Interdependence between two species through which individuals of either one or both species benefit is called positive interdependence. The benefits to the individual of any species through this relationship may be continuous or transitory, obligate or facultative. Both partners in this type of relationship may be in close contact; one may be inside some specific area or attached to the surface of the other’s body. Positive interdependence is divided into three major types: mutualism, Commensalism, and proto-cooperation.
A. MUTUALISM
Mutually beneficial relations between species are called as mutualism. In this type of interdependence, the two individuals of different species enter into some sort of physiological exchange, and both species are benefited from the relationship. Some common examples of mutualism are- Pollination by animals, role of animals in the dispersal of fruits and seeds, specific association of lichens, symbiotic nitrogen fixers, Mycorrhizae, association of algae with different animals, etc. Let us discuss these relationships in detail.
(I). Pollination by insects OR Reproductive Interdependence: Insects like bees, moths, butterflies, etc. visit flowers for nectar and help in pollination. Thus, each of the two species has an interdependent relationship in which both benefit from this relationship.
(II). Dispersal of fruits and seeds: Many species of birds move from tree to tree and eat fruits. Seeds of these fruits germinate quickly after passing through the guts of these birds. Some of these birds carry away small fruits with them to the places of their roosting, where the seeds of these fruits are dropped on the ground along with the leftovers. Field – sparrows and finches eat away seeds of grasses and help in their dispersal. After passing through these birds’ intestine, those seeds germinate quickly, even in a little diverse condition.
(III). Lichens are plant associations formed by specific algae and fungi. The algal and fungal parts of lichens are called Phycobiont and Mycobiont, respectively. The Phycobiont absorbs water from the surrounding air, which the Phycobiont utilizes during the food preparation process. Both partners share the food prepared by the Phycobiont. Lichens help form soil by causing the weathering of rocks as they grow abundantly on them. We have already discussed that lichens serve as pollution indicators, as they cannot survive in a polluted atmosphere.
(IV). Symbiotic Nitrogen Fixers: These are specific bacteria called Rhizobium that trap atmospheric nitrogen and offer it to the plant in exchange for food and shelter. The roots of leguminous plants bear root nodules that host these bacteria. There are also many nitrogen-fixing algae.
An association of two species of organisms where both partners benefit from each other is called symbiosis. It is the mutually beneficial, co-evolved association of a species with other species. Symbiosis is one of the most powerful functions of ecosystems because new life forms have been reported to originate due to the interdependence of totally unrelated species of organisms. The associations of an alga and the fungus to form lichen, the association of Rhizobium bacteria that live in the root nodules of the leguminous plants, and the association of the water fern Azolla and the nitrogen-fixing Cyanobacterium Anabaena are essential examples of symbiotic relationships. In all these associations, the group’s organisms benefit equally from each other.
Azolla is an aquatic fern that offers shelter to Anabaena, a Cyanobacterium, in the cavities of its leaves. It fixes atmospheric nitrogen and excretes nitrogenous compounds into the leaf cavities of the fern. As reported by some farmers, the decomposition of Azolla plants in crop fields increases productivity by up to fifty per cent. An unusual leguminous plant named Sesbania rostrata bears nodules on the nodes of its stem. This plant is used as a biofertilizer by farmers.
(V). Mycorrhizae: Mycorrhiza (Plural- Mycorrhizae) is a symbiotic association of certain fungi with the roots of certain seed-bearing plants. Mycorrhizae structures are made by fungi either on the surface of the host or within the host. These structures are found either on roots or on the surface of pines, oaks, and hickories and inside these structures (Endotrophic Mycorrhizae). Some Mycorrhizae are found to be associated with ectophytic allies with the roots of many plants and serve the functions of root hairs. In this case, fungi occur internally in the root tissues. The Mycorrhizae enhance water and nutrient uptake, resulting in the proper growth and development of the plant.
(VI). Association of algae with different animals: The outer tissues of some sponges, Coelenterates, Mollusks and even worms are inhabited by specific algae called as Zoochlorellae. Some brown or yellow cells of unicellular organisms remain present there. These cells are called as zooxanthellae. The algae produce nitrogenous compounds that are beneficial to the host. In exchange, they obtain materials that are metabolic products of host animals. The unicellular alga chlorella vulgaris lives within the gastrodermal cells of hydra. Alga provides food and oxygen to Hydra and receives shelter, nitrogenous wastes and carbon dioxide in return.
B. COMMENSALISM
Commensalism is the association of members of different species in which only one benefits, and neither is harmed. In other words, Commensalism is defined as the association of two species in which one lives attached to another but does not harm it. In this type of association, two or more populations live together without going into any type of relation about some physiological change. Some noteworthy examples of Commensalism are detailed below.
(I). Lianas: The rooted vascular plants that climb tall trees and shrubs and reach great heights to receive sunlight are called lianas. In other words, woody vines are called as lianas. These are usually found in dense forests growing in moist tropical climates. These plants often have particular climbing organs like thorns, tendrils, etc. Mechanical tissues of stems of some of these plants provide strength for climbing without any special organ. Bauhinia, Ficus and Tinospora species are a few examples of these plants.
(II). Epiphytes: An epiphyte is a plant that grows in air attached to the branch of another plant or tree. These are photosynthetic plants that depend on other plants just for anchorage. These plants have particular tissues called velamen in their aerial roots that help absorb moisture from the air. Other roots help the plant in anchorage. Orchids differ from lianas in that they do not grow in soil. Orchids, Bromeliads, Hanging Mosses, Usnea, and Alectoria are popularly known epiphytes. Orchids show immense genetic diversity.
(III). Other Associations: Many species of organisms have been found to establish temporary relationships with the species of other organisms. For example, several species of animals have been found to depend on trees for food and shelter. Squirrels, monkeys, birds, insects, snakes, etc. are such animals. Some species of microorganisms are found to live inside the intestines of animals. The bacterium E. coli is found in the human colon.
C. PROTOCOOPERATION
The association of organisms of two different species in which both organisms’ benefit is called protocooperation. An important example of protocooperation is the association of the Sea anemone and the hermit crab. The crab carries the anemone to fresh feeding sites, and the anemone, in turn, protects the crab from its enemies.
Negative Interdependence
Negative interdependence is a relationship between two species in which one species harms the other. This type of relationship can be categorized as exploitation, antibiosis, and competition.
A. EXPLOITATION The relationship between two species in which one species uses the other species for food, shelter, or support is called Exploitation.
A.1. Exploitation for shelter Some animals raid the nests of other animals and exploit them for their shelter. We have the well-known example of a cuckoo that never builds its nest. It lays its eggs in the nest of a crow. Similarly, the cow-birds never build their own nests and raid the nests of other birds to lay their eggs. This type of lifestyle is called nest parasitism. Among ants, there are some cases where one species uses the habitat of other ants. For example, Polyergus ant workers raid the nests of Formica ants.
A.2. Exploitation for food: Dependence of one species on the other species for the purpose of food may be put into the following categories-
I. Parasitism. The relationship between individuals of two species in which an individual of one species derives its food from the body of the individual of the other species is called parasitism. The organism that derives food from the body of another organism is called a parasite. A parasite may either be an endo-parasite or an ectoparasite. The endo-parasite lives in the body of its host. Ascaris lumbricoides, Tape-worms (Taenia solium), Liver flukes, Malaria parasite (Plasmodium species), Entamoeba histolitica etc. are endo-parasites. The parasite that depends on the body of its host and derives its food while living outside its body is called an ectoparasite. Mosquitoes, Leeches, etc., are ectoparasites. Both plants and animals adopt this type of interdependence. Some fungi and bacteria adopt the parasitic mode of nutrition, as green plants do, and they cannot synthesize the food on their own. Some fungi species parasitise animals and plants and cause diseases in them. Some species of plants derive food from the plants of other species. These plant parasites are classified as root parasites and stem parasites. Cuscuta (Dodder plant) is a total stem parasite, while Orobanche, Conopholis, and Epifagus are total stem parasites.
II. Predation. The state of the interdependence of an individual of one species on the individual of some other species in which one organism catches and kills the other organism for its food is called as predation. Most predators are animals and birds, but some plants, such as fungi, are also considered to be predators. Hawks, Eagles, Bhujangas, Swallows, and (sometimes sparrows) other birds are powerful predators during specific seasons. Some noteworthy examples of predation are mentioned below-
(i). Browsing and grazing by rabbits, snails, goats, sheep and cattle. The function of animals eating primary producers is called herbivory. Such animals are called as herbivores. In an ecosystem, herbivores limit the populations of primary producers and help maintain the balance and stability of the ecosystem. Rabbits, insects like grasshoppers, cattle like cows and goats, Mollusks etc., are herbivores. On the other hand, the primary producers trap solar energy alone and prepare food for the whole ecosystem.
(ii). Seeds and seedlings are destroyed by some cattle, rodents, and birds.
(iii). Feeding ducks, fish, muskets, etc., on aquatic plants and filter feeding of some animals on diatoms, flagellates and different algae.
(iv). Carnivorous Plants: Feeding insectivorous plants on insects – Some insectivorous plants are Nepenthes, Drosera, Utricularia, Dionea, Venus’s fly trap, Sun dew, Sarracenia, etc. These insectivorous plants are adapted uniquely to attract, trap, kill and digest their victims. These plants synthesize Proteolytic enzymes that kill and digest the insects caught in their trap, which in most cases often remain the modifications of leaves.
(v). Carnivorous Animals or Animals feeding on the flesh of other animals: The condition of food transfers in an ecosystem in which an animal eats the flesh of a herbivore or meat eater animal is called Carnivory. Carnivory helps increase the complexity of ecosystems and the flow of nutrients and energy to different levels. Carnivore animals, including Protozoans, sponges, hydroids, comb jellies, groups of worms, and crustaceans, are examples of filter feeders. Filter feeders are organisms that do not eat the flesh of animals directly. Other carnivores include spiders, insects, centipedes, lampreys, sharks, bony fishes, amphibians, most reptiles, and many birds and mammals.
The animal that captures and preys upon another animal is called a predator. The animal on the other end is called a prey. This type of relationship between two species is called a Prey-Predator relationship.
B. ANTIBIOSIS
Some organisms create harmful and challenging conditions for the lives of other animals. This behaviour is called as antibiosis. This behaviour is usually seen in the microbial world. The production of antibiotics is a very common trend among microorganisms. Some microbes secrete such chemicals or antibiotics that remain antagonistic to other organisms. The production of hydroxylamine, a toxin by microcystins, algae that causes the deaths of fish and cattle, is a well-known example of antibiosis. The hypersensitive reactions involving the interaction among microorganisms are an example of antibiosis.
C. COMPETITION The relationship between species involving a struggle among organisms for resources like nutrients, water, space, sunlight, etc., is called competition.
Competition may occur between –
(i). Members of two or more species,
(ii). Individuals of a given species. Trees, herbs, shrubs, and lianas face tough competition in a forest for sunlight, nutrients, water, and agents that pollinate and disperse fruits and seeds. There exists a tough competition among higher carnivorous animals. These animals always pass through a competition to obtain food. It often happens that two or more animals try for the same prey. Since many ecosystems are running imbalanced, prey is scarce for these animals. The tigers of Sundarbans have been reported to eat fish and crabs without appropriate food.
