Posts Tagged ‘genetic drift’

Biogeographic And Genetic Factors In Northern Rockies Wolf Populations


       Biogeographic and Genetic Factors in Northern Rockies Wolf Populations

Ken Fischman, Ph.D.(Genetics)   April 9, 2008


         As we all know, Fish & Wildlife (USFW) has proposed delisting the wolf populations of Idaho, Montana, and Wyoming. They have recently estimated total wolf populations in these three states to be 1,242, with 92 breeding pairs.  Wolf population in Idaho is currently estimated at 672 ( Nadeau et al., 2007), with 42 breeding pairs (Bangs, Personal Communication, 3/8/07).

         Idaho Fish & Game proposes maintaining a minimum of 100 wolves in their state, with a minimum of 15 breeding pairs (IDFG, 10/07). 

         This paper examines these two population targets, some recent thinking about the genetics and biology of wild animal species, and finally the possible biogeographic and genetic consequences of the minimum number of wolves projected by the Idaho authorities.

          First, let’s look at the effects of population size on the viability of a species. The Hardy-Weinberg Principle states that allele frequencies (that is different forms of a gene) will not vary over time in a population. However, this principle only holds for a population sufficiently large to overcome the tendency for Genetic Drift to change gene frequencies. 

         Genetic Drift refers to the tendency for genes to become more common or more rare in successive generations.  It has no preferred direction, and it tends to sweep genetic variants out of the population with time.  It thus opposes Mutation, which introduces novel variants into the population, thus increasing species resiliency.  i.e. the ability to resist demographic shocks and adapt to changed conditions.

         Small populations often show a Founder Effect, in which one or more gene variants increasingly predominate as inbreeding increases. Inbreeding Depression, results from an increase in homozygosity. That is the state in which there are two identical copies of genes.  Homozygosity increases the possibility of recessive alleles expressing themselves. Some of these will turn out to be valuable, but most will be deleterious, thus weakening the genetic fitness of the population.

         Wolf packs, due to their size, and if they are effectively isolated from other wolf populations, also may show a Founder Effect, thus adversely effecting the long-term survival of the population. The increase in homozygosity would in turn be responsible for high frequencies inherited diseases.  This is something that wolf biologists should be on the watch for.

          What counts in small populations is not their census size, but the Effective Population size or number of breeding individuals in the population. The literature on wolves indicates that most packs have only two Effective Breeders.  However, the latest data on Yellowstone packs show multiple breeders within some packs (Smith, 2006).  I am curious as to whether this is also true for other northern Rockies wolf populations.  On the other hand, can this situation be attributed to something unique about Yellowstone wolves, such as access to large prey populations or lack of stress?  An answer to this question would be important for calculating viable population numbers.

         Let’s examine some of the other possible consequences of small population size.  Small populations in particular are prone to large fluctuations in size. Therefore, it is important to consider this in some detail.  Why does this occur?

           Inbreeding is one source of changes in population size.  The smaller the population, the more likely it is that related individuals will breed with each other, and their offspring will have a far higher number of homozygous genes.

         That is hard to detect, but its consequences, although sometimes subtle, can be severe.  Michael Soule showed that inbreeding of Poland Swine lead to deleterious effects in as soon as the second generation.  The inbreeding resulted in decrease in piglets/litter, decreased survival of newborns, and skewed sex ratios.  As we shall see, this last may be of particular importance for wolves.

         As I previously stated, inbreeding leads to lack of genetic diversity, which in turn may result in inability to adapt and evolve under changing conditions.

         There are also Demographic effects.  One such effect would be fluctuations in the size of small populations. Another is possibility of imbalance of sexes, and increasing chances of a same sex generation. For example, 3 offspring’s chance of being the same sex is 0.25 or 1/4.  If a population remains at a low level for several generations, then a same sex generation becomes almost inevitable.

         Here is an example of a problem resulting from such a situation. One of the last populations of Kakapo, an extremely rare, flightless parrot, was found on an island off the coast of New Zealand.  There were 18 of them. Unfortunately, they were all male.

         A small population is also susceptible to  Environmental effects, such as forest fires, disease, climate change, etc

         Anthropogenic effects loom large for small populations.  These are events such as: accidents, hunting, killing of so-called problem Wolves, and illegal takings. This last is of particular importance in Idaho.  Since wolf reintroduction, 59 cases of illegal hunting/killing have been documented by ID F&G.  Due to its furtive nature, this is probably only a portion of such deaths. 

         Also, last year, 45 Wolves were deliberately killed by ID F&G, USFW, and ranchers. These deaths add up to more than 6% of the Idaho Wolf population.

         All of these effects contribute to population decrease, both directly and indirectly by damaging the social structure of a pack.  Most anthropogenic effects are indiscriminate acts with unforeseen consequences.  For instance one of the pack members killed might be the Alpha female.

         There are of course important Genetic consequences, resulting from the decreases in population size brought about by these demographic, environmental, and anthropogenic effects. As previously stated, inbreeding and small population size will increase the degree of Homozygosity. Recessive genes only express themselves when there is a double dose of them. And, because most harmful genes are recessive, this will result in a larger number of unfit animals and a greater number of deaths.

         Homozygosity also results in an increase of Monomorphisms and an associated  decrease of Polymorphisms, which are multiple functional alleles in a population. This can be damaging, especially if genes effecting the Immune system are involved.  The situation can also lead to a continuous and therefore increasing fixation of deleterious genes. We give this the fanciful name of “Muller’s Ratchet,” which is the continual loss of individuals with the smallest number of deleterious genes because there are so few such individuals.  Due to the elimination of these individuals, generation by generation, the population’s genetic load of damaging genes increases, thus decreasing its adaptive fitness, and leading to its eventual extinction.

         As the expression of recessive alleles become more common due to increase in homozygosity, a species becomes less fit because they are less diverse and therefore more subject to mass die-offs due to disease, etc.  Heterozygosity, in contrast, is increased by outbreeding, leading to improved adaptive fitness of the animals.                

          I would like to put the 673 wolves in Idaho in demographic and geographical perspective.  The size of Idaho is 82,751 square miles. That works out as one wolf for every 123 square miles.  The Human population is more than 1,240,000, which means one wolf for every 1,842 people.

         The chief prey of Idaho wolves are Elk. Their 2006 numbers were estimated  by IDFG as 102,706. The Wolf population of Idaho is actually very small in comparison.  There are 153 Elk for every wolf. 

         Geneticists and Biogeographers find it useful to employ the term Minimum Viable Population or MVP.  This is defined as the smallest population size likely to persist indefinitely in a particular area.

         Here is a little history lesson.  Main and Yadov(1971) examined marsupial populations on several Australian offshore islands and came to the conclusion that a minimum of 200 – 300 animals was necessary to maintain those populations.  However, they also concluded that MVP differs from one species to another, and according to conditions.

          Conservation Geneticists usually consider that a population of less than 500 individuals is endangered, Keep in mind however, that what is important to species preservation is not total population, but the number of Effective Breeders, and many Conservation geneticists recommend a minimum of 50  breeder pairs.

          If we assume an average of ten wolves/pack, with one breeding pair, this would extrapolate to a population of 500 wolves.

         It is of course self-evident that larger populations are usually safer for the viability of a species or sub-population of a species. The key question remains as to what is the minimum number of individuals that would put a population at risk of extinction. 

         All of the factors I have previously mentioned are involved in such a determination, but perhaps another factor is more important in this situation. That is whether or not there is such a thing as a megapopulation of wolves in the northern Rockies.

         USFW speaks of a northern Rocky wolf megapopulation, connected by wolves dispersing from packs.  The megapopulation they describe extends from Canada, western Wyoming and Montana to central Idaho, and from there to northern Utah, a distance of approximately 800 miles.

         Such a connection is particularly problematical between central Idaho and Northern Utah, yet the US FW has conflated what appear to be two distinct areas. I do not know of any data that supports this idea. Lets examine the evidence for the existence of such corridors:

         Since wolf reintroduction, and through the winter of 2006, eight wolves traversed between northwest Montana and central Idaho. Of those, only three have successfully bred. Attempts made by wolves to move between the central Idaho and Yellowstone populations have fared even worse. Only one wolf completed the journey in eleven years since reintroduction (Robinson, 2006)

         These numbers of dispersing wolves are so small that they are likely to have little or no effect on gene flow between these populations.  Additionally, the fact that in such a long time, span, only three of the nine dispersing wolves bred, makes it likely that a larger number of wolves would be necessary if their movement between these regions could be successfully translated into significant gene pool effects.

         To settle the question as to whether these so-called corridors have a real effect, it would be best to do comparative genetics studies between populations rather than to continue to track lone wolves.        

         Much has been made of the rapid increase in wolf numbers since the initiation of wolf recovery in the 1990s. This increase has been cited by the USFW as a sufficient reason to remove them from the Endangered Species list.

         Is this population increase truly remarkable, or is it due to the rapid filling of an ecological niche for a keystone predator that had been nearly empty for well over a century?  Only time will tell, but I suspect that as these niches fill, rates of increase in wolf population will slow down. This is especially liable to occur because many of the conditions that led to wolf extinction in the lower 48 in the first place, are recurring. Hunting, culling of so-called problem wolves, and illegal takings may result in destruction of the intricate social fabric of wolf packs, putting them at an even greater risk of a second extinction.  Just the other day, someone in eastern Idaho shot two wolves because they “were near his ranch.”  (KPBX radio, 4/3/08).

         It is a frequent mistake to assume that current trends will persist indefinitely into the future.  To assume that wolf populations will continue to increase at present rates is as biologically naive as were the assumptions of homeowners and Wall Street investors who have lately discovered that ever-increasing housing values are an illusion.  No one escapes the laws of Nature indefinitely.

                  So we turn to the key question of what is the Minimum Viable Population for Canis lupis?  It is important to realize that MVP has two corollaries, having to do with population size and time: (1) The smaller the population, the more likely it will go extinct within a certain time period.  (2) The longer the time period, the more likely extinction is for a population of any size.

         Mark Shaffer, in his studies of Grizzly populations in national parks, suggested that a 95% chance of persistence for 100 years would be a reasonable goal.

         Conservation Geneticists have recently set more stringent parameters of a 99% chance of persistence for 1,000 years.

         In reality, viability is too complex an issue to be reduced to a single number.  A population of some specified size might be viable under one set of circumstances, but not under another set, or viable for one species, but not for another.

         Soule and Gilpin (1987) came to the conclusion that theoretical numbers cannot be relied on, only real data, analyzed in complex ways and checked against real-life situations can be relied upon.  They called this method Population Viability Analysis (PVA).

         So, how large a population is sufficient to insure viability?  Soule stated that arguing from theory, several lines of analysis produce estimates of several thousand or larger. He said “ I am assuming a 95% expectation of persistence, without loss of fitness, for several centuries.  My guess is that it would be in the low thousands.  …estimates below this range should be an automatic signal for scrutiny.” (Soule. 1987)

         However he leaves us with the following warning:  “…anyone who applies the few thousand estimate to a given species, citing this author as an authority, deserves all the contempt that will be heaped on him or her.”

Considering all the evidence accumulated, it is clear that the wolf management plans of the federal and state agencies are not based on sound scientific and genetic data or theory.  If they are carried out as presently planned, they will undoubtedly lead to genetic impoverishment and possibly to a second extinction of wolves in the Rocky Mountain region.