Dynamic parameters of a community: birth rate, death rate, growth rate of the community

Lecture 6.

Dynamic parameters of community reflect the processes proceeding in the community for certain time interval. The basic parameters from them: birth rate, death rate, growth rate of the community.

Birth rate is a number of the new species who have appeared in the community for a time unit in the result of breeding.

There are the maximal and actual birth rates. The maximal birth rate is a maximal realization of a birth opportunity in the absence of limiting factors of environment. Actual birth rate is a real realization of the birth opportunity.

There are absolute and specific birth rates. Absolute (general) birth rate or velocity of the birth rate is expressed by relation:

DN / dt,

where dN is a number of species (eggs, seeds and etc.), which were born for certain time interval dt.

Specific birth rate (B) is a relation of the birth rate velocity to initial number (Ndt):

B = dN/(Ndt).

This size depends on intensity of species breeding: for bacteria - hour, for insects - week or month, for large mammals - year.

Death rate (velocity of the death rate) is a number of the species who have lost in the community for a time unit (from predators, illnesses, an old age and other reasons). Death rate is a size return to the birth rate.

There are the minimal and actual death rates. The minimal death rate is a minimally possible size of the death rate. Actual death rate is a real size of the death rate.

There are absolute and specific death rates. The absolute (general) death rate or death rate is defined by the attitude:

dN / dt.

Where dN is a species number, who have lost for a time interval dt.

Specific death rate (d) is a relation of death rate to initial number (Ndt):

D = dN / (Ndt).

Rate of a growth community is a change of the community number in unit of time. Rate of a growth community can be positive, zero and negative. It depends on parameters of birth rate, death rate and migration (immigrations and emigrations). The rise (profit) of number occurs in the result of the birth rate and immigration of species, and reduction (loss) of number is in the result of the death rate and emigration of species.

There are absolute and specific growth rates of community.

Absolute (general) growth rate is expressed by the relation:

dN/dt,

where dN is a change of the community number for a time interval dt.

Specific growth rate is a relation of growth rate to initial number (N):

dN/(Ndt).

In the absence of limiting factors of environment specific growth rate is equal to size r which characterizing properties of the community and it is called specific (congenital) growth rate of the community or biotic potential of a kind:

r = dN/(Ndt) или dN/dt = rN.

The value of biotic potential is differed for different species. For example, roe is capable to make for a life of 10-15 kids, female of melliferous bee - 50 thousand eggs, a fish-moon - to 3 billion of calves.

However in the nature, due to action of limiting factors, the biotic potential of community is never realized completely. Usually its size is defined as a difference between birth rate and death rate in the population:

R = b - d.

Where b is a number of born species, d is a number of lost species in the community for the same period of time.

When b = d, r = 0 and the community is in stationary conditions. When b> d, r> 0, number of the community increases. When b <d, r <0, number of the community is decreased. The formula d = b - r makes it possible to define death rate which it is difficult to measure directly, and it is possible to simply define r by direct supervision.

Growth rate can be expressed in the form of a curve of population growth. There are two basic models of the community growth: J-shaped and S-shaped.

The J-shaped curve reflects unlimited exponential growth of the community number, not depending on density of the community. Such type of growth is possible while biotic potential of the community (r) is realized completely. It proceeds while the competition for resources is low. However after excess of environment capacity (limiting density of saturation, limiting number) (K) there will be a sharp decrease in number.

S-shaped (logistical) curve reflects logistical type of the growth depending on density of the community at which growth rate of the community decreases in process of number growth (density). If the number of community reaches the maximum the growth rate decreases to zero.

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