Explain the types of relationship between organisms in an ecosystem
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TYPES OF INTERACTIONS WITHIN ECOSYSTEMS
Populations of different organisms live together in a community which together with the nonliving factors of the environment forms an ecosystem. The organisms interact among themselves in various ways for survival. Earlier in unit one, we had observed that in a trophic level, a carnivore would feed on a herbivore. That is an example of a predator/prey interaction. Various other forms of interactions between organisms within the same community will be looked into in this unit.
TYPES OF INTERACTIONS
Interactions between organisms that characterize particular communities have risen as a result of their evolutionary trend. The plants, animals, protists, fungi and bacteria that live together in communities have changed and adjusted to one another continually over a period of millions of years. Many communities are very similar in their species composition and appearance over wide areas. For example, the open savanna that stretches across much of Africa includes many plant and animal species that coexist over thousands of square kilometers. Interactions between these organisms, some of which have evolved over millions of years, occur in a similar manner throughout these grassland communities. Some of these interactions include:
- Predator/Prey interaction
- Competition (Interspecific and Intraspecific)
While mutualism and commensalism are positive interactions others, like parasitism, predation and competition are negative interactions.
Predator-prey interactions are interactions between organisms in which one organism feeds on the other. A good example is a lion hunting a gazelle in an open Savanna of Africa. The predators have natural characteristics and developed recognizable strategies that help them hunt and capture their prey. The strategies that prey use to avoid being eaten are not so obvious. Some animals however, employ chemical defences to ward off predators. Venomous snakes, lizards and fishes are well known examples. Also, bees, wasps, predatory bugs, scorpions, spiders and many other arthropods have chemicals that they use to defend themselves and to kill their prey.
The dart-poison frogs of the family Denrobatidae produce toxic alkaloids in the mucus that covers their skin so powerful that a few micrograms will kill a person if injected into the bloodstream
Among plants¸ obvious structures are developed such as thorns, spikes, prickles to discourage browsers like vertebrate herbivores from feeding on them. Some grass deposit silica in their leaves to make them too tough to eat. Plants like in the potato and tomato family (Solanacea) are rich in alkaloids and steroids, while some others like in the milk weed family (Asclepiadeceae) tend to produce a milky sap that deters herbivores from eating them.
SYMBIOTIC INTERACTIONS (SYMBIOSIS)
Symbiotic relationships are those in which two or more kinds of organisms live together in often elaborate and more or less permanent relationship. Examples of symbiosis include lichens, which are associations of certain fungi with green algae or Cyanobacteria, and Mycorrhizae formed from the association between fungi and the roots of most kinds of plants. Here the fungi expedite the absorption of certain nutrients by the plants and the plants provide the fungi with carbohydrates.
The major kinds of symbiotic relationships include:
In nature, the individuals of one species are often physically attached to those of another. For example, epiphytes are plants that grow on the branches of other plants. In general, the host plant is unharmed while the organism that grows on it benefits.
Marine barnacles grow on other often actively moving sea animals and are thus carried passively from place to place. The barnacles gain protection from predation and by the free movement, reach new sources of food. On land commensal interactions occur between certain birds called Oxpeckers and grazing animals, such as cattle or Rhinoceros. The birds spend most of their time clinging to the animals, picking off parasites and other insects and carry out their entire life cycles in close association with the host animal. Cattle egrets in association with Fulani cattle provide a good example of this type of relationship in Nigeria.
In mutualism, each organism involved in the symbiotic relationship benefits. Leafcutter ants in the tropics are known to remove a quarter or more of the total surface of the plants in a given area. They do not eat the leaves but take them to their underground nests where they chew them up and inoculate them with the spores of particular fungi. These fungi are cultivated by the ants and brought from one specially prepared bed to another where they grow and reproduce. In turn, the
fungi constitute the primary food of the ants and their larvae. Ants and aphids provide another example. Aphids are small insects that suck fluids from the phloem of living plants with their piercing mouthparts. They extract a certain amount of sucrose and other nutrients from this fluid but much is excreted in an altered form through their anus as honeydew. The ants carry the aphids to new plants where they come in contact with new sources of food and then use the honeydew as food.
Parasitism is a symbiotic relationship which is harmful to the prey organism and beneficial to the parasite. The parasite is much smaller than the prey and remains closely associated to it.
Both vertebrates and invertebrates are parasitized by members of many different phyla of animals and protists.
Parasites can be internal or external. Internal parasites of humans include tapeworm, roundworm and hookworm. The internal parasites are generally marked by much more extreme specialization than the external ones. The more closely the life of the parasite is linked to that of its host, the more its morphology and behavior are likely to have been modified during the course of its evolution. Consequently, the structure of an internal parasite is often simplified and unnecessary armaments and structures are lost as it evolves. External parasites live on the bodies of vertebrates mainly birds and mammals. They include lice and fleas. They have developed adequate structures to attach themselves to the hairs or feathers of mammals and birds. Invertebrates are also parasitized. Many fungi and some flowering plants are parasitic on other plants and a few are pests of crops. A heavy infection of parasites called the “parasite load” may eventually cause the death of the host, directly or indirectly, by weakening it so that it succumbs to a predator or disease.
Competition as an interaction occurs among organisms within a community of an ecosystem when they require the same resource that is in short supply called resource competition, or when the organisms seeking a resource harm one another in the process even if the resource is not in short supply. This is called interference competition.
Competition can be interspecific or intraspecific.
Interspecific competition refers to the interactions between individuals of different species, both of which require the same resources that is in short supply. Interspecific competition is often greatest between organisms that obtain their food in similar ways. We find as a result that green plants compete with other green plants for sunlight, nutrient and water, herbivores with other herbivores and carnivores with carnivores. In addition, competition is more acute between similar organisms than between ones that are less similar.
Intraspecific competition occurs when individuals of a single species or of the same species compete for a resource in short supply. Food and mating partners provide examples of causes for intraspecific competition. Competition for space may occur in some animals such as for nesting sites, wintering sites or sites safe from predators.
SELF ASSESSMENT EXERCISE
Differentiate between interspecific and intraspecific competition.
Explain what is meant by predator-prey interaction.
Organisms within ecosystems interact among themselves at both interspecific and intraspecific levels. The interactions can either be negative or positive. Interactions occur over resources especially when in short supply.
- Interactions among organisms of communities have evolved over years of species co-existence.
- Interactions can be interspecific (between species) or intraspecific (among same species).
- Some interactions are negative eg predation and parasitism while some others are positive eg mutualism and commensalism.
- Some organisms have developed various means to ward off predators.
- Competition among organisms at interspecific or intraspecific levels occurs over resources especially when in short supply.
TUTOR MARKED ASSIGNMENT
- Name some defence methods used by organisms to protect themselves against predation..
- What symbiotic relationship will you give to the following?
- Ants and aphids
- Lice and birds
- Fleas and humans
- Cats and rats
- Cattle egret and cattle