Interaction between Species

No organism exists independently of other organisms. The producers, consumers, and decomposers of an ecosystem interact with one another in a variety of complex ways, and each forms association with other organisms. There are four basic kinds of interaction between species: competition, in which the outcome is negative for both groups; symbiosis, which benefits both groups; and  predation  and parasitism, in which the outcome benefits one and is detrimental to the other (Table T).
Each type of interaction affects evolution, the persistence of species, and the overall diversity of life. It is important to understand that organisms have evolved together so that predator, parasite, prey, competitor, and symbiont have adjusted to one another. Human interventions frequently upset these adjustments.

Competition
Competition occurs when two or more individuals attempt to use an essential common resource such as food, water, shelter, living space, or sunlight. When this type of competition occurs, the more fit species will win out and persist, and the less fit will lose, must evolve into a slightly different niche, and migrate to a different geographic area or become extinct. This is known as the competitive exclusion principle, which states that two species that have exactly the same requirements cannot coexist in exactly the same habitat; one will always exclude the other.  

                                    

Niches: How Species Coexist

Taken to its logical extreme, we could imagine an Earth with very few species, perhaps one green plant on the land, one herbivore to eat it, one carnivore, and one decomposer. If we added 4 species for the ocean and 4 for fresh waters, we would have only 12 species on our planet. Yet about 1.5 million species have been named and scientists speculate that many more millions may exist; the number is so great, we do not have even a good estimate of it. How can they all coexist? The answer is their niches—the species’ way of life or functional role in an ecosystem.Species that require the same resources can coexist by utilizing those resources under different environmental conditions. They are said to have different ecological niches.
An ecological niche takes into account all aspects of the organism’s existence—that is, all physical, chemical, and biological factors that the organism needs to survive, remain healthy, and reproduce—what it does for living: what it eats, what eats it, what organisms it competes with, and how it interacts with and is influenced by the abiotic components of the ecosystems in which it is found and the role it plays in the energy flow and matter recycling in an ecosystem ( its occupation ). The niche represents the totality of an organism’s adaptations, its use of resources, and the lifestyle to which it is suited. 

Fundamental Niche 

A species’ fundamental niche is the full potential range of physical, chemical, and biological conditions and resources it could theoretically use if there were no direct competition from other species. But in a particular ecosystem, species often compete with one another for one or more of the same resources. 
Realized Niche
The lifestyle that an organism actually pursues and the resources that it actually uses make up its realized niche.
Understanding a species’ niche can help us to prevent it from becoming prematurely extinct and to assess the impact of development or changes in land use (i.e., change in environment). Cutting a forest may drive away prey and eliminate the niche of the wolf.   

 
Symbiosis
Symbiosis is a close, long-lasting, physical relationship between two different species. In other words, the two species are usually in physical contact and at least one of them derives some sort of benefit from this contact.  Each partner in symbiosis is called a symbiont. Two species can benefit one another and enhance each other’s chance of persisting.
Symbiosis is widespread and common; most animals and plants have symbiotic relationships with other species. Symbiosis therefore affects biological diversity.                                      

 
Predation
Predation is the third way in which species interact. It occurs when one organism, known as a predator, kills and eats another, known as the prey. The predator benefits from killing and eating the prey and the prey is harmed. Predation can increase diversity of prey species by reducing the abundance of the dominant prey. In this way, predators can contribute to species evenness by keeping the dominant species from overwhelming others via competitive exclusion.                                    
Parasitism
Parasitism occurs when one species (the parasite) feeds on part of another organism (the host) by living on or in the host. In this relationship, the parasite benefits and the host is harmed. Parasitism can be viewed as a special form of predation. But unlike a conventional predator, a parasite usually is smaller than its host (prey). It also rarely kills its host, and remains closely associated with, draws nourishment from, and may gradually weaken its host over time. Parasitism also promotes biodiversity by helping prevent some species from becoming so plentiful that they eliminate other species through competition.                                                   


When one considers all the interactions among the organisms in a community, however, it is obvious that some species benefit at the expense of others. If one species declines during a disturbance such as a drought, other species that compete with it may flourish during that period. Thus, it appears that ecosystems with greater species richness are more likely to contain species that are resistant to any given disturbance.

Environmental Factors That Influence Biodiversity

Species are not uniformly distributed over Earth’s surface; diversity varies greatly from place to place. What lives where depends on a number of interrelated factors. Factors  are: Climate, geologic features, soil type, topographic characteristics (slope, direction of slope, elevation, etc), vegetation, and animals. These factors influence the number and types of plants. The plants in turn influence the soils and the number and types of animals. If environmental conditions were constant over time and space, the existing dominant species would become more dominant. They would increase at the expense of rarer species, following the competitive exclusion principle. But a habitat that includes a variety of local environments is likely to offer a refuge to rarer species, thus leading to greater diversity. The more diverse habitat allows more niches, and more species persist. Also, evolution helps increase the number of species, hence biodiversity to a great extent.

The number of species also varies over time as well as space.

Factors that tend to decrease biodiversity:

1. Environmental stress
2. Extreme environments (conditions near to the limit of what living things can withstand)
3. A severe limitation in the supply of an essential resource
4. Extreme amounts of disturbance
5. Recent introduction of exotic species
6. Geographic isolation.

Of course, people also affect diversity. In general, urbanization, industrialization, and agriculture decrease diversity, reducing the number of habitats and by simplifying habitats.

Wallace realms: Biotic Provinces

In 1876, A.R.Wallace suggested that the world could be divided into six biogeographic regions on the basis of fundamental features of the animals found in those areas which in fact, are known as Wallace’s realms.

Wallace’s realms are: Neartic (North America), Neotropical (Central and South America), Palaearctic (Europe, northern Asia, and northern Africa), Ethiopian (central and southern Africa), Oriental (the Indian subcontinent and Malaysia), and Australian.

A biotic province is a region inhabited by a characteristic set of taxa (species, families, orders), bound by barriers that prevent the spread of distinctive kinds of life to other regions and the immigration of foreign species. In a biotic province, organisms share a common genetic heritage but may live in a variety of environments as long as they are genetically isolated from other regions.