The Concept of Metapopulation is one that offers ecologists or scientists an understanding of the population dynamics in the field of wildlife management. This concept has provided great knowledge on species that exist either naturally or artificially in fragmented habitats and how these populations can be managed. A metapopulation brings together the several distinct populations of a species. In a metapopulation, a population may go extinct if it cannot withstand the population size fluctuations. The metapopulation models that are used in conservation biology must always include:
(a) Information on which patches provide suitable habitats for species
(b) The most likely extinction and colonization within a habitat
Through metapopulation models, the biological diversity can be analyzed, protected and conserved.
The term metapopulation refers to a given species population which is separated into spatial populations which interact at a certain level with each other. Currently, so many wildlife species are facing extinction due to habitat fragmentation or habitat loss. The habitats have for a long time been experiencing human disturbance which has resulted to the formation of small areas of the habitat, that are mostly isolated from each The need to conserve the threatened species has made it necessary to develop effective means through which further habitat fragmentation is prevented, and conservation of the remaining species population and habitats promoted. When species within fragmented habitats have viable populations, a metapopulation model can be developed to conserve them appropriately. The Grizzly bear (Ursus arctos) and the Killer whale (Orcinus orca) are two examples of species that are facing the problem of habitat fragmentation. In this paper, the two species population dynamics will be discussed.
Definition and description of the Metapopulation Concept
The metapopulation concept focuses on small populations of a species within a much larger population, which may locally become extinct but have the ability to re-colonize. The concept was first introduced in 1954 by two scientists, Andrewartha and Birch (Andrewartha and Birch 1954; p. 657). A population is made up of smaller local populations but within the local populations, trends usually have different directions. The local populations are usually joined together by dispersal, and the dispersal events in turn encourage re-colonization of populations that have experienced local extinction. Frequent dispersal that may prevent the occurrence of local extinctions results to a single population which is only spatially distributed. Infrequent dispersal on the other hand results to the extinction of a regional metapopulation. In conservation biology, the regional persistence of a given species which occupies fragmented habitat patches is solved by metapopulation dynamics. The Richard Levin’s concept of metapopulation that tried to describe insect pests in the farmlands based on their population dynamics is considered to have failed in addressing stochastic found in metapopulation (Hanski, I. 1991). Therefore, other metapopulation models that prove both stochascity and small migration rates cause persistence have been developed. The rate of dispersal among habitat patches determines the persistence. For any given population model, it is important to match a population to a model based on the species mobility.
Population dynamics of the Grizzly Bear (Ursus arctos) and Killer Whale (Orcinus orca)
These two species of mammals are threatened by habitat loss and destruction due to human activities. The grizzly bear is the most widely spread bear in the world and its found in three continents (Asia, North America and Europe).The habitats consist of the forests, tundra and the dry deserts. Since they are solitary, the bear requires a wide home range. Human activities have isolated the populations, which has created a great problem for this species. It is successful management of the remaining population that has saved the species from becoming extinct. The population dynamics of the species provides the changes in numbers, individual weights and age of individuals in a population. The study also looks at the processes (either environmental or biological processes) that are related to these changes. The population decline is also addressed.
The Grizzly bear is solitary and it occupies a large home range (Snyder, S.2003). Currently, the population has been isolated from each other and individuals can be found in fragmented habitats. The adult bear home ranges may overlap based on the sex and relatedness of the individuals, and the males have larger home ranges. The males usually disperse from their natal homes. The females have smaller home ranges and may disperse, or establish home ranges that overlap or are adjacent to their natal homes. In early 1990s the bear population in Scandinavia experienced a bottleneck from which it has recovered from now. The population has increased both in number and in range. The grizzly bear home range distribution is found in areas with the least human influence, where the distribution being is not random. The Scandinavian bear population is now expanding. Bears mature sexually between the age of 4-6 years, and their home ranges increase during the mating season (Mid-may to early July). Females give birth to a litter of 1-3 cubs during winter (Jan-March). The bears can live up 30 years, and the females often experience reproductive senescence at the age of 27 years.
The Killer Whale is found in the Arctic and Antarctic waters, and it’s the largest member species of the Oceanic dolphin family. The whale populations have declined due to habitat loss and death caused by whaling. The Killer whales are opportunistic predators. Local populations of the whale have lost their habitats as a result of human activities. Killer Whales are widely distributed in the world’s oceans and major seas, and can be found in areas that are away from the coast (about 800 km). Killer whales form groups which make up a community. Both the resident populations and transient populations exist. Sexually maturity in females occurs after attaining 8 years of age, while males mature at 15. The maximum longevity is estimated to be 50-60 years. The females have a 50.2 years mean life expectancy, while births occur between the month of October and March. The maximum longevity is estimated to be 50-60 years (Robin, W. 2002).
Habitat disturbance and fragmentation
Human activities have caused habitat degradation and fragmentation which has threatened the existence or survival of the two species. The killer whale is threatened by the depletion of prey species, pollution, whaling activities and conflicts with the fishing vessels. The release of chemicals in water has caused whale poisoning and death, one example being the presence of PCBs (Polychlorinated biphenyl’s) that have been disposed off in the water. The Killer whale source of food is being depleted at a very high rate, increasing the possibility of food for the whales. For instance, the salmon fish stocks which are the main source of food for the whales have been declining, while other prey species like seals and sea lions have declined in Alaska and Aleutian Island waters. When whales lack enough food, they draw energy from the blubber which then magnifies the negative effects of pollutants in an individual. Food scarcity reduces the reproductive rate of the Killer whales hence low populations of the whales. This coupled with subsistence whaling and commercial whaling has decreased the number of the surviving killer whales drastically. The Killer Whales compete with the fishermen for the decreased fish stocks, which has made them to be killed in order to minimize the competition. The spilling of oil in ocean waters has made some areas inhabitable and human noise from shipping activities or drilling has interfered with the acoustic communication and echolocation in the killer whales. The oil spills have led to death, low reproductive rate, and food shortage due to the death of prey species (Szakci, J .1999)
The Grizzly bear habitats have been destroyed and fragmented since the late 1840s in North America, when gold mining made people clear large tracts of grizzly bear habitats to create space for their settlements. The Grizzly bears are also captured and killed by humans as a source of food. In addition, they have been targeted as a source of fur that is sold in the market. Since they are very aggressive, they are considered a threat to human life and therefore have been killed in large numbers. This has decreased the number of mature surviving individuals and future reproductive individuals (off springs).Grizzly bears have also been killed through train and car accidents. For instance, grizzly bears in Montana and Banff National Park in USA are often killed by trains as they try to scavenge for leaking grains from cars. Within the parks, grizzly bears are killed on the roads due to unmitigated road access.
Principles that influence Population growth and survival
The Killer whales and Grizzly bear population’s growth or survival is affected by physical, chemical and biological principles. Food availability affects a population’s growth and survival, because it provides energy for the body functions and physical activities undertaken by the animal. Lack of food sometimes causes death through starvation. The Killer Whales feed on fish, squids and marine mammals. For the whales to get all their nutritional requirements to reproduce and survive, food needs to be adequate. Over fishing in Killer whale habitats by man has reduced food sources for the whales, and especially when the whales tend to have specialized feeding. Food shortage has reduced the reproductive rate of killer whales, a situation that has denied the species potential reproductive individuals in future. The Grizzly bears have had their habitats cleared which have depleted their food resources, both plants and animals. They prey on mammals such as bison, caribou, sheep, deer and mouse. Disappearance of habitats has reduced the population of the prey species, hence reduced food resources.
Competition is another principle that affects the population growth and survival of a species. The grizzly bear has been observed to compete with the gray wolves for food, which sometimes creates an interspecies conflict between the bears and the gray wolves. Stiff competition for scarce food resources, puts at risk its survival. The killer whales have been considered in the past to reduce the fish stocks in the oceans, which have made the species to be viewed as a competitor for fish as food by the humans. Though the ability of killer whales to compete effectively for food might have increased it survival, many of them have been killed by humans as a measure to minimize the competition. The grizzly bears have a biological ability to eat a lot of food and store it as fat, in preparation for winter when the bears go into false hibernation. They feed on a lot of food which is converted into hundreds of kilograms of fat, which is then used as a source of energy when they hibernate during the winter. The survival of the bear during winter is guaranteed by the stored fat reserves when food is very scarce during this period. The grizzly bear aggressive behavior has increased the survival rate of the young ones, since they are very aggressive when it comes to protecting their young ones. This ensures that the young ones survive up to the reproductive age, which increases the number of individuals within the population.
Space is a very important factor that influences the survival of a species based on the home ranges. The wide home ranges of both the killer whales and grizzly bears provide enough space for the animals to feed, mate and interact freely with each other. Habitat fragmentation has greatly reduced the species home ranges. This means that they can no longer get adequate food or mate freely, therefore reducing the population number due to low reproductive rate. The reproductive individuals lack mates or have to travel long distances before they get mates. Problems caused by large predators’ retention
The retention of large predators in a small habitat can give rise to a high mortality rate in the prey species or extinction of some prey species. Large predators consumer large quantities of food over a given time period. Where a large number of predators may inhabit a small fragment of the habitat with a low population of prey, the prey numbers decline. For the ecosystem to balance, the population of a prey species in the habitat needs to be proportional to the population of predators. If this is not the case, then the ecosystem becomes unstable. A high number of predators may consume the prey population at a very high rate, which will decrease the prey population and in some cases may cause extinction. Another problem caused by the retention of large predators is intense interspecies competition. Competition between species is experienced when more than one species exploits similar sources of food. Like earlier stated, large predators consume large quantities of food and when large number of predator species feed on a given prey species, the prey population may decline or become extinct. In a natural ecosystem, the predator – prey relationships ensure that both the prey and the predator populations are balanced. When a many predator species feed on one or few prey species, the prey population decreases which alters the proper biological functioning of the system.
Large predators have wide home ranges. This means that they require a lot of space for them to obtain adequate food to reproduce and survive. Retaining large predators requires large habitats and when large predators exist in a given habitat, they require a lot of space for their survival. Natural habitats that fail to provide enough space for the predators are most likely to experience conflicts between animals or between animals and humans. While each individual tries to acquire space it scrambles for the scarce resources within the small habitats. Retaining large predators in a small area gives rise to space-related problems or even conflicts.
Effects of Killer whale and Grizzly bear depletion on the community
Depletion of these two species in the ecosystem causes ecosystem imbalances in the natural community. A natural community is made up of different species which have various ecological roles. For each ecosystem, only a given number of organisms that checks each others population can co-exist freely. Having the proper population in a given natural community ensures that there balance which is vital for proper functioning of the ecosystem. The removal or decrease in number of a species population interferes with the natural functions of an ecosystem. When the species population is decreased through depletion, the natural community functions are disturbed. Large predators are vital in an ecosystem’s food chain. A food chain keeps all species or individuals at a balance because the prey populations are controlled by the species that feed on it. For instance, while plants are producers, the herbivores are primary consumers which then become a source of food for the predators. Through this process all the species populations are kept at a stable population. Depletion of a given species will increase inappropriately the population of another species. This imbalance creates a bigger ecological problem for the natural community. For instance, the killer whales feed on salmon fish and decrease in whale populations will lead to the increase in salmon fish populations beyond the level that the habitat or ecosystem can support. This will lead to overexploitation of salmon fish food resources. This makes the marine ecosystem or community experience instability. The grizzly bears feeds on bison, deer and caribou. It therefore checks the population the population of its prey species when it feeds on them. The decline in the number of grizzly bears in an ecosystem will therefore increase the prey population beyond the proper limit, and this means that the high number of the prey population will deplete its available food sources.
Demonstration of the metapopulation principle
The metapopulation principle has been used in real-words cases. To demonstrate the application of the principle in wildlife conservation, four real-world studies will be discussed;
(a) Reintroduction of Wild dogs in South Africa’s Kruger National Park
The lycaon pictus, which is an endangered wild dog, was to be introduced in 1997, in Kruger National Park (South Africa) (Don caster, P., et al.2006). The metapopulation principle had hedges introduced in the park to divide the populations. This measure was taken to protect the species from any uncertainty of a catastrophe in the park. The population was introduced into small scattered park reserves with the sub populations being managed as one single metapopulation. Translocations were to be applied based on the metapopulation model, in order to ensure gene flow and prevent inbreeding. This would also reduce the risk of extinctions. The model advocated for the use of frequency of exchange, determined by the wild dog’s natural reproductive life span (5 years). Persistence was then assured by reducing the species inbreeding by two thirds. Each area (reserve) sustained a pack of 10-20 animals with a home range size of 537km. The areas were made up of a savannah woodland habitat, where five reserves sizes covered 370-950km2 the wild dogs are protected by enclosing electrical fences around the reserves. The fences have also reduced conflicts between the wild dogs and the livestock families. Through this metapopulation management programme, the biodiversity conservation has been promoted. The genetic integrity of the metapopulation is maintained by moving single sex groups of the wild dogs between the reserves.
(b) A metapopulation principle in Population Viability Analysis of the Blue Karner Butterfly
The Blue Karner Butterfly (Lycaeides Melisa samuelis) is an endangered species. This species depends on wild blue lupine for its survival. The efforts to increase the population of this species led to the development of a metapopulation model that was developed by incorporating a patch structure, which would act as a representation of areas with lupine (www.coryi.org). The butterfly is found in northern states of America (New York, Minnesota ), and during its larval stages, the individuals depend on wild blue lupine (lupinus perennis).The butterfly is to be introduced in the northern edge area patches according to the model, which serve as the most suitable habitats for the butterflies. Some patches which are inhabited by the butterflies create a metapopulation for the species. A metapopulation model to be developed will be based on data that indicates the presence or absence of individuals across the patches at Fort McCoy. This area is made up of oak savannah where blue lupine is common. The Karner blue butterfly is favored by the blue patchy distribution, which are used to model a metapopulation.
(c) A metapopulation model of Bobcats in Eastern Florida Flatwoods Eco-region
The Bobcat metapopulation is made up of small populations in different places, where each population is found in a large habitat block connected to the other by the wildlife corridors (www.coryi.org). The corridors act as linear habitats whose function is to connect the blocks together. Through the corridors, the individuals are able to travel across the habitat blocks. The corridors run through areas of the habitat that are not inhabited (referred to as hostile lands). In the Bobcat habitats, freshwater bodies, marshes and high density development areas make up the hostile lands. The habitat blocks are preferred by the Bobcats and are made up hardwood forests, hardwood swamps and pine forests. Individuals can freely move from one block to another in order to breed. The gene flow between the bobcat’s small populations is encouraged by the movements, and inbreeding is also prevented.
(d) Metapopulation model of the California Least Tern
This bird that was once abundant has declined due to human encroachment into its habitats, as well as nesting habitat destruction. The breeding grounds have reduced, making the birds to nest on mudflats which are far away from the ocean or in man-made landfills where there is minimal human interference (www.earthplatform.com/population/ecology?Terms=population%20ecology%20host:www.ramas.com). Pollution, dredging and development along the coastal breeding regions have destroyed the terns fishing grounds. The few remaining nesting sites are found on different areas, including the military grounds of the US Navy and the Marine Corps .The birds have these areas protected by the and Wildlife service with collaboration from the military organizations. The birds are able to move between the small habitats areas that are currently under intense protection, to breed and to feed. The locations are on remote beaches away from human interference. Through the small populations, the birds can now boost their population and survival rate.
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