Lecture # 13 2/19/98
Minimum Viable Populations
Corresponding Readings in Primack, Richard B. Essentials of Conservation Biology.
Chapter 12: pages 324-327 & 331-336
The concept of Minimum Viable Population, or MVP, implies that there is some threshold that will ensure a high likelihood of the survival of that population for some given time interval.
Definition: The MVP is the smallest isolated population having a [90% or 95%] chance of surviving for [100, 500, or 1000] years despite the foreseeable effects of demographic, environmental and genetic stochasticity, as well as natural catastrophes.
This term has come into wide usage because the U.S. Forest Service was enjoined by Congress in 1976 to maintain "viable populations" of all its native vertebrate species in each National Forest.
The process of determining MVPs is complex and inexact. However, it is an important goal, and research efforts continue to refine the process. The notion of setting a "hard target" for MVP has been viewed as too arbitrary, resulting in a transition to the use of Population Viability Analysis, or PVA, which attempts to identify the risks to small populations and set appropriate population estimates. Think of PVA of the process and MVP as the product.
There are two classes of factors which are included in PVA:
1) Deterministic Factors (usually species characteristics, cannot be changed)
- per capita growth rate, r
- type of survivorship curve
Example: Florida panthers number about 30 individuals. Encounters with cars and trucks are the main cause of mortality. Birth rates < death rates; PVA model indicates densities will fall to a few individuals in about 30 years.
2) Stochastic Change Factors (risks of rarity)
a) Demographic Risks
- Demographic stochasticity (chance variation in births and deaths)
- Environmental variation (year to year)
- Catastrophes (episodic extreme events)
b) Genetic Risks
- Inbreeding (expression of harmful alleles) Basis of "50" rule
- Drift (chance loss of genetic variation) Basis of "500" rule
Fluctuations in population size as a result of these factors will result in a high risk of extinction for small populations. PVA is a process for evaluating all of the interacting factors that affect risk of extinction for a single population. It is an attempt to identify the factors that are important for the survival of a population, and concentrate on those that seem to be most critical to the population in question.
Vortex: Vortex is a computer model which simulates the growth of a population for a specified time period, usually 100 years. Because each simulation or "run" includes random variation, no two runs are the same. Typically one does 100 runs and examines the average outcomes. Vortex takes into account the deterministic and stochastic factors listed above.
An example was provided in class using Vortex to estimate the risks to the Laysan Island monk seal population.
By incorporating demographic and stochastic factors, PVA is effective in identifying risks and for answering "what if" questions. This is probably a better idea than trying to determine exact MVP thresholds. Be wary of N>2000 estimates in the scientific literature, since these are determined using the "500" rule for effective population size.