Lecture 10: Small Populations

Question 1

Minimum viable population (MVP)

Answer 1

“the smallest isolated population having a 99% chance of remaining extant from 1000 years despite the foreseeable effects of demographic, environmental, and genetic stochasticity, and natural catastrophes” (Shaffer 1981)

Question 2

Minimum dynamic area (MDA)

Answer 2

the area of suitable habitat necessary for maintaining the minimum viable population

Question 3

How can Minimum dynamic area be estimated?

Answer 3

MDA can be estimated from the home range size of individuals and colonies

Question 4

What are some examples of minimum dynamic area (MAKE SURE THIS IS THE RIGHT QUESTION BEING ASK FOR THE ANSWER****)

Answer 4

-E.g., studies suggest that 100-1,000 km squared are needed to maintain small populations in Africa - large carnivores require 10,000 km squared reserves - E.g., Mountain Lions (puma concolor) in the Santa Monica Mountains California

Question 5

Small populations are subject to rapid declines and extinctions for three main reasons:

Answer 5

1. Loss of genetic variability 2. Demographic fluctuations 3. Environmental fluctuations

Question 6

Allele

Answer 6

• one of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome - i.e. is alternative forms at particular gene

Question 7

Genetic drift

Answer 7

• AKA random genetic drift - the change in the frequency of an allele in a population due to random sampling

Question 8

Effective population size (Ne)

Answer 8

• the size of a population as estimated by the number of breeding adults - assumes males and females are all breeding adults, are equal in number, and all contribute equally to the next generation

Question 9

Heterozygosity

Answer 9

• the proportion of individuals that inherited 2 different alleles for a particular gene

Question 10

What equation do you use to calculate heterozygosity?

Answer 10

H = H_t/H_o = [1 – 1/(2N_e)]

H_t = H at some time t

H_o = H naught - the heterozygosity you started with

Question 11

E.g. of heterozygosity question:

A population of 50 breeding individuals woud retain 99% of its original heterozygosity after 1 generation. Use the equation to calculate the heterozygosity.

Answer 11

H = [1-1/(2*50)] = 1.– 0 0.01 = 0.99

Question 12

What is the equation for Heterozygosity remaining after t generations?

Answer 12

H_t = H^t

Question 13

What is an example of heterozygosity remaining after t generations?

Answer 13

E.g., after 2 generations, our population of 50 would have a heterozygosity of:

H₂= 0.99² = 0.98

• significant losses of genetic variability can occur quickly if the population is small

Question 14

Inbreeding depression

Answer 14

• a condition that occurs when an individual receives two identical copies of a defective allele from each of its parents
• leads to the expression of deleterious recessive alleles
• results in higher offspring mortality, fewer offspring, offspring that are weak, sterile or have low mating sucess

Question 15

Genetic rescue

Answer 15

• Definition: improved population fitness that results from interbreeding with immigrant individuals
• loss of genetic diversity through genetic drift and inbreeding can be devastating for populations
  - E.g., population size and heterozygosity were at dangerously low levels for the Florida Panther
  - In 1995, translocation of 8 adult females from Texas led to a dramatic population recovery

Question 16

Population fluctuations

Answer 16

N_e = t / (1/N_{<span>1</span>} + 1/N₂ +… + 1/N_t)

N_e = the amount of time that we measure it (t) / (1/ # of individuals in the first time period + 1/# individuals in the second time period +…+ 1/N_t)

• this equation is the harmonic mean
• N_e is affected by many factors
• N_e may also vary over time
  - especially true for certain species, such as butterflies and annual plants
• N_e in a fluctuating population falls between minimum and maximum populations sizes

Question 17

Population fluctuations example

Answer 17

• e.g., if we were to monitor a population of Karner Blue Butterflies (Lycaeides melissa samuelis) for 5 consecutive years with population sizes of 10, 20, 100, 20, and 10 breeding individuals:

         N<sub>e</sub> = 5 / (1/10 + 1/20 + 1/100 + 1/20 + 1/10) = 16.1

• Note that N_e is much closer to the smaller population size
• a single year of a small population size will drastically lower N_e
• the harmonic mean is ALWAYS going to be less than the arithmetic mean

Question 18

Population bottleneck

Answer 18

• pronounced drop in population size followed by a population recovery
  - rare alleles are lost if no individuals possessing those alleles survive and reproduce
  - the resulting decline in heterozygosity leads to a drop in the average fitness of the population
• the longer a bottleneck lasts - the worst the effects/ the more severe the effects on the heterozygosity of that population

Question 19

Founder effect

Answer 19

• when a few individuals leave one population to establish a new population
  - a special category/type of bottleneck

Question 20

population bottleneck examples

Answer 20

• E.g., the European Bison underwent a severe population bottleneck

FINISH SLIDES!!!!!!

Question 21

Stochasticity

Answer 21

• random variation leading to uncertainty of outcome

Question 22

Demographic stochasticity

Answer 22

• chance variations in the sex ratio of offspring or the survival and reproductive success of individuals can prevail over what is expected on average
• especially important for small populations

Question 23

Allee effect

Answer 23

• a correlation between population size or density and the mean individual fitness (often measured as per capita population growth rate) of a population or species
• e.g., if population density is very low, finding a mate can be very difficult or impossible

Question 24

Environmental stochasticity

Answer 24

• random variation over time in environmental conditions that affect reproduction or survival of individuals
  - not as dependent on population size as demographic stochasticity
  - more dependent on size of geographic range of a speices
  - E.g., droughts, storms, earthquakes, fires, disease outbreaks

Question 25

Extinction vortex

Answer 25

• the smaller a population becomes, the more vulnerable it is to further demographic variation, environmental variation, and genetic factors that tend to lower reproduction, increase mortality rates, and so reduce population size even more, driving the population to extinction