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Allee Effect is when a relationship is identified between the size of the population or density and the fitness of an individual found within it. A strong Allee Effect is noticed when the population is low or critical with a negative growth rate and a weak Allee Effect is noticed when the population is low but the growth rate is noticed to be positive. It has two manifestations and three mechanisms.
The allee effect reduces the speed of the expansion of range for a population and thus prevents biological invasions. Mate limitation is one of the most common mechanisms that cause the allee effect on plants as well as animals.
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Key Terms: Population, Rate, Regions, Density, Fitness, Income
What is Allee Effect?
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Allee effect is a process of identifying a relationship between population size or density and the average person’s fitness. It is supposed to be greatly common and happens in regions with a growing population. Allee Effect is categorised by the condition of density dependence, type is comparatively lower. A region where the population shrinks to lower densities will notice a higher allee effect and when the proliferation rate is going up and is positive, then the allee effect will tend to be weak. If the proliferation rates are going down at a low density but the rate is positive, then it is called a null hypothesis.
Some of the factors contributing to Allee Effect are:
- The property to exhibit density or small population size of a particular region is one of the main factors to differentiate between the two types of Allee Effect.
- A population that will have a critical population size or density, where population growth is negative, will have a strong Allee Effect.
- However, a region with a low per capita growth rate and a low population size but having a positive per capita growth rate will show a weak Allee Effect. So a population that hits a level lower than the threshold will become extinct.
Schematic Representation of Allee Effect
Allee noticed that being in a collection made land isopods numbers better, however, their surviving rate decreased when they were subjected to isolation. These facts, however, contradicted the Malthusian paradigm and the Logistic model. So after making note of both kinds of Allee effects, we get the following:
- An Allee effect is a mutual relationship, often positive, between the size of the population and absolute average individual fitness over a particular period.
- This relation, however, may take the population density to a critical point where the population ceases to exist.
Per Capita Growth Rate vs Population Size
Manifestations
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The allee effect shows two manifestations:
- Component Allee Effect: This is shown when a population has a positive relationship between some components of fitness.
Component Allee Effect
- Demographic Allee Effect: This is found when the component Allee effect shows a positive relationship between population size and the per capita population growth.
Demographic Allee Effect
Mechanism of Allee Effect
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There are three mechanisms of the allee effect, namely,
- Ecological Mechanism
- Human Induced
- Genetic Mechanisms
Ecological Mechanism:
The characteristics of Ecological Mechanism are:
- Men Cooperative defence, Mate limitation, cooperative feeding and environmental conditioning are the factors contributing to the Allee Effect.
- Mate limitation is when finding a suitable mate for sexual reproduction becomes difficult due to the lower population. This is found in planktons, sessile vertebrates and plants.
- Cooperative defence is when the group adopts anti-predator behaviour to fight against predators. Less energy and time is spent on foraging because of this.
- Cooperative feeding is when the group searches for survival food.
- Environmental conditioning is when the individuals work together for improving the environmental conditions for the benefit of their species.
Human-Induced:
The characteristics of Human Induced Mechanism are:
- The killing that is done by humans on the population results in the species’ extinction.
- Rare species are costlier than the common species due to being more desirable.
- This results in an anthropogenic allee effect where the common species survive and the rarer species die out.
Anthropogenic Allee Effect
Genetic Mechanisms:
The characteristics of Genetic Mechanism are:
- When the population size becomes low, the diversity in their genes is lost.Allee effect is noticed under such circumstances.
- So, the decrease of the population size results in the decrease of the fitness of the species.
Genetic Allee Effect and their Interaction with Ecological Allee Effect
Examples of Allee Effect on Different Taxa
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Following are the examples of the Allee effect:
- Fruit flies have the potential to attack 400 crops globally, making them one of the most dangerous pests. Sterile males are used to controlling them, which tends to show the Allee effect. However, To avoid the Allee effect, natural enemies are released in a large number.
- The Allee effect can result in a species’ extinction, when the density is low, as the mating encounters become low.
- Schooling fishes should be done with group-living animals, as a high harvesting pressure might lead to potential population disintegration.
- Some endangered species have way fewer numbers to locate responsive mates, and sometimes they have an unequal sex ratio.
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Things to Remember
- Allee Effect is categorised by the condition of density dependence, his type is comparatively lower.
- A region with low population density experiences a higher allee effect as compared to regions with higher population densities.
- The property to exhibit density or small population size of a particular region is one of the main factors to differentiate between the two types of Allee Effect.
- Cooperative defence, mate limitation, cooperative feeding and environmental conditioning are the factors contributing to the Allee Effect.
Sample Questions
Ques. Define the Allee Effect. (2 marks)
Ans. The Allee effect is a system in biology identified with the aid of correspondence among population size or density and therefore the average person’s fitness that is categorised by way of the essence of dependence on density which is relatively lower. Factors like mate predicament, environmental conditioning, cooperative feeding and cooperative defence contribute to the Allee Effect.
Ques. What are the Manifestations of the Allee Effect? (3 marks)
Ans. There are two kinds of Manifestations of the Allee Effect, namely,
- Component Allee Effect: This manifestation holds true when there's a positive association between some fitness components in a population. When the population is low, the component allee effects sum up and produce a demographic allee effect.
- Demographic Allee Effect: This manifestation holds true when the component Allee effect proves to have a positive association between the population size and the per capita increase. When the population is high, the competition for resources causes the depletion in the demographic Allee effect.
Ques. How is the Allee Effect related to the population? (3 marks)
Ans. Allee Effect is related to the population factor in the following ways:
- Strong Allee effect can be found in a region with a large population density. In other words, regions with negative population growth show strong allee effects.
- Weak Allee Effect becomes noticeable in regions with a low per capita growth rate and a low population size. However, its per capita growth rate is positive.
- Population below the threshold point comes to an end.
Ques. Explain the Ecological Mechanism. (4 marks)
Ans. The Ecological Mechanism is when the Cooperative defence, Mate limitation, cooperative feeding and environmental conditioning are the major factors that contribute to the Allee Effect. Mentioned below are the details:
- In Ecological Mechanism, mate limitation is easily noticable which is troubles arising for finding a suitable mate for sexual reproduction due to lower population.
- Cooperative defence is another factor contributing to allee effect which can be seen when a certain population adopts anti-predator behaviour. This behaviour is adopted to fight against the predators.
- Cooperative feeding is yet another factor contributing to the allee effect which can be seen when a certain group searches for survival food.
- Environmental conditioning occurs when the individuals work together for improving the environmental conditions for the benefit of their species.
Ques. Explain the Human-Induced Mechanism. (2 marks)
Ans. The Human-Induced Mechanism can be seen by the killing of species of animals done by humans such that the species’ population results in its extinction. Rare species of animals tend to cost more than the common species due to possession of more unique and desirable traits that could possibly benefit some categories of population. This results in an anthropogenic allee effect where the common species survive and the rarer species die out.
Ques. Explain Genetic Mechanism. (2 marks)
Ans. Genetic Mechanism is observed when the size of a population becomes too low that the diversity in their genes is lost. Hence, a decrease in the size of a population of a certain region results in the decrease of the fitness of the species and the loss of genes.
Ques. What is the Allee Threshold? (2 marks)
Ans. The Allee threshold is the population density and below this point, the growth is observed to be negative. This usually depends on sex ratio, the life history parameters determining survival and reproduction and age structure. The Allee threshold depends on both Frequency and Density. The Allee threshold depends on the age structure because immature individuals die out in younger populations sometimes before reproducing. So the younger populations need persistence of higher calibre than their predecessors.
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