Inter-specific relationships between two (or more) species can be discovered in any community and belong to two main categories —symbiosis and antagonism.
Symbiosis means ‘living together’. It is a beneficial relationship between two different species in which one or both the species are benefited and neither species is harmed.
Symbiotic relationships include commensalism (one species benefited, another called host not affected), proto-cooperation the species are benefited, the relationship is favourable to both but not obligatory) and mutualism (both the species or symbionts are benefited, the relationship is favourable to both and obligatory).
Commensalism occurs when one species is benefited from a symbiotic relationship. All communalistic relationships are facultative, as the commensalism neither harm nor help their hosts: die hosts also appear neither to resist nor to foster the relationship in any way. Examples of commensalism showing more or less continuous contact with the host are offered by a great variety of epiphytes and epizoans. All epiphytes use trees only for attachment and manufacture their own food by photosynthesis.
In Vanda, an epiphytic orchid, special kind of aerial roots hang freely in the air and absorb moisture with the help of their special absorptive tissue called velamen. Sessile invertebrates that grow on plants or other animals represent many permanently fixed commensals. For example, hydroids like Hydractinia live as commensals on the gastropod shells occupied by crabs.
Barnacles attached to the skin of whales provide another example, an association, which provides wider distribution and feeding opportunities for the sessile crustaceans. Examples of commensalism without continuous contact also occur.
An interesting example of a commensal living within its host is that of a small tropical fish, Fierasfer. This species finds shelter in the doaca of a sea cucumber, moving out for food and returning to the cloacal cavity at its own will. Tie suckerfish, Echeneis, attaches to the underside of a shark, thereby securing protection, wide pro-graphical dispersal and scrapes of food. The shark neither benefits nor suffers in any respect.
Protocooperation is a relationship between two species, which is favourable to both but not obligatory. The association of a crab and a coelenterate shows an interesting example of potocooperation. The sea anemone, Adamsia palliata, grows on the back of the hermit crab, Eupagurus prideauxi, or is sometime ‘planted’ there by the crab. It protects the crab with the help of its nematocysts that prevent the approach of predatory fish, which feed on the crab.
The sea anemone is transported by the crab from place to place and obtains particles of food when the crab captures and eats another animal. In this case the crab is not absolutely dependent on the coelenterate, or vice-versa. Hence the association, through favourable to both, is not obligatory for their existences.
Mutualism occurs when both the species benefit from a symbiotic relationship. Mutualism may be facultative, where the species involved in the association can exist independently. It may be obligatory, where the relationship is imperative to the existence of one or both the species. Mutualism may occur between two animal species, between two plant species, or between animal and a plant species. An example of obligatory mutualism between two animal species, without continuous contact, is the association between aphids and dairy ants (Fig. 2.2).
Dairy ants keep the tiny green aphids (plant lice) as food suppliers. Aphids secrete honeydew, a sugar and protein mixture, on which the ants depend. A common species of garden ant, for example, places aphids on the roots of com. Aphids feed there and the ants thereafter “milk” these “ant cows” by gently stroking them. At the approach of winter the aphids are carried into the ant nest and are put back on com roots the following spring. Thus, ants obtain food from the aphids, and the aphids in turn secure protection, food and care from the ants.
Lichens such as Graphis, Parmelia and Cladonia exhibit a more intimate form of mutualism between two plant species. Each lichen is a symbiotic association between a fungus and an alga. In many species of lichens, the algal symbiont is Trebouxia. The algae manufacture food for themselves as well as for the fungus. The fungus in turn contributes water and carbon dioxide that enable the alga to synthesize food. If they are separated from their association, they lead a precarious life, more particularly the fungus.
Nitrogen fixing mutualistic bacteria of the genus Rhizobium infects the roots of many leguminous plants. They enter the roots through the root hair and the infected root cells respond by developing root nodules (Fig. 2.3). In these root nodules, the host plant provides nutrients for the bacteria and the bacteria in turn contribute fixed nitrogen to the host plant, which becomes independent of supplies of nitrogen from the soil.
There are many examples of symbiotic mutualism between an animal and a plant species. In green hydra, Chlorohydra viridissima, the cells of gastrodermis contain many unicellular algae, oochlorellae, which are transferred by the host eggs from one generation to another. The algae supply food and oxygen, and in turn receive protection, water and other materials essential for continued photosynthesis.
Pollination of flowers by insects is yet another manifestation of plant animal mutualism. In many cases, the relationship between the insect mouth parts and the shape the flower is complimentary with an adaptation to each other. For example, bee- pollinated s, such as Orchidaceae, usually have zygomorphic flowers.
These are tubular, so the bee can adjust itself to the flower shape and then push its head into the flower. Similar adaptations occur butterfly and moth- pollinated flowers and pollinator’s mouth parts. Interdependence of the yucca moth, Pronuba yuccasella, and the yucca flower is very fascinating. These are co-evolutionary relationships. In some cases flower species have coevolved to avoid competition among themselves flowering at different times of the year, or by attracting different species of insect as their pollinator.
The antagonistic relationships are manifested through parasitism, predation, competition and antibiosis between two different species. Parasitism is a relationship in which one species (parasite) is always benefited at the cost of other species (host). A parasite derives nourishment from the host and in many cases finds protection and living space on or inside the host.
The host is usually larger than the parasite and 1 efficient parasite does not kill its host. Many parasites require a secondary host for dispersal completion of their life cycle. For example, mosquito species serve as vectors for the protozoan malaria parasite, Plasmodium. Animals that become infected and serve as a source from which animals can be infected are known as reservoir hosts. Animals may also be parasitic on plants.
Nematodes infest the roots of plants. Wasps or gnats, especially on oak, roses form galls. A variety of insect larvae are leaf miners, wood borers, cambium feeders, and fruit eaters. Plants themselves may be parasites either on other plants or on animals. Bacteria and fungi are among the most important disease-producing organisms in animals.
Filamentous bacteria, Actinomycetes are chiefly decomposers in the soil. Some of them produce chemicals to reduce severe competition for food from other microbes. Man in combating several bacterial diseases uses these chemicals called antibiotics (e.g., streptomycin). Some basidiomycetes such as Puccinia graminis and Ustilago maydis produce rust of wheat and smut in com respectively.
Parasitic adaptations are too many and beyond the scope of this treatise. The size of the parasite also influences its mode of reproduction. The host-parasite relationships have great ecological significance and are influenced by a number of environmental factors. The density of host population also affects the parasite population density.
The term predation is generally used to describe the killing and eating of one species by mother. Typical predation occurs when a carnivore kills a herbivore or another carnivore for food. The study of prey-predator relationships is very fascinating and examples of this relationship may be discovered in many natural communities.
Allelopathy or chemical competition occurs when one organism uses chemicals to cause harm 10 another. This phenomenon is widely reported in terrestrial plants (Seigler, 1996). The occurrence of chemical competition has also been reported in aquatic plants (Gopal and Goel, 1993). In general sense, allelopathy means causing of injury (pathy) to other organisms (allelo) by chemical means, but it can be more complex.
For example, a common alga, Chlorella, produces a bactericide that not only kills bacteria but also retards the growth of Daphnia, which feed on Chlorella. A chemical produced by a diatom, Nitzschia, slowed the division rate of Chlorella grown in the same culture. This type of interspecies antagonism probably exerts control on the abundance of different phytoplankton species in water bodies, and in some cases may influence the seasonal succession of species, so common in nature.
There are many examples of chemical interaction among crop plants. Barley, Hordeum vulgare may inhibit germination and growth rate of several weeds, even in the absence of competition for nutrients or water. Allelopathic effects of crop plants against weeds, called heterotoxicity, may be of great benefit in agricultural systems.
Some researcher suggested that the abundant oils in the leaves of eucalyptus trees in Australia promote frequent fires in the leaf litter, killing the seedlings of competitors. The study of allelopathic agents is currently a very active field in plant ecology, and it is too early to say how much the distributional patterns of plants are determined by interactions involving toxins or antibiotic substances.
Competition is another manifestation of antagonistic relationship. Competition may occur among species and between species. However, in nature competition may not be always apparent although it is occurring. Increased growth rate of some tree species after the removal of other species provides direct evidence of competition for water, light and nutrients. Individuals of the same species may compete for food, living space and mate. Competition for food also applies to animals of different species that depend on the same type of food.
Competition may result in death of some competitors, but this is usually from fighting or being deprived of food rather than being killed for food as in predation, or by disease as in extreme parasitism. The severity of inter-specific competition depends on the extent of similarity or overlap of resource requirements of different organisms and the shortage of supply in the habitat. Such a competition may have several effects on the populations of competing individuals.
Schoener (1983) divided competition into six categories. These are consumptive competition (based on the utilisation of some renewable resource), pre-emptive competition (based on the occupation of open space), overgrowth competition (occurring when one organism grows over another thereby depriving it of light, water, or some other resource).
Chemical competition (by production of a toxin acting at a distance), territorial competition (defense of territory), and encounter competition (involving transient interactions over a resource resulting in loss of time or energy, physical harm, or theft of food).
These competition mechanisms are defined in terms of the capabilities of organisms and the habitats in which they occur (Ricklefs and Miller, 2000). In terrestrial environments, consumptive competition is most common. However, pre-emptive and overgrowth competition are more common in marine habitats. Territorial and encounter con-petitions occur only among animals of all habitats, predominating in terrestrial ones.
Darwin emphasized that competition is usually most intense between closely related species or organisms. As they have similarity in structure and habits, the competition is more severe among species of the same genus than between species of different genera. Substitution experiments, developed by Wit (1960), are helpful for studying plant competition. In such experiments, ratios of two plant species are varied, but their total density is kept constant.
The results are portrayed on replacement series diagrams showing the relative strengths of inter-specific and intra-specific competition. Experimental work with agriculture crops suggested these competitive interactions are very acute in field populations. Experiments with oats (Avena) and other plants have demonstrated strong asymmetry in inter-specific competition.
It is the most critical factor confining a species to a particular niche. According to the competitive exclusion principle of Gause (1934), stabilised populations of more than one species cannot simultaneously and completely occupy an ecological niche. Thus, inter-specific competition results in the segregation of species into different niches.
When two populations compete, it is likely that one of them is more strongly affected by competition than the other. This is called asymmetrical competition. In plants, root competition for nutrients and water is generally symmetrical, whereas shoot competition for light is asymmetrical. Examples of asymmetrical competition are available in animal kingdom also (Resetarits, 1997).