What is a “Biotic Community”?

We already know that no habitat in this nature contains the individuals of only one species. In fact, a population of a single species cannot survive by itself because there is always an interdependence of one form of life on another at least, for food; any species requires one more species on which it can feed. Even the plants which are autotrophs by nature, cannot survive on their own as they also require the soil microbes to break down the complex organic matter to supply them with the essential minerals and nutrients and then the plants also require a pollinating agent (insects) for pollination. There is thus always an interdependence among the organisms in this nature or in a given habitat such that a kind of living community (biotic community) comes into being. This natural association of the interdependent populations of different species inhabiting a common environment or habitat as a viable, self-contained unit is called a biotic community, or biocoenosis. In short, a biotic community may be considered as a multi-species population.

Population Interactions- Ecological significance:

Population interactions are inherently interspecific in nature because, these interactions essentially arise from the interaction of populations of two different species. They could be beneficial, detrimental or neutral (neither harm nor benefit) to one of the species or both.  To understand them in a better way, let’s assign a ‘+’ sign for beneficial interaction, ‘-‘sign for detrimental and ‘0’ for neutral interaction and then try to look at all the possible outcomes of interspecific interactions through the table given below:

Table: Population Interactions

From the above table, we can easily understand that           both the species benefit in mutualism and both lose in competition in their interactions with each other. In both parasitism and Predation only one species benefits (parasite and predator, respectively) and the interaction is detrimental to the other species (host and prey, respectively). The interaction where one species is benefitted and the other is neither benefitted nor harmed is called commensalism. In amensalism on the other hand, one species is harmed whereas the other is unaffected. Predation, parasitism and commensalisms share a common characteristic – the interacting species live closely together. Let’s briefly discuss about the very nature of these interactions…

(i)   Predation: What would happen to all the energy fixed by autotrophic organisms if the community has no animals to eat the plants? We can think of predation as nature’s way of transferring to higher trophic levels the energy fixed by plants. When we think of predator and prey, most probably it is the tiger and the deer that readily come to our mind, but a sparrow eating any seed is not less a predator. Although animals eating plants are categorized separately as herbivores, they are, in a broad ecological context, not very different from predators.

      Besides acting as ‘conduits’ for energy transfer across trophic levels, predators ecologically play other important roles as well. They keep prey populations under control. But for predators, prey species could achieve very high population densities and cause ecosystem instability. When certain exotic species are introduced into a geographical area, they become invasive and start spreading fast because the invaded land does not have its natural predators. The prickly pear cactus introduced into Australia in the early 1920s caused havoc by spreading rapidly into millions of hectares of rangeland. Finally, the invasive cactus was brought under control only after a cactus-feeding predator (a moth) from its natural habitat was introduced into the country. Biological control methods adopted in agricultural pest control are based on the ability of the predator to regulate prey population. Predators also help in maintaining species diversity in a community, by reducing the intensity of competition among competing prey species. In the rocky intertidal communities of the American Pacific Coasts the starfish Pisaster is an important predator. In a field experiment, when all the starfish were removed from an enclosed intertidal area, more than 10 species of invertebrates became extinct within a year, because of interspecific competition.  

      If a predator is too efficient and overexploits its prey, they the prey might become extinct and following it, the predator will also become extinct for lack of food. This is the reason why predators in nature are ‘prudent’. Prey species have evolved various defenses to lessen the impact of predation. Some species of insects and frogs are cryptically-coloured (camouflaged) to avoid being detected easily by the predator. Some are poisonous and therefore avoided by the predators. The Monarch butterfly is highly distasteful to its predator (bird) because of a special chemical present in its body. Interestingly, the butterfly acquires this chemical during its caterpillar stage by feeding on a poisonous weed.

      For plants, herbivores are the predators. Nearly 25 per cent of all insects are known to be phytophagous (feeding on plant sap and other parts of plants). The problem is particularly severe for plants because, unlike animals, they cannot run away from their predators. Plants therefore have evolved an astonishing variety of morphological and chemicals that make the herbivore sick when they are being eaten by them inhibit feeding or digestion disrupt its reproduction or even kill it. This can be exemplified through an obnoxious weed called as Calotropis, growing in abandoned fields. The plant produces highly poisonous cardiac glycosides and that is why we would never see any cattle or goats browsing on this plant. A wide variety of chemical substances that we extract from plants on a commercial scale (nicotine, caffeine, quinine, strychnine, opium, etc.,) are produced by them actually as defences against grazers and browsers.

(ii)  Competition: When Darwin spoke of the struggle for existence and survival of the fittest in nature, he was convinced that interspecific competition is a potent force in organic evolution. It is generally believed that competition occurs when closely related species compete for the same resources that are limiting, but this is not entirely true. Firstly, totally unrelated species could also compete for the same resources. For instance, in some shallow South American lakes visiting flamingoes and resident fishes compete for their common food, the zooplankton in the lake. Secondly, resources need not be limiting for competition to occur; in interference competition, the feeding efficiency of one species might be reduced due to the interfering and inhibitory presence of the other species, even if resources (food and space) are abundant. Therefore, competition is best defined as a process in which the fitness of one species (measured in terms of its ‘r’ the intrinsic rate of increase) is significantly lower in the presence of another species. It is relatively easy to demonstrate in laboratory experiments, as Gause and other experimental ecologists did, when resources are limited, the competitively superior species will eventually eliminate the other species, but evidence for such competitive exclusion occurring in nature is not always conclusive. Strong and persuasive Abingdon tortoise in Galapagos Islands became extinct within a decade after goats were introduced on the island, apparently due to the greater browsing efficiency of the goats. Another evidence for the occurrence of competition in nature comes from what is called competitive release. A species, whose distribution is restricted to a small geographical area because of the presence of a competitively superior species, is found to expand its distributional range dramatically when the competing species is experimentally removed. Connell’s elegant field experiments showed that on the rocky sea coasts of Scotland, the large and competitively superior barnacle Balanus dominates the intertidal area, and excludes the smaller barnacle Chathamalus from that zone. In general, herbivores and plants appear to be more adversely affected by competition than carnivores.

(iii) Parasitism: Considering that the parasitic mode of life ensures free lodging and meals, it is not surprising that parasitism has evolved in so many taxonomic groups from plants to higher vertebrates. Many parasites have evolved to be host-specific (they can parasitize only a single species of host) in such a way that both host and the parasite tend to co-evolve; that is, if the host evolves special mechanisms for rejecting or resisting the parasite, the parasite has to evolve mechanisms to counteract and neutralize them, in order to be successful with the same host species. In adaptations such as the loss of unnecessary sense organs, presence of adhesive organs or suckers to cling on to the host, loss of digestive system and high reproductive capacity. The life cycles of parasites are often complex, involving one or two intermediate hosts or vectors to facilitate parasitisation of its primary host. The human liver fluke (a trematode parasite) depends on two intermediate hosts (a snail and a fish) to complete its life cycle. The malarial parasite needs a vector (female Anopheles mosquito) to spread to other hosts. Majority of the parasites harm the host; they may reduce the survival, growth and reproduction of the host and reduce its population density. They might render the host more vulnerable to predation by making it physically weak. Isn’t it correct to say that an ideal parasite should be the one to be able to thrive within the host without harming it?

      Parasites that feed on the external surface of the host organism are called ectoparasites. The most familiar example of this group is the lice on humans and ticks on dogs. Many marine fish are infested with ectoparasitic copepods. Cuscuta, a parasitic plant that is commonly found growing on hedge plants, had lost its chlorophyll and leaves in the course of evolution. It derives its nutrition from the host plant which it parasitizes. The female mosquito is not considered a parasite, although it needs our blood for reproduction because, it does not live in permanent association with the human host all the time which is an essential criterion for being a parasite.

      In contrast, endoparasites are those that live inside the host body at different sites (liver, kidney, lungs, red blood cells, etc.). The life cycles of endoparasites are more complex because of their extreme specialisation. Their morphological and anatomical features are greatly simplified while emphasizing their reproductive potential.

      Brood parasitism in birds is a fascinating example of parasitism in which the parasitic bird lays its eggs in the nest of its host and lets the host incubate them. In nature, it is seen live in case of cuckoo laying her eggs in Crow’s nest. During the course of evolution, the eggs of the parasitic bird have evolved to resemble the host’s egg in size and colour to reduce the chances of the host bird detecting the foreign eggs and ejecting them from the nest. This can be easily observed in nature by watching the movements of the cuckoo (koel) and the crow in our neighbourhood park particularly, during the breeding season (spring to summer) and hence, brood parasitism in live action.

(iv) Commensalism: This is an interaction in which one species benefits and the other is neither harmed nor benefited. Say for example, an orchid growing as an epiphyte on a mango branch, and barnacles growing on the back of a whale, benefit while neither the mango tree nor the whale derives any apparent benefit from the growth of such second species. The cattle egret and grazing cattle browsing in close association, a sight we are most likely to catch if we were to live in a farmed rural area, is a classic example of commensalism. The egrets always forage close to where the cattle are grazing because the cattle, as they move, stir up and flush out from the vegetation insects that otherwise might be difficult for the egrets to find and catch. Another example of commensalism is the interaction between sea anemone that has stinging tentacles and the clown fish that lives among them. The fish gets protection from predators which stay away from the stinging tentacles. The anemone does not appear to derive any benefit by hosting the clown fish.

(v)  Mutualism: This interaction confers benefits on both the interacting species. Lichens represent an intimate mutualistic relationship between a fungus and photosynthesizing algae or cyanbacteria. Similarly, the mycorrhizae are associations between fungi and the roots of higher plants. The fungi help the plant in the absorption of essential nutrients from the soil while the plant in turn provides the fungi with energy-yielding carbohydrates.

      The most spectacular and evolutionarily fascinating examples of mutualism are found in plant-animal relationships. Plants need the help of animals for pollinating their flowers and dispersing their seeds. Animals obviously have to be paid ‘fees’ or so called allowance for the services that plants expect from them i.e. dispersal of its pollen grains. Plants offer rewards or fees in the form of pollen and nectar for pollinators and juicy and nutritious fruits for seed dispersers. But the mutually beneficial system should also be safeguarded against ‘cheaters’, for example, animals that try to steal nectar without aiding in pollination. How this is accomplished in nature is explained by the fact that keeping in view these vital plant-Animal interactions, there often involves a co-evolution of both the species together i.e. mutualists which in the context of our example, means the evolution of the flower and its pollinator species together which is tightly linked with one another. In many species of fig trees, there is a tight one-to-one relationship with the pollinator species of wasp. It means that a given fig species can be pollinated only by its ‘partner’ wasp species and no other species. The female wasp uses the fruit not only as an ovipositor (egg-laying) site, but also uses the developing seeds within the fruit for nourishing its larvae. The wasp pollinates the fig inflorescence while searching for suitable egg-laying sites. In return for the favour of pollination, the fig offers the wasp some of its developing seeds, as food for the developing wasp larvae. Similarly, the orchids show a bewildering diversity of floral patterns many of which have evolved to attract the right pollinator insect (bees and bumblebees) and ensure guaranteed pollination by it. Not all orchids offer rewards. The Mediterranean orchid Ophrys employs ‘sexual deceit’ to get pollination done by a species of bee. One petal of its flower bears an uncanny resemblance to the female of the bee in size, colour and markings. The male bee is attracted to what it perceives as a female, ‘pseudo copulates’ with the flower, and during that process is dusted with pollen from the flower. When this same bee ‘pseudocopulates’ with another flower, it transfers pollen to it and thus, pollinates the flower. Here in this example, we can see how co-evolution operates actually in nature. If the female bee’s colour patterns change even slightly for any reason during evolution, pollination success will be reduced unless, the orchid flower co-evolves to maintain the resemblance of its petal to the female bee…

Concept of “Key-stone” species in a biotic community & their ecological significance:

Connecting concepts: What are “Keystone & Critical link species”?

Such animal or plant species, living in a particular habitat, whose presence in that very habitat is so important that it influence the survival and existence of many other species in the same habitat, are referred to as key stone species. Say for example, a few animals produce long-lasting changes which alter or maintain their environment. They are called keystone species. These species play crucial role in regulating the relative abundance of other species.

Examples:  Elephants and crocodiles can be quoted as few examples of keystone species in the following sense:

(i)   Elephants damage trees by browsing the lower branches, stripping the bark and uprooting them. This prevents forests from encroaching on the grasslands where grazing beasts and other herbivores flourish.

(ii) Crocodiles by their swimming movements dig and maintain deep pits free of rooted swamp vegetation. Fishes and other aquatic animals survive dry periods by gathering in these pits.

Critical Link species: Since, only a few species can actually function as keystone species. Several other species play an important role in supporting network species by providing food or acting as pollinators of flowers or for that matter, acting as dispersal agents of seeds and fruits or facilitate in the absorption or circulation of nutrients say, microbes in the soil. Such species are called critical link species because; these serve to link different species thereby, forming either a food chain or food web as such in the nature.  For example, Mycorrhizal fungi are critical link species which help the vascular plants in obtaining nutrients from the soil and organic residues. Tropical rain forests are rich in critical link species due to high degree of zoophily and zoochory.

What is ‘Ecotone’ and ‘Edge Effect’?

The adjacent biotic communities do not ways have sharp lines of demarcations between them. There are usually transition zones between any two habitats which comprise or house the species from both the adjacent habitats. This transition zone is what we call the ecotone. An ecotone often has some populations from each adjacent community and some species which are characteristic to itself only. Therefore, the total number of species is often greater in the ecotone (transition zone) than in the adjoining communities. The presence of a greater number and diversity of species in this transition or ecotone region is referred to as “edge effect.”

For example, the ecotone between grassland and the forest will have few species from both the communities in addition to those which are specific to ecotone conditions. Those species which live primarily in ecotones or spend maximum time in this region called “edge species”.