Relationships among Organisms : Positive and Negative Interactions

Relationships among Organisms

Most of the ecologists are in favour of the use of the term symbiosis, which literally means 'living together', in its broader sense.

Odum (1971) used the term 'symbiosis' in its broader sense and preferred to group all the types of symbiotic interactions into two major groups. These are:

1. Positive Interactions

A type of interaction, where populations help one another, the interaction being either one-way or reciprocal. Their benefit may be in respect of food, shelter, substratum, transport etc. These include (i) commensalisms (ii) Protocooperation and (iii) Mutualism.

2. Negative Interactions

Where members of one population may live at the expense of members of the other population compete for foods, excrete harmful wastes, etc. These include

(i) Competition, (ii) Predation,  (iii) Parasitism, and (iv) Antibiosis.

Positive Interactions

1) Mutualism

Here both the species derive benefit. The two populations enter into some sort of physiological exchange. The following are some common examples of mutualism.

i) Symbiotic Nitrogen fixers

This is a well known example of mutualism, where the bacterium Rhizobium form nodules in the roots of leguminous plants, and lives symbiotically with the host. Bacteria obtain food from the higher plant and in turn fix gaseous nitrogen, making it available to plant.

ii) Mycorrhizae

A symbiotic association between a fungus and a root of a higher plant is called Mycorrihza, which may be (i) Ectotrophic, where fungal hyphae are natural substitute of root hairs absorbing water and nutrients from soil e.g. Pines and Oaks or (ii) Endotrophic, where fungi occur internal to root tissue. e.g. Orchids and members of Ericaceae

iii) Lichens

These are examples of mutualism where contact is close and permanent as well as obligatory. Their body is made up of a matrix formed by a fungus, within the cells of which an alga is embedded. Usually the fungal groups are from Ascomycetes or Busidiomycetes and the algal groups are species of blue greens. The algal groups takes up the photosynthetic function. The fungal groups are concerned with reproduction. The fungus makes moisture as well as minerals available, whereas alga manufactures food. Neither of the two can grow alone independently in nature. Lichens grow abundantly on bare rock surfaces.

2) Commensalism:

Commensalism refers to association between members of different species only. One is benefited without any effect on the other. Some common examples are:

i) Lianes

Lianes are common in dense forests of moist tropical climates. They maintain no direct nutritional relationship with the trees upon which they grow. On the basis of the type of device used for climbing their support, lianes may be leaners, thorn lianes, twiners or tendril lianes. Common lianes are species of Bauhinia, Ficus and Tinospora.

ii) Epiphytes

Epiphytes are plants growing perched on other plants. They use other plants only as support and not for water or food supply. They differ from lianes in that they are not rooted into the soil. Epiphytes may grow on trees, shrubs, or larger submerged plants. They grow either on the trunks or leaves. Epiphytes are most common in tropical rain forests. Many orchids, Usnea and Alectoria are well known epiphytes.

iii) Epizoics

Some plants grow on the surfaces of animals. For example, green algae grow on the long, grooved hairs of the sloth. Similarly,Basicladia (Cladophoraceae) grows on the backs of freshwater turtles.

Negative Interactions

These include the relations, in which one or both the species are harmed in any way during their life period. Some (Clarke, 1954) prefer to call such types of associations as 'antagonism'. Such negative interactions are generally classified into three broad categories, as exploitation, antibiosis and competition which are discussed in detail as follows:

I) Exploitation

Here one species harms the other by making its direct or indirect use for support shelter, or food. Thus exploitation may be in respect of shelter or food.

1. Shelter

The so-called 'parasitic birds' as cuckoo and cowbird never build their own nests and female lays eggs in the nest established by birds of another usually smaller species.

2. Food

The various relationships in respect of food may belong to:

a) Parasitism

A parasite is the organism living on or in the body of another organism and deriving its food more or less permanently from its tissues.

There are some parasitic vascular plants. Species of Cuscuta (total stem parasites) grow on other plants on which they depend for nourishment. Young stem twines around the host stem from which adventitious roots develop that finally penetrate the stem of the host, establishing relationship with its conducting elements. The specialized roots are called haustoria.

Other examples of such association are total root parasites as Orabanche, and Epifagus (Orobanchaceae) which are found on roots of higher plants. Rafflesia is found on roots of Vitis. Members of the family Loranthaceae (Viscum album, Loranthus sp) are partial stem parasites. They grow rooted in branches of host

 trees. Others like Santalum album, are partial root parasite. Their roots are attached to host plants. Majority of parasites are microorganisms, of which fungi, bacteria, mycoplasmas, rickettsias and viruses parasitise plants as well as animals.

b) Carnivorous Plants

A number of plants as Nepenthes, Darlingtonia, Drosera, Utricularia, Dionaea consume insects and other small animals for their foods. They are also known as insectivorous plants. They are adapted in remarkable ways to attract, catch and digest their victims. Their leaves or foliar appendages produce proteolytic enzymes for digestion of the insects. The carnivorous habit in plants is said to be an incidental feature of their nutrition, since they possess green leaves and carryout photosynthesis.

II) Antibiosis

The term 'antibiosis' generally refers to the complete or partial inhibition or death of one organism by another through the production of some substance or environmental conditions as a result of metabolic pathways. Here none of them derives any benefit. These substances and or conditions are harmful (antagonistic) to other organism. The phenomenon of antibiosis is much common among microbial world. Production of chemicals that are antagonistic to microbes - the antibiotics is well known.

Bacteria, actinomycetes and fungi produce a number of antimicrobial substances which are widespread in nature. Antagonistic substances are also reported in some algae, as for example in cultures of Chorella vulgaris, some substance accumulates which inhibits the growth of the diatom, Nitxschia frustrulum. Pond 'blooms' of blue-green algae especially Microcystis are known to produce toxins such as hydroxylamine which causes death of fish and cattle.

The term antibiosis would also include such phenomena as hypersensitive reactions that involve the interaction between microorganisms, particularly pathogenic ones, and harmful to one or both.

III) Competition

Competition occurs when individuals attempt to obtain a resource that is inadequate to support all the individuals seeking it, or even if the resource is adequate, individuals harm one another in trying to obtain it. The resources competed for can be divided into two types :

(i)    Raw material such as light, inorganic nutrients, and water in autotrophs and organic food and water in heterotrophs.

(ii) Space to grow, nest, hide from predators. etc.

The competition may be

(i)   Intraspecific: Occurring between members of the same species of the population.

(ii) Interspesific: Occurring between different species of population. Competition thus is usually between members of the same trophic level.