The population dynamics of the grizzly bears Literature Review - Essay Example

Literature Review There has been a great deal of study into the nature of what is now termed metapopulations, first proposed by Richard Levine in 1969, and how this model of species study can help increase understanding of the population dynamics of the grizzly bears. A metapopulation is defined as a collection of small populations of a single species that exist nearly completely separated from each other. An example of this could include the distribution of grizzly bear populations with a pocket in the North Cascades and a pocket in Yellowstone National Park as well as a few populations in places in between. Although they are separate, they remain linked together through such processes as dispersal and migration and therefore manage to affect each other in various ways. Although the population of each group can fluctuate through death and birth rates, with their own probability of going extinct, these groups can also be supported by other groups through occasional contact. The dynamics of these groups are different not only because their living environments and landscapes are not always equally suitable, but also because of their proximity, or lack of it, to other groups. Limited contact with other groups can force one of these smaller groups to become extinct, leading to the collapse of the entire species if too many groups die out. There are several different ways in which a metapopulation might be organized though, not all fitting this classic definition. For instance, the group could have a core group, from which several subgroups branch off and / or rejoin later. There are also situations in which all members of a species exist in small patches that remain more closely entwined with each other, saving each other from extinction and freely passing members from one patch or subgroup to another. There are also situations in which subgroups are sparsely distributed over adequate environments to such an extent that they have no capacity to reach each other and some appropriate living areas remain void of this species. The metapopulation study of the grizzly bear must include population dynamics, habitat fragmentation, underlying principles that would affect the bears, population growth and survival, issues faced in trying to re-introduce bears into protected areas and how the absence of bears would affect these communities. Reports indicate that while the grizzly bear population was well over 100,000 individuals at the turn of the 19th century, there are fewer than 1,000 living today in the few protected areas of the North American continent left open to them. The largest concentration of grizzly bears is believed to be located along the Great Continental Divide. It is believed that between 440-680 bears live in this 22,950 square mile region, about 200 of which are believed to live in the Glacier National Park area. In addition, up to 268 grizzly bears had been cited in Yellowstone National Park in 2002. More than the numbers of current grown bears, though, population dynamics depends in large part upon the number of female members and their ability to produce cubs, thus ensuring the future of the species. Studies in the Selkirk Mountains Grizzly Bear Ecosystem indicated bear densities of 2.33 bears for every 100 kilometers while litter size of these individuals averaged 2.22 cubs per female. Females typically had a litter every 3 years. The average litter size for female bears in Yellowstone was 1.96. Equally as many bears lost in one year in Yellowstone were as due to natural causes as they were to human intervention. These bears were lost to management removal, hunting, relocation or, in the case of bears living in Canada, predator control in heavily ranched areas. Still other bears were lost to unknown causes. These effects of human intervention in the bear population can also be seen in the other areas in which bears are protected. In addition, the conditions in which the bears are now forced to live are becoming increasingly incapable of supporting large populations of grizzly bears. Habitat fragmentation has had a great effect on the range grizzly bears can enjoy as well as the capacity of the land to support them. The adult home ranges can vary, but the low end is typically in the 45.5 square mile range while some bears will range as far as almost 900 miles. Habitat fragmentation refers to the dividing of various habitat ranges as a result of clearing and fencing off land for specific human uses. As people move into the areas previously inhabited by bears, fencing off fields in order to keep livestock safe or clearing ground to plant crops, the bears begin to lose their ability to freely traverse these ranges, contributing to more and more bears trapped into increasingly shrinking areas of uninhabited land areas. This increased bear population in smaller areas not only increases the demand upon the plant and animal resources available to the individual bear who originally claimed this range as home, but also takes on the burden of additional bears, all with the same needs as the first in terms of food resources and capacity. In addition, bear populations begin to break down as the gene pool also begins to shrink and chances that many individuals could succumb to illnesses increase. These more isolated populations also begin to experience greater danger from further human encroachment and accessibility. Finally, bears hungry from a lack of available natural resources in a given area have begun turning to the creatures that have brought this situation upon them, the humans, for food. Campers and hikers careless about bringing food into the woods have often unwittingly enticed hungry animals into their campsites, encouraging more and more bears to approach camps whether they smell food or not. The argument can also be made that shrinking wildlife areas have encouraged more human visitors to seek the same places bears have been seeking as places of refuge. In this case, the increased reports of bear/human interaction can be attributed not to more bears approaching people, but instead to more people encroaching upon the bears. This interaction, regardless of the driving force, in turn, leads to more human involvement in controlling or ‘solving’ the bear problems. As mentioned, habitat fragmentation is leading to a general decline in the available resources available to a growing grizzly bear population; however, there remain several other challenges to the long-term survival of the grizzly bear species. Not only is the available land space for food resources shrinking through habitat fragmentation, but these food sources are also being depleted within the smaller range. One of the largest sources of fat rich food for grizzly bears is the whitebark pine nut. Although the bears cannot climb into the trees in order to bring the cones down to eat, they are able to dig up the cones squirrels have collected and hidden in middens underneath the leaf litter on the forest floor. However, whitebark pine is being reduced as lumber companies continue to cut down larger trees for profit. Grizzly bears often depend upon bison and elk for sources of digestible protein and energy, but these, too, are being greatly reduced by hunting and an increasing population of carnivores in the immediate area. Another source of protein is the cutthroat trout, which, again, is threatened by various outside influences. Finally, the bears will eat cutworm moths when they migrate into their area, typically in the winter months. Because of their effects on human crops, these insects are also in danger. These ranges are also shared with black bears and quickly escalating populations of grey wolves, further adding to the drain on ungulate populations and decreasing the chances that grizzly bears will continue to find adequate food supplies to support growing cubs. Decreasing food sources are not the only challenge, either. Decreasing ranges and limited access to other populations reduces the genetic makeup of the bears in a given region, especially in regions such as the Cabinet-Yaak and the Northern Cascades, where only very small populations of grizzly bears, if any, still remain. Without adequate gene pool diversity, the population will be unable to continue. One solution to the problems of both gene pool diversity and limited food resources seems to be obvious in the possibility of relocating bears to larger wilderness areas or exchanging bears among different populations. However, there are several problems faced by bears and humans in considering whether or not to relocate an animal. The most limiting human consideration is the expense of tracking, capturing, sedating and physically relocating a particular bear. The possibility of relocating bears is further prohibited by a lack of overall success in the program, with many bears not taking to the relocation efforts and having to be removed anyway. Since many of these bears are transferred because they are creating problems in their home area, it should not be surprising to note that these bears, once they have survived relocation, will continue with their disruptive behavior in their new habitat. For the bears, there remain many challenges to overcome once the transfer has been made. The bear wakes up in an entirely new place with no memory of where to find food, shelter or any of the other things it needs. As a result, many bears suffer extreme stress in trying to adjust to the new region, sometimes to the point of death. In addition, they must compete with resident bears already located in the area, increasing chances of mortality as a result of injuries sustained while fighting. Finally, if the bear is introduced into an area as a means of enriching the gene pool of that bear population, there remains no guarantees that the bear will survive to mating season or be successful in attracting a mate in the face of new competition. There are numerous issues that would need to be faced should grizzly bears be removed from the communities in which they live. For example, studies have shown that the grizzly bears’ diet is comprised in part of army cutworm moths. This particular insect is known as a pest throughout much of North Dakota and other plains states. This insect climbs on the plants, grazing on the leaves of its host and causing a great deal of damage to the plant. Large numbers of them can cause significant damage to a large variety of crops including winter wheat, alfalfa, canola, mustard and flax. Other plants they feed on include sugarbeets, garden plants, many types of weeds and grasses. These moths migrate in the winters to the lands occupied by the bears, preferring to take refuge on the talus slopes where bears have been seen feeding energetically upon them. It is estimated that bears can eat as many of 40,000 moths per day, significantly reducing the number of moths available to attack the crops being grown elsewhere. Without the bears, these moths would be much more difficult to control. Bears also help with the distribution of fruit and berry seeds and assist in keeping the gray wolf population, a population that has seen little difficulty in maintaining and population numbers, held to manageable levels. These aspects of the grizzly bear population problem have been researched in several articles that address the metapopulation principle as a portion of their discussion. A report regarding the issues facing Glacier National Park addresses the metapopulations of several species, including the grizzly bear, that make this region home. In particular, the report provides ample evidence of biological extinction as well as the role humans play in eradicating species from the planet. The special concerns of smaller populations, such as that represented in the grizzly bear population, are discussed as well as the sizes and characteristics of these populations. A study by Craig Pease and David Mattson (Demography of the Yellowstone Grizzly Bears, 1999) found that grizzly bear management efforts should concentrate most upon the threats currently posed to the whitebark pine. The demographic study concluded that grizzly bear populations fell drastically in years when the whitebark pine harvests were scanty and rose when there were plenty of nuts available. Rather than actually growing in recent years, the study showed that the number of bears in the park had actually remain relatively stable from 1975 through 1995, with yearly increases or decreases in concordance with the whitebark pines. Another area of concern mentioned in the report is the threat the human population poses to the grizzly bears as more and more humans penetrate the inner regions of the area. The idea that grizzly bears should be relocated in order to increase a population’s chances for survival is the topic of a special report prepared by the government regarding whether this practice should be enacted to strengthen the bear population in the Bitterroot Ecosystem. It was believed that an infusion of new DNA in the form of bears relocated into the area would support the bear population and ensure its survival, but the question centered more around whether or not this population should be encouraged to grow. There remained a great deal of concern over whether a larger bear population would pose too much of a threat to the existing and growing human population in this region. Meanwhile, another study, published by William Fagan in 1999 (Weak Influences of Initial Conditions on Metapopulation Persistence Times), did not support the idea that relocating bears would have any effect at all on the current population. By analyzing the metapopulation model with stochastic catastrophes, the author of this study concluded that variation in the initial dispersion has very little effect on the group’s ability to survive in a normal environment. However, when exacerbating circumstances arise, such as illnesses or other additional threats, variation at initial dispersion becomes more and more important. Further, the author argues that if initial variation has very little effect upon the survival of a group, then artificially mixing the group would serve as little or less purpose, given the probability that a transplanted bear would experience life-threatening difficulties in adjusting to his new environment. As should remain clear, the model of metapopulations, as well as the most appropriate way of conserving the grizzly bears, requires a great deal more study to be truly understood. Read More