Download Lecture 15 - Small Populations

Introduction to Wildlife & Fisheries Conservation
WFSC 304
Lecture 15: Small Populations
The population is a typical unit of management. Your job as a conservation
biologist is to prevent extirpation of existing populations of native (some game)
species that are struggling or to establish new populations of endangered species.
Your job as a natural resource manager is to maintain ample populations of utility
species (e.g. game) for harvest. Both jobs rely on understanding population
biology, ecology and genetics and using that knowledge to manage populations.
The former almost always is about the biology of small populations, and
sometimes the latter is as well (e.g. Atwater’s PC).
Minimum Viable Population (MVP)
• Schaffer (1981) MVP: “A minimum viable population for any given species in any
given habitat is the smallest isolated population having a 99% chance of remaining
extant for 1000 years despite the foreseeable effects of demographic,
environmental, and genetic stochasticity, and natural catastrophes.”
Populations closer to zero are far more prone to go extinct (extirpation) because a
random jump in population numbers, or string of jumps mostly negative, could
cause the population to hit zero, which is an absorbing state. Zero population size
can only be increased by immigration, if there is any.
See demo (Stochastic demographic drift simulation.xls).
Population viability analysis (PVA)
– estimate the likelihood of a population’s extinction over time
– indicate the urgency of recovery efforts
– identify key life stages or processes that should be the focus of recovery efforts
– compare proposed management options and assess existing recovery efforts
– endangered species management to develop a plan of action
Example from book:
Minimum Dynamic Area (MDA)
•
Area needed to maintain the population….
(thought experiment—what data would one need to estimate)
3 Major Risks to Small Populations
•
Loss of genetic variability
•
Demographic fluctuations
•
Environmental stochasticity
In general, populations have a
greater chance of persisting when
they occupy large habitats
Why?
 more abundant resources
 environmental impacts can
be patchy
 more niche space for
organisms
Why is genetic variation necessary for natural populations?
After Falk DA, Knapp E, Guerrant EO (2001) available here: https://www.nps.gov/plants/restore/pubs/restgene/1.htm
1. Genotypes partly determine organisms' physical form and function
2. Genetic diversity helps organisms cope with current environmental variability
(e.g. MHC loci)
3. Diversity within populations reduces deleterious effects of breeding among
close relatives
4. Genetic diversity is the basis for adaptation to future environmental change
Add that heterosis (vigor through outbreeding) is important as well. MHC is a sub
example of this general effect.
Loss of Genetic Variation within Populations — a function of population size
1.
Genetic Drift - gene
frequencies within a
population that change
over time due to random
processes; increases in
smaller populations; each
generation retains just a
portion of the gene pool
from the previous
generation. Just as with
extirpation, alleles
starting near zero
frequency are the ones
most prone to disappear.
Rare alleles go first. Why?
2.
Founder effects - colonization of a new area by a small group of individuals;
the individuals of the group represent a smaller proportion of the gene pool from
the source population. Rare alleles unlikely to be represented by a small group of
founders.
3.
Genetic bottleneck - reduction of a population to a very low level resulting in
a) loss of certain alleles, especially rare ones, and b) reduction in the amount of
variation in genetically-determined characters
4.
Inbreeding depression - loss of fitness due to genetic recombination
(reproduction) among closely related individuals; smaller populations have a
higher probability for the expression of deleterious recessive alleles
Ne
Within generations it is important to know the number of breeding males and
females. In which case in generation t, the Ne = 4N♀N♂/( N♀+N♂)
Among generations, low numbers are particularly corrosive of genetic variation, a
dynamic captured by the harmonic mean: Ne = t/(1/N1 + 1/N2 … + 1/Nt)
[live DEMO; be able to do the calculations on your own]
What major group of animals is notable for temperature dependent sexual
differentiation?
Ne is such a foundational parameter. From it you can predict the genetic variation
(proportion heterozygous individuals) remaining in a population in the next
generation (or over time if carried out; Wright 1931): H = 1-1/(2 Ne)
Do examples in your head. Always explore extremes—what if N is large? What if
small?
So genetic variation in a population can be decreased by:
 genetic drift
 emigration
 directional or stabilizing selection (not in book)
Genetic variation can be increased by:
 mutation
 immigration
 disruptive or divergent selection (not in book)
Understand by drawing fitness functions (on board; not in book)
Three modes of instantaneous selection (top
row) and transition of trait distribution before
(middle row) and after (bottom row) selection,
indicating directional shift and reduced
variance in trait mean (on left), reduced
variance only (middle) and increased variance
(on right). If instantaneous pattern of
selection varies across time or space we call
that divergent selection and it increases
(phenotypic & genetic) variance.
Consequences of low genetic variation
Why would
bottlenecks
reduce hatching
success?
Why is inbreeding costly?
Why would 10% of the
populations show no cost of
inbreeding?
Inbreeding increases exponentially the chances that deleterious mutations are
homozygous.
Review: homozygous, heterozygous—terms for individual genetic variation: none
or some. For diploid organisms an individual is hoz if A1A1, hez if A1A2
Example on left:
Florida panthers mating
with each other
Example on right:
Florida panthers mating
with Texas cougars
Best—maintain large
enough populations to
maintain standing genetic
variation
Lack of gene flow since 1964
Note differential impact of disease on young
What demographic/genetic effect is caused by the disease episodes?
What effects did this have on the population?
Case study: Florida panthers (video link; to time 16:00 only)
What are the pros and cons of supplemental food for wildlife?