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It's becoming harder to conserve large, unbroken tracts of wilderness. Is there another way for conservation biologists to ensure the survival of a species? In this lesson, you'll learn about habitat fragmentation and metapopulations.
You may remember that the Theory of Island Biogeography basically showed how two geographic variables - distance and ecosystem size - can affect one ecological variable: species diversity. But ecology isn't just the study of species diversity, and species diversity is affected by more than just two variables. For instance, what if we're talking about habitats that can receive immigrant species from more than one source, as is usually the case with non-island habitats? Habitat destruction and fragmentation by human development also raises questions about how to best preserve remaining habitats so that they can still support wild populations of even the largest and most ecologically sensitive species. These types of questions led ecologists to explore more complicated models of ecology, which included more variables and studied more than just species diversity.
Preserving very large areas of natural habitats is great from a conservationist's point of view. This is because large areas of uninterrupted wilderness help to ensure that organisms within the area have enough room to maintain a range large enough to support a given population. Take, for example, the wild bison herds in Yellowstone National Park. Yellowstone Park is only a small fraction of the original range that the American Bison used to roam; however, it is large enough to support two separate herds of bison with a total population that ranges between 2,300 and 4,500 animals.
Creating new national parks the size of Yellowstone is now pretty much impossible in most parts of the United States because, aside from the national parks and some state parks, very few large tracts of wilderness remain undisturbed.
Human activities have reduced natural habitats and, in many cases, fragmented them into small, sometimes isolated, patches. These patches of natural habitat create a number of questions for conservation biologists. Some of these include:
These types of conservation questions led ecologists to extensively study fragmented habitats and patchy environments. Let's take a look at how conservation biologists approach these problems by using a theoretical animal that we'll call Egan's Tree Snake.
The study of populations in patchy environments led to the emergence of metapopulation theory. This theory describes a way in which several, small and somewhat isolated populations in a patchy environment can ensure the survival of the species in a larger, general area. Metapopulation theory is mostly dependent on the existence of metapopulations, or groups of local populations that are connected by immigration. The main idea of metapopulation theory is that in a patchy environment, you can have lots of small populations of a single species. From time to time, populations will go locally extinct within a given patch, but the species will still exist in other patches. If the rate of migration is high enough, individuals from other patches will eventually recolonize and repopulate the empty patch. In this way, the species will inhabit different patches at different times but will maintain a stable metapopulation and presence in the area.
However, there are a number of other assumptions that are made in metapopulation theory. The first assumption is that immigration events between individual populations must be infrequent because if immigration is occurring on a daily or weekly basis, then there is essentially just one population. The second assumption is that local extinctions within a patch are likely to occur eventually. If a patch is large enough to support a population indefinitely without any extinction, it is a stable population on its own and therefore not dependent on a larger metapopulation for survival. The third assumption is that colonization events must occur at least as frequently as extinction events over a long period of time. If, for example, extinctions are occurring at twice the rate of colonizations, then, eventually, the population will be extinct in all patches.
There is some pretty good evidence that metapopulation theory is at work in some habitats. Perhaps the most compelling evidence is from a number of English ponds where core samples have demonstrated that a particular species of snail has undergone several cycles of colonization and extinction within the same pond. This shows that these ponds are interconnected to others by immigration, that local extinctions do occur from time to time and that recolonization also occurs in the same patches.
There are two main reasons why metapopulation theory is important in conservation biology. The first is that metapopulation theory allows for smaller patches of habitat to be considered for preservation as long as other similar patches exist within the area. If a biologist only looked at the patch size of a single habitat and determined that Egan's Tree Snake would eventually go extinct there, it might not be considered for preservation, but if it's big enough to serve as a functioning habitat within a metapopulation, it might be worth preserving.
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The second reason why metapopulation theory is important in conservation biology is that it highlights the importance of migration to the survival of a species in an area. As a result, wildlife corridors, or routes that animals can use to migrate between different patches of natural habitat, are now often preserved or built between natural habitats that would otherwise be isolated. Wildlife corridors usually refer to narrow bands of land that are at least similar to the habitats they connect. However, in some cases a tunnel or underpass can also serve as a wildlife corridor. If a road is being built through a natural habitat that would be a barrier to one of the species living there, like our tree snake, elevating the road or building tunnels underneath it can effectively create wildlife corridors underneath the road.
Let's review. Beginning in the 19th century, people started to realize that natural habitat destruction was a problem and that something needed to be done about it. As a result, the United States and other countries around the world began to put aside large tracts of natural habitats and preserve them as national parks. These national parks have been extremely successful and are prime examples of what can be achieved with conservation biology.
As time passes, there are fewer and fewer large, undisturbed tracts of land that aren't already protected. Natural habitats are becoming more fragmented by human activities. This raises a series of challenges to conservation biologists, who have the goal of maintaining at least some natural habitats of all types in an attempt to preserve healthy populations of as many different species as they can.
The study of populations in patchy environments led to the emergence of metapopulation theory. This theory describes a way in which several small and somewhat isolated populations in a patchy environment can ensure the survival of the species in a larger general area. Metapopulation theory is mostly dependent on the existence of metapopulations, or groups of local populations that are connected by immigration. The main idea of metapopulation theory is that in a patchy environment, you can have lots of small populations of a single species. From time to time, populations will go locally extinct within a given patch, but the species will still exist in other patches. If the rate of migration is high enough, individuals from other patches will eventually recolonize and repopulate the empty patch.
Metapopulation theory influences the decisions that conservation biologists make. Smaller patch sizes can be considered for preservation if there are other similar patches nearby. Conservation biologists are also acutely aware of the necessity of wildlife corridors, or routes that animals can use to migrate between different patches of natural habitat. These wildlife corridors are now often preserved or built between natural habitats that would otherwise be isolated.
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