Biological Interactions: Positive and Negative Interactions in an Ecosystem (.PPT)

Interactions in an Ecosystem

Interactions in an Ecosystem: Organisms living together in a community influence each other directly or indirectly under natural conditions. All the vital process of living such as growth, nutrition and reproduction requires such interactions between individuals in the same species (intraspecific) or between species (interspecific) These inter or intra relationships of individuals in a population or community of an ecosystem is called biological interactions or population interactions.

The interaction between organisms may not be always beneficial to all the interacting counter parts. Based on whether, the interaction is beneficial to both interacting species or harmful to at least one interaction species, the ecological of biological interactions are classified into two categories.

(I).   Positive interactions

(II). Negative interactions

(I). Positive interactions:

In positive interactions, the interacting populations help one another. The positive interaction may be in one way or reciprocal. The benefit may be in respect of food, shelter, substratum or transportation. The positive association may be continuous, transitory, obligate or facultative. The two interacting partners may be in close contact in such a way that the tissues intermixed with each other; or they may live within a specific area of the other; or attached to its surface.  Different types of positive population interactions are:

(1). Mutualism

(2). Commensalism

(3). Proto-cooperation

(1). Mutualism:

Mutualism, also called as symbiosis, is also a positive type of ecological interaction. Mutualism is a symbiotic association between two organisms in which both the interacting partners are mutually benefitted. Mutualism is different from proto-cooperation in the sense that mutualism is obligatory and none of the partners of mutualism can survive individually. In mutualism, the organisms enter into some sort of physical and physiological exchange.

Examples of mutualism:

(a). Lichens: lichens are the symbiotic association between algae and fungi. The body of lichen composed of fungal matrix in which the algal cells are embedded. The fungi provide protection to algal components and also provide moisture and nutrients to them. The algal components in turn will supply carbohydrates for fungus.

Learn more: General Characters of Lichens

(b). Symbiotic nitrogen fixation: mutualistic interaction can be seen in the symbiotic nitrogen fixation of Rhizobium associated with root nodules of leguminous plants is the best example. Similarly other microorganisms associated with plants such as Alnus, Casuarina, Cycas for nitrogen fixation are also belongs to mutualism.

Learn more: Cycas: Morphology, Anatomy & Reproduction

(c). Mycorrhizae: they are the symbiotic association between fungi and the roots of some trees. Fungal components help in the absorption of water and minerals by the plant. The plant in turn supplies foot to fungal components.

(d). Pollination by animals: Bees, moths, butterflies etc. derive food from the nectar of plants and in return bring out pollination

(e). Seed dispersal by animals: Fruits are eaten by birds, and other animals and the seeds contained in them are dropped in the excrement at various places.

(f). Zoochlorellae and Zooxanthellae: Zoochlorellae and Zooxanthellae are unicellular microscopic algae that symbiotically live in the outer tissue of some sponges, coelenterates and mollusks. Algae are autotrophs and they can prepare food by photosynthesis. Algae obtain materials released by metabolism of host animals for their photosynthesis. Chlorella vulgaris is a unicellular green alga which lives in the gastro-dermal cells of Hydra. Algae through photosynthesis provide food and oxygen to Hydra, which in turn provide shelter, nitrogen wastes and CO₂ to Chlorella.

(g). Association between termites and Trichonympha: Termites feeds on wood, however they cannot digest the cellulose in the wood. Trichonympha is a protozon which lives in the gut of termites. Trichonympha can produce digestive enzymes and they digest cellulose of wood. Trichonympha in turn obtain food and shelter from termite.

(2). Commensalism:

Commensalism is a positive type of ecological interaction between two species in an ecosystem.  In commensalism, the association occurs between members of two different species where one species is benefited the other is neither benefited nor harmed. Here the two populations live together without entering into any kind of physical exchange, and one is benefited without any effect on the other.

Examples of commensalism:

(a). Climbers and lianas such as Bauhinia, Tinospora etc., which are rooted in the soil but climb over large trees. These climbers use other trees as support to get enough sunlight, more than that, the supporting plants do not have any positive or negative effect.

(b). Epiphytes: They are the plants which growing on the surface of other large plants. They use other plants only as a support and not for water or food supply. They are different from lianas in that they are not rooted in the soil. Example: Orchids, Mosses, Nephrolepis, Usnea, green algae growing on the surface of snails, microbes such as bacteria and protozoans live within the body cavity of other animals.

(3). Proto-cooperation:

Proto-cooperation is a positive type of population interaction and it is also called as non-obligatory mutualism. Proto-cooperation is a less extreme type of population interaction. In proto-cooperation, two species interact favourably with each other, though both of them are able to survive separately. It is a temporary association where both the interacting partners get benefited. It is different from mutualism in the sense that, the association is not essential for the survival of any of the species.

Example for proto-cooperation: Association between hermit crab (Eupagurus prideauxi) and sea anemone. The sea anemone is carried by the carb to fresh feeding sites and the crab is in turn protected from enemies by sea anemone.

(II). Negative interactions:

In negative interactions, one of the interacting populations is benefited and the other is harmed. In negative interaction one population may eat members of the other population, compete for foods or excrete harmful wasters. Different types of negative population interactions are:          

(1). Ammensalism

(2). Parasitism

(3). Predation

(4). Cannibalism

(5). Competition

(1). Ammensalism:

Ammensalism is a negative type of population interaction. In ammensalism one species is harmed or inhibited other is neither benefitted nor harmed. Some authors prefer to use the term antibiosis for commensalism. Antibiosis is the partial or complete inhibition or death of one organism by another through the production of some substances or environmental conditions as a result of its metabolic pathway.  In antibiosis none of them derives any benefit. The process of antibiosis is common in microbial populations and the chemical substances produced by microbes for antibiosis are generally called as antibiotics.

Examples of ammensalism

(a). Chlorella vulgaris produces a toxin (chlorellin, an antibiotic) which is harmful to other algae.

(b). Larger and more powerful organism excludes another organism from its source of shelter or food is also a type of ammensalism

(c). Algal blooms such as red tide or green blooms are also example of ammensalism.

(2). Parasitism:

Parasitism is a negative type of population interaction. Parasitism belongs to the ‘exploitation’ category of negative population interactions. In exploitation, one species harms the other by making its direct or indirect use for shelter or food. A parasite is the organism living on or in the body of another organisms and deriving food form its tissues. The harmed one is called host, the benefitted one is called parasite. A parasite usually takes a host which is usually larger than its body size. Usually a specialized parasite does not kill the host at least until it has completed its reproductive cycle. Those organisms which derive their nourishment only partly and remain in contact with their host only for a short period of their life cycle are not true parasites (examples: mosquitos). Some parasites requires more than one host to complete its life cycle and such parasites are called heteraceous parasites (example Puccinia, Malarial parasite).

Examples of parasitism:

(a). Cuscuta is a total stem parasite which lives on the surface of other large plants. They are devoid of chloroplasts and hence they cannot prepare their own food. Thy have specialized absorptive structures called haustoria. In the case of complete parasite, the haustoria will be inserted into the phloem tissue of host plants and they absorb the prepared food materials from the host phloem.

(b). Rafflesia, Orabanche and Conopholis are complete root parasites

(c). Loranthus and Viscum (Loranthaceae) are partial stem parasites. They bear leaves with chlorophylls and hence they can prepare their own food. The haustoria of partial parasite are attached to the xylem of host plants. Form the xylem of host plants, partial parasite absorbs water and minerals and they prepare their own food by photosynthesis.

(d). Santalum album and Thesium are partial root parasites. Their roots are attached to the host plants.

(e). Microorganisms such as bacteria, virus, fungi, mycoplasma, protozoans etc. which cause many diseases in human and other animals and plants are parasites.

(f). Hyperparasites: Parasitic microbes growing in or on other parasites are called hyper parasites.

(3). Predation:

Predation is a negative type of population interaction and it belongs to the ‘exploitation’ category of negative population interactions. In predation, one species kill and feeds on another species. The killer species is called predator and the one who dead are called prey. The predators are usually larger and power-full than prey. Predation is very important in community dynamics and it helps to maintain the constancy of number of different trophic levels in the ecosystem and thereby maintain the stability of ecosystem.

Examples of predation

(a). Lion, tiger and Beer are predators of forest ecosystem. They predate herbivores

(4). Cannibalism

Cannibalism is a negative type of interaction of individuals in the same population. In cannibalism, bigger individual of a species kill and feeds on smaller individual of same species. Cannibalism is a natural method of population control in the ecosystem.

(5). Competition

Competition is the association of two or more species; each species is adversely affected by the presence of other species in respect of food, shelter, space, light etc. Competition occurs when individuals attempt to obtain a resource that is inadequate to support all the individuals seeking it or even if the resources are adequate individuals harm one another in trying to obtain it. The resources in the environment for which the individuals compete include raw materials for life such as water, light and nutrients, space for occupying and selection of mates for sexual reproduction. The competition in the ecosystem may be of two types:

a.      Intraspecific competition

b.      Interspecific competition

(a). Intra-specific competition: It is the competition occurring between the individuals of the same population (competition within population). It is also called as scramble competition. Intra-specific competition is an important density dependent factor regulating population size. Intra-specific competition is also responsible for the even distribution of individuals of the species in an ecosystem.

(b). Inter-specific competition: It is the competition occurring between populations of different species whose requirements are common and inadequate in the ecosystem (competition between population). It is also called as contest or interference competition.

Key concepts:

What are biological / ecological / population interactions?
What are the different types of population interaction in the ecosystem?
What is meant by mutualism (symbiosis)?
What is meant by commensalism?
What is proto-cooperation?
What is ammensalism?
What is parasitism?
What is predation?
What is cannibalism?
What is meant by competition in an ecosystem?
Differentiate intraspecific and interspecific competition

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