The Population Ecology and Population Genetics

March 24, 2009 by  

By Charles Gorecki

Lesser Snow Goose is one of three species of snow geese; the other two are the Ross’s Goose and Greater Snow Goose. All three species are migratory waterfowl that fly south each year for the winter and then back north to the arctic tundra in the spring to reproduce. The life history traits of the lesser snow goose will be examined to understand their evolutionary changes in the presence of man. Snow geese have interesting mate selection practices which lead to complex gene flow patterns between populations. It is essential to evaluate snow goose fecundity, survival rates and clutch size to gain a better understanding of this dynamic species. In order to understand the changing behavioral patterns in lesser snow geese populations, we must thoroughly evaluate their complex and adaptive population ecology and population genetics.

The Lesser Snow Goose, Chen caerulescens caerulescens, weighs about 5.5 to 5.9 pounds. Until as recently as 1961 it was thought that the blue goose was a separate species, but it is now known that “blues” and “snows” are actually the same species. This rare trait is known as “plumage dimorphism” and is found in only a few other bird species (Batt 1998). Blue Geese and Snow Geese nest in mixed colonies and often breed to produce viable offspring. The white-phase lesser snow goose is all white except for its primary flight feathers, which are all black due to the pigment melanin. The pigment melanin increases structural strength and reduces wear to these feathers. Young white-phase geese are a gray-blue color their first year, then they get their adult plumage. The blue-phase lesser snow goose has a white head and the body is a combination of gray, blue, silver, brown and white, giving it a silvery-blue appearance from a distance. The young blue-phase geese are a mottled gray all over for their first year. Both color phases have pink legs and bills as adults and gray bills and yellow legs as adolescents. A distinctive grinning patch is present on the bills of adults in both color phases.

Lesser snow geese are long lived birds, up to 20 years of age that mate for life and do not breed as yearlings (Francis et al. 1992). Mate selection occurs in their wintering grounds and is finalized on the migration back north to their breeding grounds. Breeding between color phases is common, however it depends on the color of the females parents. This means that a female lesser snow goose will mate with a male white goose if its parents were white and it will mate with a male blue goose if its parents are blue geese. Similarly, if the female came from mixed colored parents it will mate with either a blue or white phase male (Cooke et al. 1995). Female lesser snow geese are philopatric returning to the same breeding colonies at a high frequency each year and bring their mate there with them regardless of their mate’s original colony (Kuznetsov et al. 1998). According to a study done by Cooke and his co-authors, they banded 27,341 females and 28,448 males as fledglings at the La Perouse Bay colony between 1969 and 1990, of which 2082 females (7.6 percent) and only 79 males (0.3 percent) were reencountered at the breeding colony ages two and older. This confirms a significant female based philopatry while males exhibit no significant philopatry (Cooke et al. 1995). Breeding pairs will also return to the same area as previous years, which can be detrimental to breeding success due to recent degradation of most breeding grounds (Francis et al.1992).

This degradation of habitat didn’t always exist. In fact, in 1969 there were less than a million lesser snow geese and every year the tundra could recover from the feeding behaviors on the nesting areas1. Now, however, with more than 6 million snow geese, the nesting grounds are being destroyed at a rate much faster than they can recover (Ben-Ari 1998). It is estimated that one-third of the suitable nesting areas have been converted into salt plains and mud flats, another third is badly degraded with the remaining third heavily grazed but not yet damaged (Cooch et al. 2001). This rapid change in population size has occurred due to several reasons, including post World War II agriculture, reduced hunting pressure and the establishment of wildlife refuges.

Picture of an enclosure protecting habitat from lesser snow geese in La Perouse Bay and the surrounding area has been destroyed by feeding lesser snow geese in their breeding grounds

The increase in the population size of lesser snow geese is due in large part to the post World War II agricultural developments. Before these developments the snow geese would fly from there breeding grounds in the north all the way to their wintering areas along the coast of the Gulf of Mexico. When the geese arrived there they had low fitness and affects of density dependency would keep the flocks at low level. Then, in the 1950’s the salt marshes where the geese wintered were seriously degraded and they were forced to find other food sources. This caused the geese to move inland to rice fields and other agricultural fields were they had plenty of food. Also, with new farming practices crop residue was left behind for the geese to feed on in route to and from there breeding grounds, resulting in increased survival rates of adult geese. This in turn gave the geese increased ability for reproduction when they returned to their breeding grounds. In essence the geese were given an unlimited food supply during migrations enabling the population to soar (Ben-Ari 1998). Several other factors have helped to increase the population of lesser snow geese. Wildlife refuges have been established by federal and local agencies to protect and restore wildlife habitat. These refuges provide safe havens from hunters along with additional food sources for the geese. The hunting of snow geese has also declined in recent years because hunting these large flocks is very difficult and the notion that snow geese are not good table fare. Another factor is an expansion of the breeding range of the lesser snow geese since the 1920’s. This increase in range occurred because of unusually cold temperatures in the northern breeding colonies, forcing the geese to nest farther south (Ben-Ari 1998). These behavioral changes in the life history of lesser snow geese have given rise to their sharp population explosion due to an increase in annual survival rates of adult geese. As a result more geese were able to survive until reproductive age and contribute young to the ever increasing population.

This trend can only last so long, because the survival rate for juvenile geese has sharply declined. Now, because so much of their breeding grounds have been destroyed when the young geese are born they have a reduced chance of surviving to reproductive age. This occurs because of malnourishment of the hatchlings, with less food available in the breeding grounds young starve to death or are too weak to make the annual migration south. Another factor that contributes to this decrease reproduction, is the fact that since the breeding grounds are so degraded females are venturing further away from the prime breeding grounds to raise their young. When their babies hatch parents take them to feed in areas that the food isn’t as nourishing or even inedible to young geese (Cooch et al. 2001).

The ability for a long lived herbivorous geese population to increase depends on the age of first reproduction and success rates for different age groups to produce offspring that survive to reproduce. Lesser snow geese generally reproduce for the first time in their second year and all geese that have survived to their fourth year have attempted to reproduce, with nesting success increasing after their first attempt (Cooke et al.1995). The typical clutch size for lesser snow geese is four eggs and this has been declining according to a study done by Cooke between 1973 and 1984. The average number of eggs in a clutch early in the study was five. This decline may be due to the decrease in food for the breeding female on the breeding grounds before laying eggs. In this study they also found that the optimum number of eggs to produce is six. This gives the geese the best chance for at least some of their young to survive their first year. With increased brood size, young geese have a better chance of surviving predation and an increase in at least some of the eggs hatching. The decline in clutch size is strongly dependent on per nutrient availability over time despite the directional selection favoring birds that lay larger clutches (Cooke et al. 1995). In a study by Charles Francis he evaluated the average survival rate of different age groups of snow geese at the La Perouse Bay colony. In this study it was determined that the annual mortality rate is about 58% for newly fledged goslings most of which occurs before they leave the nesting areas. It is estimated that birds have about a 20% mortality rate due to hunting in their first year. In contrast, adult birds experience about 11% mortality from hunting and a total mortality of about 18%. The mortality rate of 18% doesn’t drop off due to senescence for at least the first 10-15 years of age and it is estimated that fewer then 5% of lesser snow geese will survive to that age (Francis et al. 1992). So if the geese can survive their first year there is a high probability that they will survive to reproduce. From this data it is easy to see how the lesser snow goose population has dramatically increased and if the trend continues we will be face with an ecological disaster.

If the population of lesser snow geese continues to rise they will destroy all of their suitable nesting habitat and 10s of millions of fledgling geese will starve to death as the population gets older and this will lead to an eventual population collapse. The lesser snow goose is not the only species that is in dire straits. Many other species of plants and animals rely on the same breeding grounds and have already begun to suffer serious declines (Batt 1998). We are faced with the question of how to bring this population back down to levels where the breeding grounds can support the population. Several methods have already been implemented to reduce population size such as increased bag limits and spring hunts. Studies have been done on greater snow geese (Chen caerulescens atlantica) showing that a spring hunt can reduce the ability of females nesting success. This result is achieved by causing geese to spend more time evading hunters rather then stocking up on food for their migration back to their breeding grounds. This has lead to decreased fitness and the apparent choice of some females to not breed (Gauthier et al. 2001). It has been showed in a study by Mainguy in years when there was a spring hunt adult geese arrived at the breeding grounds with significantly reduced fat and protein reserves compared to years with no spring hunt. Although most geese survived the hunt many of them died on their migration north or arrived in the breeding grounds with reduced condition and in turn had reduced clutch sizes or no clutch at all. This study shows that a spring hunt may not only increase mortality but also reduce fecundity (Mainguy et al. 2002). Francis et al. (1992) contends that hunting is only a significant source of mortality in lesser snow geese for their first year, after which they become more adept at avoiding hunters all together and the affects of fitness are slight compared to the greater snow goose example. Francis also suggests that increased hunting pressure may have been a viable option in the 1970’s and 80’s, but at this point more aggressive methods of control need to be employed (Francis et al 1992). Some of the other, less popular methods of controlling the rising lesser snow goose population are baiting and using live decoys, commercial harvesting, trapping and culling birds on migration and wintering areas and using them for human consumption (Ben-Ari 1998). At this time it is estimated that the snow goose harvest would have to increase by more then six-fold just to bring the geese down to steady-state levels (Cooke et al. 1995). The most effective methods of control would be to poison populations by the thousands or other methods of mass genocide, both of these methods have been deemed unacceptable by society. It is clear that more aggressive methods need to be used or will be faced with millions of fledglings dying each year on the breeding grounds or a breakout of avian cholera in one of the major wintering areas which would not only effect snow geese but other waterfowl species as well (Batt 1998). It is clearly a difficult task to bring these birds back into control but something needs to be done before it is too late.

Genetic variation is common among lesser snow geese, the most obvious genetic variation among them is their plumage dimorphism. This blue-white color polymorphism is controlled by two alleles at a single locus. The blue allele (B) is incompletely dominant to the white allele (b) (Cooke et al. 1995). This results in some variation in lesser snow geese with both homozygous (BB) and heterozygous (Bb) displaying blue-phase coloration and homozygous (bb) geese exhibit white-phase coloration. The blue-phase geese range in coloration as described by Cooke on a scale from (2-6), with the higher numbers indicating an increasing level of dark plumage. Categories 2-4 are generally classified as light bellied geese and 5-6 are classified as dark bellied geese. Generally mating between two homozygous (BB) geese produces offspring that exhibit higher color values and decreasing in color values in offspring from (BB)x(Bb) pairings. The lowest color values are found in offspring from two heterozygous (Bb)x(Bb) blue geese or pairings between white geese and blue geese (bb)x(Bb) (Cooke et al. 1995). It is interesting to note that populations of lesser snow geese used to homozygous with white plumage geese in the western range and homozygous blue geese in the east and only as recently the 1950’s have the two populations coming into contact and began interbreeding2. This would signify that the two color phases evolved separately. Currently the western populations are all homozygous white and the two color phase are present in the central breeding grounds with increased frequency of blue phased geese to the nearly all blue phase geese in the eastern populations (Weckstein et al. 2002). The snow geese in the central region are dimorphic in coloration, however they are not evenly distributed in these colonies as one would expect. Since the blue allele exhibits incomplete dominance it is to be expected that there would be an increasing number of blue-phase geese in the central populations. However Cooke’s study provides evidence that something else is at work. There is a high gene flow rate in snow goose populations in the central breeding ground, so there should be a rapid increase in the ratio of blue allele frequency there. Over the course of Cooke’s study there has been a slight and systematic increase in allele frequency among female geese, however this is much slower than would be expected. The male geese from the central populations have experienced no increase in the allele frequency. This paradox is explained by the fact that the lesser snow geese have strong assortative mating preferences with respects to plumage. In the breeding colony at La Perouse Bay, which is in the central breeding grounds, 90.2% of females with two white parents returned with a white male, 78.2 percent of females with two blue parents returned with a blue mate. In cases where the female had mixed parents 62.5 percent acquired white mates and 37.5 percent blue mates (Cooke et al. 1995). With this structure of gene flow the blue-phase allele will increase at a gradual rate over a much longer period then what would occur if mate selection were purely random.

Gene flow has also been shown in populations of lesser snow geese in the western breeding range, where the geese are all white-phase. There are two breeding colonies on Wrangel Island that have been shown to exhibit a great deal of genetic variation. A study by Kuznetsov on eleven loci in lesser snow geese showed that three were monomorphic with the other eight loci exhibiting up to six alleles per locus. In the study they found that the two populations wintered in different areas with a slight overlap. The geese from southern wintering populations had fewer polymorphic loci then geese from the northern wintering populations. Genetic variation occurred at a higher frequency in female geese then in males form the two colonies with females from the southern wintering areas containing more homozygous alleles. This is supported by the female philopatry and male immigration. The larger amount of genetic variation may be due to the northern population recently coming in contact with populations from banks Island which are genetically different and also suggesting that the northern and southern populations have only recently come back into contact with each other. It is estimated that a genetic exchange between the populations may be as much as 9% per generation due to pairings between populations and extrapair copulations (Kuznetsov et al. 1998).

The lesser snow goose’s ability to adapt to post World War II agriculture has enabled their sharp rise in population. With this marked rise in population, breeding habitats have been destroyed. Now, we are now faced with an impending ecological disaster. If this problem is not corrected soon, it may be too late and the breeding ground ecosystem may be lost forever. In light of this, snow goose assortative mating has resulted in an interesting mix in gene flow. Snow goose populations mix during their wintering period and interbreed to allow for more genetic variation. From the data presented, it is clear that the Lesser Snow Goose’s problems are a product of their successful adaptability.

LITERATURE CITED

Batt, B. 1998. Snow Geese: Grandeur and Calamity On An Arctic Landscape. Ducks Unlimited, Inc., Memphis, Tennessee.

Ben-Ari, E. T. 1998. A New Wrinkle in Wildlife Management. BioScience 48:667-674.

Cooch, E., R. F. Rockwell, and S. Brault. 2001. Retrospective Analysis of Demographic Responses to Environmental Change: A Lesser Snow Goose Example. Ecological Monographs 71:377-400.

Cooke, F., R. F. Rockwell, and D. B. Lank. 1995. The Snow Geese of La Perouse Bay: natural selection in the wild. Oxford University Press, Oxford, UK.

Francis, C. M., M. H. Richards, F. Cooke, and R. F. Rockwell. 1992. Changes in Survival Rates of Lesser Snow Geese With Age and Breeding Status. The Auk 109(4):731-747.

Gauthier, G., R. Pradel, S. Menu, and J.D. Lebreton. 2001. Seasonal Survival of Greater Snow Geese and Effect of Hunting Under Dependence in Sighting Probability. Ecology 82:3105-3119.

Kuznetsov, S. B., V. V. Baranyuk, and J. Y. Takekawa. 1998. Genetic Differentiation Between Wintering Populations of Lesser Snow Geese Nesting on Wrangel Island. The Auk 115(4):1053-1057.

Mainguy, J., J. Bety, G. Gauthier, and J. F. Giroux. 2002. Are Body Condition and Reproductive Effort of Laying Greater Snow Geese Affected by the Spring Hunt? The Condor 104:156-161.

Weckstein, J. D., A. D. Afton, R. M. Zink, and R. T. Alisauskas. 2002. Hybridization and Population Subdivision Within and Between Ross’s Geese and Lesser Snow Geese: A Molecular Perspective. The Condor 104:432-436.


Comments

One Comment on "The Population Ecology and Population Genetics"

  1. niels henriksen on Sat, 2nd Jul 2011 2:25 am 

    Very interesting article. Especially the studies concerning the dimorphism. About the problematics related to the population size, 2010 saw one on the largest juvenile rates despite all the teories of increased mortality due to degradiation of the breeding grounds so the question is if there is need to be so pessimistic about the population size. Apparantly the present level gives a fantastic hunting and an execeptionel experience just watching these huge flocks. And the tundra is huge too. My guess is that the best way to deal with the population size is to keep up the liberal hunting, enjoy it, and let nature find its way

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