid growth (high intrinsic rate of increase). They thus replaced Paramecium caudatum due to the limited amount of food under the existing conditions. 

The experiment done by taking Paramecium caudatum and Paramecium bursaria gave the opposite result. Although both protozoans were competing for the same food, Paramecium bursaria occupied a different part of the culture and fed upon the bacteria without disturbing Paramecium caudatum. Thus, both ciliate protozoans could survive and reach a stable equilibrium in the same culture medium. In this case, the habitat proved to be sufficiently different for the two species to co-exist, although their food was identical. 

The most widely debated theoretical aspects of competition theory revolve around the Lotka-Volterra equations. This equation was proposed by Lotka (1925) and Volterra (1926). These equations are useful for modeling predator-prey, parasite-host, competition, or other two-species interactions. In terms of the competition within a limited space, the simultaneous growth equations of the populations can be written using the logical equation:

Where N1 and N2 are the numbers of species 1 and 2, respectively,

           K is the equilibrium level.

A is the competition coefficient indicating the inhibitory effect of species 2 on species 1.

b is the corresponding competition coefficient signifying the inhibition of species 1 on species 2. 

1. Amensalism: In this interaction, one population is harmed, whereas the other is not affected. Its symbol is (-, 0). Lawton and Hassell (1981) refer to this interaction as asymmetrical competition. Example: penicillin killing bacteria. 

1. Commensalism: In this type of relationship, among the two living organisms, one organism benefits from the other without causing any harm to it. Examples: The tree frog uses plants for protection. The cattle egrets eat up the insects residing upon the back of the cattle during their grazing time. The golden jackal following a tiger’s trail and feeding upon its prey remains when it is expelled from its pack. Its symbol is (+,-).

1. Parasitism: When parasites colonize a host, the host is said to have an infection. The parasite gets shelter and nutrition as benefits from the host, whereas the host gets a disease when the infection gives rise to symptoms that are clearly harmful to the host. Its symbol is (+, -). Example: fleas or ticks that live on dogs and cats. 

1. Predation: In this type of interaction, one population adversely affects the other by the direct attack but somehow depends on the other for survival. Its symbol is (+, -). It is of two types: true predation and herbivory.

• True Predation: In this type of predation, a predator kills and consumes a prey species. True predators are carnivores like tigers and birds of prey. Examples: tiger predating upon the deer, leopard killing livestock, etc. 
• Herbivory: In this type of predation, a herbivorous animal is a predator which feeds upon the plants (prey). Large herbivores like sheep, cattle, kangaroos, etc., are known as grazers. Other herbivores include insects, molluscs, fish, etc. 

1. Proto-cooperation: In this type of interaction, both populations benefit from their association but do not have an obligatory relationship. This means the benefits provided by both populations only increase the size or growth rate of the population but are not compulsorily important for their growth or survival. Thus, this interaction is also called facultative cooperation. Its symbol is (+, +). Example: red-billed oxpecker and black rhinoceros, crocodile bird (Pluvianus aegyptius) entering the open mouth of a crocodile and making it free from leeches.

1. Mutualism: In this type of interaction, both populations are benefitted from each other’s survival and growth and cannot survive without each other under natural conditions at the same time. Flowering plants and pollinators display both facultative and obligate mutualism. Obligate mutualism is shown by termites and endosymbiotic bacteria, parasitoid wasps, and Polyana viruses. Its symbol is (+, +).

Concept of Habitat, Ecological Niche, and Guild

Habitat: It is the place where an organism lives or where one would go to find it. The habitat can also be defined as the place occupied by an entire community. The habitat of an organism or a group of organisms (population) includes other organisms and the abiotic environment. 

Ecological Niche describes the specific role a population plays within an ecosystem. The environmental Niche can be defined by their interaction with another organism (E.g., predator or prey) or their role played in nutrient cycling (E.g., primary producer or decomposer). It not only includes the physical space occupied by an organism but also its functional role in the community and its position in environmental gradients of temperature, moisture, pH, soil, etc. 

Charles Elton (1927) first used the term ‘niche’ in the sense of the ‘functional status of an organism in its community .’The Niche in terms of microhabitat is called ‘spatial niche’ whereas, in terms of the importance of energy, relations are known as ‘trophic niche .’G.E. Hutchinson (1957) designated niche as ‘multidimensional or hypervolume niche’ which can be measured and mathematically manipulated. For example, a two-dimensional climograph, which depicts the x-and y-axes of a particular species of a bird and a fruit fly, could be expanded as a series of coordinates (x, y, and z axes) to include other environmental dimensions. 

Hutchinson (1965) differentiated Niches into two main types: fundamental Niches and realized niches.

1. Fundamental Niche is the potential set of conditions a species can occupy. It can be determined experimentally. It is the Niche before the competition, predation, etc.
2. Realized Niche: Due to the effects of competition, enemies, and biogeography in nature, the species only partially occupied the Niche. This is known as the realized Niche. It is the Niche after the competition, predation, etc.

The Niche can be further divided into two distinct aspects: alpha niche and beta niche.

1. Geographic Niche, conditions niche, or beta niche: In this type of Niche, some species live in different places due to variations in environmental tolerances. So, species with other beta niches never physically come together to interact.
2. Resource–specialization niche or alpha niche: In this type of Niche, several species can occur in the same place, which means that they have the same beta niche. Two species having different alpha niches imply that they are using different resources or are using similar resources in such a way that overlap in their use is minimized. 

Guilds: They are groups of species with comparable roles and niche dimensions within a community. For example, a group of fruit-eating birds in a rainforest, a guild of forest-floor dwelling herbs, wasps parasitizing herbivore population, nectar-feeding insects, snails living in the forest floor litter, and vines climbing into the canopy of a tropical forest.

The ecologically equivalent species are those species that occupy the same Niche in different geological regions (continents and major oceans). The species composition of the communities differs widely in different floral and faunal regions, but similar ecosystems develop equivalent functional niches wherever physical conditions are similar, regardless of the geographical location.

The ‘habitat’ can be regarded as the ‘address’ of the organism, ‘niche’ as its ‘profession’, and its trophic position in food webs as how it lives and interacts with the physical environment and with other organisms in the community. 

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