1) The Allee effect describes how individual fitness of a population decreases at low densities due to reduced cooperation for tasks like mate finding, predator defense, and environmental conditioning. 2) There are two types of Allee effects - strong and weak. Strong Allee effects occur when per capita growth rates become negative below a critical density threshold, whereas weak Allee effects occur when per capita growth rates decline but remain positive. 3) Examples of species exhibiting Allee effects include fruit flies, where sterile males are used to control populations, and schooling fish, where overfishing can cause population disintegration due to reduced cooperation at low densities.

1. BSC. (H) BIOTECHNOLOGY
SEMETER- V
SUBJECT: BIOMATHEMATICS
PRESENTATION
TOPIC: ALLEE EFFECT
PRESENTED BY:
JOYDEEP PAL (BWU/BBT/18/036)
AAKANKSHA KRISHNA (BWU/BBT/18/041)

2. HISTORY
• Allee effect described by Warder
Clyde Allee (1885-1955).

3. POPULATION REGULATION
• Two types: 1. Density dependent; 2. Density independent
• 1. Density dependent: populations, birth rate and death
rate do not change with population density. Factors: Birth
rate, death rate, nutrition…
• 2. Density independent: Population, birth rates fall but the
death rate rise.
• Factors (environmental): Flood, earthquake….
• For example: Population increasing means birth rate also
increasing but date rate also rising – Positive feedback
(which is called Allee effect).
• If Birth rate decreasing and death rate increasing with the
density– Negative feedback.

4. WHAT IS ALLEE EFFECT?
• The Allee effect is a phenomenon
in biology characterized by a correlation
between population size or density and the
mean individual fitness (often measured as per
capita growth rate ) of a population or species.
• The generally accepted definition of Allee effect
is positive density dependence, or the positive
correlation between population density and
individual fitness.
• It is sometimes referred to as "undercrowding"
and it is analogous to “depensation" in the field
of fishery sciences.

5. STRONG ALLEE VS WEAK ALLEE EFFECT

6. MECHANISM
 Due to its definition as the positive correlation
between population density and average fitness, the
mechanisms for which an Allee effect arises are
therefore inherently tied to survival and reproduction.
 In general, these Allee effect mechanisms arise from
cooperation or facilitation among individuals in the
species.
 Examples of such cooperative behaviors include
better mate finding, environmental conditioning, and
group defense against predators.

7. MATHEMATICAL EXPRESSION
• As a result of Allee’s work , a population
where the per capita growth rate declines at
low population size is said to show an Allee
effect. One of the simplest mathematical
models, a variation of the logistic equation is:
dN/dt= rN (1-N/K) (N/A-1)
• Where, A is called the Allee threshold. The
value of N(t) A is the population size below
which the population growth rate becomes
negative due to allee effect.
• N=population size; K= carrying capacity; r
= intrinsic rate of increase.

8. EXAMPLES OF ALLEE EFFECT IN VARIOUS
TAXA
• Fruit flies are considered to be one of the most dangerous pests in agriculture as they have the potential to attack
approximately 400 crops globally. Usage of sterile males is one of the techniques used to control them which creates
Allee effect.
• It is important in schooling fishes which are group living animals. If harvesting pressure is high, it may cause a potential
population disintegration.

9. ACKNOWLEDGEMENT
• I would like to thank our respected HOD sir and I also thankful to Souvik
Sinha sir to give this opportunity to present in this interesting topic.