Population Interaction
There is no such habitat on earth that is inhabited just by a single species.For any species, the minimal requirement is one more species on which it can feed. Even a plant species, which makes its own food, cannot survive alone; it needs soil microbes to break down the organic matter in soil and return the inorganic nutrients for absorption.
And then, how will the plant manage pollination without an animal agent? It is obvious that in nature, animals, plants and microbes do not and cannot live in isolation but interact in various ways to form a biological community. Even in minimal communities, many interactive linkages exist, although all may not be readily apparent..
Interspecific interactions
include competition, predation, herbivory, symbiosis (including parasitism, mutualism, and commensalism), and facilitation. We will use the symbols + and - to indicate how each interspecific interaction affects the survival and reproduction of the two species engaged in the interaction.
Symbiosis
Symbiosis is a general term for interspecific interactions in which two species live together in a long-term, intimate association. In everyday life, we sometimes use the term symbiosis to mean a relationship that benefits both parties. However, in ecologist-speak, symbiosis is a broader concept and can include close, lasting relationships with a variety of positive or negative effects on the participants.symbiosis includs parasitism, mutualism, and commensalism.
Predation, parasitism and commensalism share a common characteristic– the interacting species live closely together.
Name Description Effect Competition Organisms of two species use the same limited resource and have a negative impact on each other. - / - Predation A member of one species, predator, eats all or part of the body of a member of another species, prey. + / - Herbivory A special case of predation in which the prey species is a plant + / - Mutualism A long-term, close association between two species in which both partners benefit + / + Commensalism A long-term, close association between two species in which one benefits and the other is unaffected + / 0 Parasitism A long-term, close association between two species in which one benefits and the other is harmed + / -
Interspecific Interactions.
- Important: Interspecific interactions 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.
- Important: Assigning a ‘+’ sign for beneficial interaction, ‘-’ sign for detrimental and 0 for neutral interaction.
- Important: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.
Competition
Competition
is an interaction between organisms or species in which both require a resource that is in limited supply (such as food, water, or territory). Competition lowers the fitness of both organisms involved, since the presence of one of the organisms always reduces the amount of the resource available to the other.
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.
- Important: 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.
- Important: Totally unrelated species could also compete for the same resource
- 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.
Examples
- Example-1:Weeds growing in a garden compete with garden plants for soil nutrients and water.
- Example-2: Grasshoppers and bison in the Great Plains compete for the grass they both eat.
- Example-3: visiting flamingoes and resident fishes compete for their common food, the zooplankton in the lake
Cont...
Intraspecific & Interspecific competition
Competition among members of the same species is known as intraspecific competition, while competition between individuals of different species is known as interspecific competition.
Apparent Competition
..occurs when two otherwise unrelated prey species indirectly compete for survival through a shared predator. To be Cont...
Apparent Competition typically manifests in new equilibrium abundances of each prey species. For example, suppose there are two species (species A and species B), which are preyed upon by food-limited predator species C.
Apparent competition model
Scientists observe an increase in the abundance of species A and a decline in the abundance of species B
In an apparent competition model, this relationship is found to be mediated through predator C; a population explosion of species A increases the abundance of the predator species C due to a greater total food source. Since there are now more predators, species A and B would be hunted at higher rates than before.
- 1.:Thus, the success of species A was to the detriment of species B — not because they competed for resources, but because their increased numbers had indirect effects on the predator population...
- 1. The competitive exclusion principle postulates that two species which compete for the same limited resource cannot coexist at constant population values. When one species has even the slightest advantage over another, the one with the advantage will dominate in the long term. This leads either to the extinction of the weaker competitor or to an evolutionary or behavioral shift toward a different ecological niche. The principle has been paraphrased in the maxim "complete competitors cannot coexist"
Cont...
- 2. Georgy Gause formulated the law of competitive exclusion based on laboratory competition experiments using two species of Paramecium, P. aurelia and P. caudatum. The conditions were to add fresh water every day and input a constant flow of food. Although P. caudatum initially dominated, P. aurelia recovered and subsequently drove P. caudatum extinct via exploitative resource competition. However, Gause was able to let the P. caudatum survive by differing the environmental parameters (food, water). Thus, Gause's law is valid only if the ecological factors are constant.
Cont...
Outcome of Competition
Evidence for competitive exclusion occurring in nature is not always conclusive.
NCERT
Strong and persuasive circumstantial evidence does exist however in some cases.
Interference Competition-The 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. Hence, the feeding efficiency of the Abingdon tortoise was reduced due to the interfering and inhibitory presence of the goats even though resources i.e grazing land was abundant.
Outcome of Competition
1: Exclusion
Competitively inferior one will be eliminated eventually (Gause’s ‘Competitive Exclusion Principle’), if resources are limiting, but not otherwise. More recent studies do not support such gross generalisations about competition.
2: ‘Resource Partitioning’.
Species facing competition might evolve mechanisms that promote co-existence rather than exclusion. One such mechanism is ‘resource partitioning’.
If two species compete for the same resource, they could avoid competition by choosing, for instance, different times for feeding or different foraging patterns. MacArthur showed that five closely related species of warblers living on the same tree were able to avoid competition and co-exist due to behavioural differences in their foraging activities.
3: ‘Character Displacement’.
Commparison of allopatric versus sympatric populations of species shows evolutionary evidence of competition in nature. Example: Galápagos finches.
When two species occur on the same island (sympatric populations), they tend to exhibit greater differences in morphology (shape of beak) and resource use than when found on different islands (allopatric populations). “Tendency for characteristics to be more divergent in sympatric populations of two species than in allopatric populations of the same two species is called character displacement.
So..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.
-
Outcome-1
Exclusion or Extinction -
Outcome-2
‘Resource Partitioning’ -
Outcome-3
‘Character Displacement’
Predation
Definition-
Predation
is a nature’s way of transferring to higher trophic levels the energy fixed by plants.
- Example.1: Tiger and the deer
- Example.2: Sparrow eating any seed is also an example of predation.
- Example.3: Although animals eating plants are categorised 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 play other important roles. They keep prey populations under control. But for predators, prey species could achieve very high population densities and cause ecosystem instability. Explanation.....
The term "but for predation" means "without Predator."
The sentence is merely saying that without predation, prey population growth is more likely to be at a level which leads to ecosystem instability.Refernce from English Grammer : But for the storm, the train could leave Boston. = Without the storm, the train could leave Boston
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..
Biological control methods
The prickly pear cactus introduced into Australia in the early 1920’s 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.
- Moth:Cactoblastis cactorum
NCERT EXCERPT
Predators also help in maintaining species diversity in a community, by reducing the intensity of competition among competing prey species.
Keystone
as Predator
Robert Paine's Experiment
In the rocky intertidal communities of the American Pacific Coast 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
Keystone species
are those species whose importance to an ecosystem’s structure, composition, and function is disproportionately large relative to their abundance. These species can be of any life form, but they have in common an effect on their environment that is always greater than what can be expected based on their biomass .
Keystone Species
Term coined by Robert Paine.
- Example:Sea otters are a keystone species.They feed heavily on large herbivorous invertebrates such as sea urchins.When sea otters keep sea urchin populations low, the huge algae (called kelp) can grow more readily and form forests that are home to a diversity of fish and invertebrates.
- Result:If sea otter populations are reduced, then the near-shore community is overrun with sea urchins and species diversity is reduced.
Why predators in nature are ‘prudent’ ?
If a predator is too efficient and overexploits its prey, then the prey might become extinct and following it, the predator will also become extinct for lack of food.
Camouflaged
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..
Pray with..
Warning Colouration
Pray-Predation
Warning Colouration
Some Pray are poisonous and therefore avoided by the predators.-Dart frogs (Phyllobates and Dendrobates) found in tropical rain forests of South America are highly poisonous as well as brightly coloured to be easily noticed. Predators usually avoid them.
Two types of Mimicry..
1. Batesian mimicry
First Types
Batesian mimicry
The mimic is defenseless. It has, however, resemblance to a dangerous or unpalatable model so that the predator usually does not prey upon it, e.g., Viceroy Butterfly mimics unpalatable toxic Monarch Butterfly.
Two types of Mimicry..
Mullerian mimicry
Second Type
Mullerian mimicry
It is resemblance to two animal species, especially insects, both unpalatable/ferocious, to their mutual benefit, e.g., Monarch Butterfly and Queen Butterfly
NCERT EXCERPT
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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
'Prudent’in Nature
PRE.DATORS
For plants, herbivores are the predators.
PREDATION
Herbivores are the predators
Predation may involve two animal species, but it can also involve an animal or insect consuming part of a plant, a special case of predation known as Herbivory
PREDATION
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 chemical defences against herbivores
Cardiac glycosides
Many plants produce and store chemicals that make the herbivore sick when they are eaten, inhibit feeding or digestion, disrupt its reproduction or even kill it. You must have seen the weed Calotropis growing in abandoned fields. The plant produces highly poisonous cardiac glycosides and that is why you never see any cattle or goats browsing on this plant
Defences
Plants evolved an astonishing variety of morphological and chemical defences against herbivores. Thorns (Acacia, Cactus) are the most common morphological means of defence. 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.
PARASITISM
Co-evolution
parasites have evolved to be host-specific
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 parasitise only a single species of host) in such a way that both host and the parasite tend to co-evolve.
- Co-Evol: If the host evolves special mechanisms for rejecting or resisting the parasite, the parasite has to evolve mechanisms to counteract and neutralise them, in order to be successful with the same host species.
Special adaptations for Parasitism
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1
Loss of unnecessary sense organs
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2
Presence of adhesive organs or suckers to cling on to the host
-
3.
Loss of digestive system
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Important
- High reproductive capacity.
Very Important
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 (mosquito) to spread to other hosts. The female mosquito is not considered a parasite, although it needs our blood for reproduction.
Parasites
reduces population density of Host.
Majority of the parasites harm the host; they may reduce the survival, growth and reproduction of the host and reduce its population density.
- MCQ: Parasites might render the host more vulnerable to predation by making it physically weak.
- 1: Parasites that feed on the external surface of the host organism are called ectoparasites. The most familiar examples of this group are the lice on humans and ticks on dogs.
- 2: Many marine fish are infested with ectoparasitic copepods.
Examples of Parasitism
- Cuscuta: a parasitic plant that is commonly found growing on hedge plants, has lost its chlorophyll and leaves in the course of evolution. It derives its nutrition from the host plant which it parasitises.
- Brood parasitism : 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. 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.
‘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 emphasising their reproductive potential.
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Commensalism
An orchid growing as an epiphyte on a mango branch
‘This is the interaction in which one species benefits and the other is neither harmed nor benefited.'
- Ex.: 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.
The cattle egret and grazing cattle in close association, a sight you are most likely to catch if you live in farmed rural areas, is a classic example of commensalism.
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The cattle egret and grazing cattle in close association, 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 insects from the vegetation 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
Mutualism
This interaction confers benefits on both the interacting species.
‘MUTULISM'
Lichens represent an intimate mutualistic relationship between a fungus and photosynthesising algae or cyanobacteria. 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.
Picture : Lichen on tall trees. Walking through the forest surrounding by these towering giants, I was inspired by the sheer amount of lichen clinging to the trees. I especially found the patterns it took on the trees, forming thick lines up the sides.
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Plant-Animal Relationships-
The most spectacular and evolutionarily fascinating examples of mutualism are found in plant-animal relationships.
‘Plant-Animal Relationships'
Plants need the help of animals for pollinating their flowers and dispersing their seeds. Animals obviously have to be paid ‘fees’ for the services that plants expect from them. 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.
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‘plant-animal interactions'
often involve co-evolution of the mutualists, that is, the evolutions of the flower and its pollinator species are 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 oviposition (egg-laying) site but uses the developing seeds within the fruit for nourishing its larvae.
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‘plant-animal interactions'
The Blastophaga 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.
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‘Sexual deceit’
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.
‘Pseudocopulates’
In Mediterranean orchid Ophrys, One petal of 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, ‘pseudocopulates’ 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 you can see how co-evolution operates. 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.
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