Conservation Biology

1. Minimum Viable Population and
Effective Population Size
Conservation Biology

2. Minimum Viable Population
 A minimum viable population size (MVP) can be
defined as the smallest size required for
a population or species to have a predetermined
probability of persistence for a given length of time

3.  The probability for long-term persistence of a
species depends on whether the species can avoid
the erosion of genetic variability that can occur in
small populations.
 When genetic variation is reduced, the ability of a
species to adapt to environmental change may
become restricted.

4.  In these cases, genetic variability can be
substantially reduced through inbreeding (mating
between close relatives) and genetic drift (random
changes in gene frequencies).
 Inbreeding and genetic drift both can result in an
increased chance for the transmission of harmful
traits to subsequent generations, which ultimately
affects population and species viability.

5. Genetic bottlenecks
 When the individuals are genetically weak, in any
case of disease they can not survive because of
interbreeding and having no resistance to diseases.
 This is known as genetic bottlenecks.

6. Franklin Rule
 One of the earliest attempts to define a minimum
lower threshold that would prevent the loss of
genetic variability in a species was made in 1980 by
Australian geneticist Ian Franklin and American
biologist Michael Soulé.
 They created the “50/500” rule, which suggested that
a minimum population size of 50 was necessary to
combat inbreeding and a minimum of 500 individuals
was needed to reduce genetic drift. Management
agencies tended to use the 50/500 rule under the
assumption that it was applicable to species
generally.

7.  In general, the results of Population Viability Analysis
modeling indicate that species with high reproductive
capacities, such as arthropods and rodents, can
accommodate lower MVPs than species with lower
reproductive capacities, such as redwood trees and
large mammals and some birds.

8. Demographic Stochasticity
 The PVA ( Population Viabilty Analysis) model also
incorporates environmental and demographic
stochasticity.
 Demographically stochastic events are random
fluctuations in population variables, such as sex
ratios and number of births or deaths.

9. Environmentally Stochasticity
 Environmentally stochastic events are random
events, such as severe weather, floods, fires, and
other ecological disturbances.

10.  Depicting such events with PVA has the effect of
increasing the model’s MVP estimate, because both
types of phenomena have the potential for reducing
population size, either by increasing the death rate
or increasing the annual variability with respect to
the birth rate.

11.  Estimating MVP with PVA allows scientists to
determine which biological parameters (e.g., hunting
pressure, disease, habitat loss, inbreeding) will have
the greatest impact on the extinction probability of a
given species. This information can provide
environmental managers with a set of quantitative
targets for the minimum critical area required to
support a viable population.

12.  One major limitation of PVA is that it requires large
amounts of data to make realistic predictions.
Therefore, some researchers argue that using a
single, universal MVP (such as the 50/500 rule)
would streamline conservation efforts. Others,
however, maintain that MVPs must be carried out in
a case-by-case fashion, because the circumstances
that characterize extinction risk differ among species.

13. Effective Population Size
 The effective population size is the number of
individuals that can contribute to the next generation.
It is denoted by Ne.
 Over aged are not included.
 Adults that can not increase progeny not included.

14. Factors which affect the Ne
 Unequal sex ratio can affect the effective population
size.
 For say, we have
 1:4 of female and male in case one.
 2:7 of female and male in case two.
 5:9 of female and male in case three.
So we have all different ratios.
When sex ratios are not equal or near to equal the
population size can not defined as effective population
size.

15. Reproductive Output of females
 The reproductive output of female can affect the
population size.
 For example, A goat can give birth to 3 baby goats
 On the other hand, second goat can give birth to
only one.
 So, when reproductive output is different a
population size can affect.

16. Population Fluctuation
 Many individuals can migrate from one populated
are to another populated group so, population
number and size could be affected by their
immigration or emigration.