Introduction to Wildlife & Fisheries ConservationWFSC 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 habitatsWhy?

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 function2. 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 1964Note differential impact of disease on youngWhat 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?