REPRODUCTION POTENTIAL

 

bulletmajor fluctuations in numbers; can go from endemic to epidemic very quickly à \ we want to be able to predict pop’n change
bulletthis depends on two major forces: reproductive potential and environmental resistance

 

 

REPRODUCTIVE POTENTIAL 

 

1.        fecundity (e)- ability to produce young (# of eggs varies from few to hundreds of thousands), it is the # eggs ♀ is capable of producing ( not the usual or actual #) i.e. under ideal conditions (i.e. a ♀ has not suffered ill from environment)

 

2.        sex factor (s)- proportion of ♀ (individuals capable of bearing young)  i.e. even ♂ & ♀ = 0.5
parthenogenesis - ability of ♀ to reproduce without ♂ (fertilization);
partial parthenogenesis common in hymenoptera (larch sawfly ♀ = 0.96) and
complete parthenogenesis occurs in homoptera (balsam woolly adelgid ♀=1.0)

 

3.        polyembryony (p) - single egg can produce one/few/many individuals – hymenoptera

 

4.        length of development period = number of generations (n)- time from egg to reproductive adult; e.g. fruit fly as short as 2 weeks (i.e. ~25 generations in a year), many are 1 year, env. Conditions can prolong to 2 or even up to 4 years,  periodic cicada has a life cycle of 17 years!  Latitude & altitude are factors: Nantucket pine tip moth has 4 generations/yr in Georgia, 3 in Virginia, 2 in Ohio, and 1 in Ontario

 

The standard equation of reproductive potential = dN / dt (= change in population over change in time) = Nrn

Where,

            N = original pop’n

            r = product of fecundity = (# eggs per female * sex factor * polyembryony)

            n = # of generations

 

However, that approach to the equation is confusing.  I have thought of another way to present the same notion.  Those with a keen interest in watching sports on TV will appreciate it (e.g. the sports network ESPN).

N(e*s*p*)n

 Where,

            N = original pop’n (traditionally the value of 1 is used, but any size can be used)

            e = # eggs per female

s = sex factor (proportion of females)

p = polyembryony (# of young produced by one egg, e.g. do you typically get twins or triplets?)

            n = # of generations

 

in example:

 

spruce budworm: e=359, s=0.5, p=1

            start with 2 individuals for 1 generation (Ontario) … = 2 * (359*0.5*1)1 = 359

            start with 2 individuals for 4 generations (Georgia) … = 2 * (359*0.5*1)4 = 2,076,289,020

 

some wasp x: e=359, s=0.5, p=4 (e.g. quadruplets)

            start with 2 individuals for 1 generation … = 2 * (359*0.5*4)1 = 1,436

            start with 2 individuals for 4 generations … = 2 * (359*0.5*4)4 = 26,576,499,000

 

balsam woolly adelgid: e=248, s=1, p=1

            start with 2 individuals for 1 generation … = 2 * (248*1.0*1)1 = 496

            start with 2 individuals for 4 generations … = 2 * (248*1*1)4 = 7,565,484,032

            start with 496 individuals for 3 generations … = 496 * (248*1*1) 3 = 7,565,484,032

  

Thus insects have tremendous potential of creation,  note the factors:

fecundity (max. eggs/female),

sex factor (proportion of females, budworm vs. adelgid),

polyembryony (twins? Quadruplets?, budworm vs some wasp), and

number of generations - this is a factor of time (1 year vs. 4 years) and rate of development (warmth, or accumulated heat)

 

This potential is counter balanced by environmental resistance