POPULATION GROWTH CURVE

           Introduction

• Population growth curve is something that shows the pattern of the growth of a population over  time  i.e., a growth curve is an empirical model of the evolution of a quantity  over  time. 

• Growth curves are widely used in biology for quantities such as population size or  biomass , individual body height or biomass. Values for  the measured property can be  plotted on a graph as a function of time.

• In theory , any kind of  organism could take over  the Earth just by reproducing.

•  But in practical, all living  organisms need some specific resources such as nutrients and proper environmental conditions to survive and reproduce.

• The essential resources are not unlimited, and a population can only reach a size that match the availability of resources in its local environment.

TYPES OF POPULATION GROWTH CURVES

 There are mainly two types of population growth curves-

     1. Exponential growth curve( J-shaped)

                                  &

    

      2. Sigmoid growth curve(S-shaped)

EXPONENTIAL GROWTH CURVE

• —  In this type of growth form, the population grows exponentially, and after attaining the peak value the population may abruptly crash.

• This increase in population is continued till large amount of food materials exist in the habitat.

• — After sometime, due to increase in population size the food supply in the habitat becomes limited which ultimately results in decrease in population size.

• For example, many insect populations show explosive increase in numbers during the rainy season, followed by their disappearance at the end of the season.

• The following equation exhibits J-shaped growth:

             dN/dt = rN                                             

                                         

             where,   

                                    

            dN/dt represents rate of change in population.

             r=biotic potential

             N= population size 

SIGMOID GROWTH CURVE 

      

• The sigmoid growth curve is also known as Logistics growth curve which is S-shaped.           

• When a few organisms are introduced in an area, the population increase is very slow in the beginning i,e., the positive acceleration phase or lag phase.            

• In the middle phase the population increase becomes very rapid which is known as logarithmic phase.     

• Finally in the last phase the population increase is slowed down i,e. negative acceleration phase.

 —           

• The level beyond which no major increase can occur is referred to as saturation level or carrying capacity which is denoted by K.—            

• In the last phase the new organisms are almost equal to the number of dying individuals and thus there is no more increase in the population size.         

• The J-shaped (sigmoid) growth form is represented by the following equation:

         dN/dt = rN(K-N/K)                   

                   =rN(1-N/K)

                     Here,

                    dN/dt is the rate of change in population size.

                     r= biotic potential

                     N= population size

                    (K-N/K) is for environmental resistance.

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CARRYING CAPACITY

                                    

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• The carrying capacity of a biological species in an environment is the maximum population size of the species that the environment can sustain indefinitely , given the food , habitat , water and other necessities available in the environment.

• — In population ecology, the carrying capacity is defined as the environment’s maximal load.

•  Carrying capacity was originally used to determine the number  of  animals that could graze on a segment of land without destroying it. Later the idea has been expanded to more complex populations.