Then, why is it that when a forest is torn apart into little pieces, not many come to the rescue? Forests—much like Picasso’s painting—are intricate and complex portions of the globe where all components collaborate to form a complete system. Problems arise because many of these forests are patched and separated by a sea of human civilization. These fragments of forest are unhealthy and do not represent what a true large forest could be. Unfortunately, the number of these patches is increasing. Many scientists call these patches of forest, islands, because although they are not isolated by large bodies of water, they are separated by regions where there is sparse exchange of organisms or energy.
Throughout the study of living things, islands have been studied to examine species interactions and the changes in a system through time. In David Quammen’s book The Song of the Dodo: Island Biogeography in an Age of Extinctions, he justifies this by saying, “
“Biogeography is the study of the facts and the patterns of species distribution” in David Quammen’s book. In this field of science, one is concerned with the location of all types of organisms, but also where they do not occur. Biogeography can usually be described by the formation of new species, the extinction of species, and separation of species due to geographical change such as the formation of mountains or the interruption of a forest by a new housing complex. Therefore, biogeography studies where one can find a particular species.
After these basic questions are answered, inquisitive scientists go into the past to figure out the background story. Why are these particular species remaining while others have disappeared? More importantly, it asks: why are these species going extinct in this area or all over the world? Biogeography can also be used in such specific questions as: Why are the species on the islands of
Many of these studies revolve around islands. For example,
Island biogeography began with
On islands fewer species are present; therefore, fewer species relationships will occur. Due to their isolation, islands are prime locations for evolution to occur. With a small population, there is a limited amount of genetic differences between individuals. This causes island organisms to have a higher likelihood of straying away from their original form. Evolution is the change of organisms over time. This can occur due to new conditions on the island that the immigrant must adapt to.
For example, on the mainland, the soil is dark brown and millipedes are camouflaged as such (they are dark brown). Let’s say some of these millipedes find their way onto the island. The island soil is a dark reddish brown. Slowly, through natural selection, it will be more like that millipedes with a reddish color will survive on the island because they are better camouflaged. They will survive better than the dark brown ones so they will produce offspring that also have the reddish color. Also, with less competition, the millipedes will become bigger due to better nutrition. Over time (millennia, mind you), the millipedes on the island will be large and dark red, while the millipedes on the mainland for the most part will remain the same. Islands push the fast forward button of evolution. Therefore, it is easier to see the complete chain of events on an island. Now, mainly due to size differences, the “mechanical parts” of the reproduction of the two populations of millipedes will not fit together. As a result, they have become two separate species.
As another example, in a wind storm, a couple of beetles get lucky and land on an island instead of the ocean. The island is a complementary habitat, but there are only a couple of them present. Let’s say that the only male present has extraordinarily long front legs. His offspring will be more likely to have longer legs than the offspring of the other beetles of the mainland. The offspring on the island will mate with each other causing the long legs of the new generation of beetles to become even more pronounced. As time passes (millennia, mind you) and new generation of beetles are born and mate, the trait could become a distinguishing feature of the beetles on the island. These beetles could, then, become a new species that cannot mate with those on the mainland anymore. This is why on islands new species are more likely to form due to small species populations. This demonstrates the way that an extremely limited population size can be a catalyst for a huge shift in the characteristics of a species.
The theory of island biogeography deals with the characteristics of an island and how they affect the rate of speciation and extinction on the island. On islands, species will immigrate from the mainland or from other islands. Sometimes, the species will already be present or the new organism may be the first on the island. The latter is called a species introduction. The number of species on an island is determined by the number of different species on the island and not by population size of the species (Henderson and Whittaker).
On any island, as the number of species on an island increases, the rate of immigration will decrease. This occurs because as the number of species on the island increases, it is likely that a new species from the mainland will be a repeat and not a newly introduced species. Repeats will not increase the number of species that are present on the island. Therefore, with time and as more species are present, it will be increasingly less likely that a new species will arrive on the island and more likely that it will be a repeat. As the number of species on the island increases, the rate of extinction increases because if more species are present, there are more than can go extinct. Additionally, there will be more competition for resources. In island systems, it is more likely for a species to go extinct because it is difficult for a species population to be maintained in isolation with only a few individuals present. Reverting to the beetles from before, if the male would not have been able to find the females to mate, the population would not have continued.
Two island characteristics will determine the rate of immigration and the rate of extinction on a particular island. They are the size of the island and the distance the island is from the mainland (Quammen). Let’s set up a scenario to portray how this affects the species diversity of the island:
A group of people stand on a beach each with three small similarly-sized pebbles in their hands. A line is drawn in the sand. Three feet behind the line, three circles are drawn to represent three different islands. They are all drawn at a similar distance away from the line. More importantly, they range in size. One is large, one is medium-sized, and the last is quite small. Lining up along the barrier, each with their back turned to the circles throws each pebble over their shoulder hoping one will land in one of the circles. Ten pebbles land in the large circle, five land in the medium-sized circle, and in the smallest circle, only one pebble lands.
A larger island is more likely to have new species colonize it due to a larger surface area for them to “land.” Following suit, a smaller island would have a overall lower immigration rate. The movement of organisms is represented by the pebbles in the scenario above. The smaller island would also have a greater rate of extinction because it is likely that even though a new species appears upon the island not enough of them will be present to replenish the population with their offspring. The larger island will have a lower rate of extinction because it is more likely that the mainland will “replenish” the population with new immigrants. Additionally, it would be more likely that more of them landed on the island allowing the population size to grow quicker and the possibility of extinction to decline (Henderson and Whittaker).
Now, the group of people plays a similar game, but the circles rather than being three differently sized circles, they the same size at different distances from the starting line. In the style of bozo buckets, it is much easier for the pebbles to fall into the “islands” closer to the line than the circles farther away.
The distance of an island is from the mainland also plays are a significant role in species distribution. If an island is closer to the mainland, its rate of immigration is going to be higher than that of a farther away island because of the short distance that a species would have to travel. Therefore, smaller species or those that cannot swim or fly very well will be more likely to colonize islands that are closer. If an island is closer to the mainland, its extinction rate will be lower. This is because it will become more likely that the mainland will replenish the population size with more immigrants (Henderson and Whittaker).
It would seem that an island that is closer to the mainland and larger would be ideal in terms of species diversity. This is true. (Quammen).
More recently, most habitat fragmentation has occurred due to the activities of humans where they are separated by human dwellings or even roads; it is the process in which fragments of natural habitat are formed. Habitats that were once continuous become separated into smaller fragments. Habitat fragmentation caused by natural processes that slowly alter the layout of the environment is characterized by gradual speciation of the surrounding species. Natural processes include the formation of mountains separating a large forest into two smaller sized forest fragments. (e.g. mountains form that separate a large forest in half) These processes can occur over millions of years and allow plenty of time for the surrounding animals and plants to adapt completely. Habitat fragmentation due to human activity is often sudden. This does not allow species time to react. Therefore, fragmentation caused by humans is characterized by a large rate of extinction (“Habitat”). Although fragments used to be catalysts for evolution, today with more of them due to human activities, they actually decrease the amount of overall species diversity.
When fragments are created rapidly without allowing for evolutionary change, it is detrimental to the overall species diversity of a region. The overall amount of habitable area decreases. Plants are killed and animals must move to the remaining fragments. These remaining fragments will become very crowded, and consequently competition for resources will occur, as with population thinning (decline in population size). Few species can move between habitats (such as birds) but not many can or not many are “willing” to move across uncovered land that would increase their vulnerability to predators (“Habitat”).
A large piece of habitat is also necessary to maintain species diversity instead of a small patch. The larger an area of forest is, the more species diversity it will be able to maintain. This was discovered by Henry Allen Gleason in his article “On the relation between species and area” in 1922. In his study, he maintained patches of grass of different sizes. Each had the possibility of containing twenty-seven different types of grass. On average, a plot that was ten square meters contained ten different species of grass. A patch of twenty square meters had approximately thirteen species, compared to one of forty meters squared that had sixteen species. The best species diversity was found in the patch eighty square meters that was able to maintain twenty of the possible twenty-seven species comfortably. It was obvious that as the size of the plot increased, the number of species maintained also increased. This relationship can be used for all organisms (Quammen). In order for high species diversity to be maintained, a large area is necessary.
Additionally, a fragment is much like an island. The size of the fragment will determine the number of species present. Also it’s distance from the main forest and or other forests (or in the case of an island, mainland) will determine how many immigrants will colonize the fragment increasing species diversity. Fragments have small population sizes making them more susceptible to extinction. Small changes in environment due to climate or resources would be catastrophic in a small environment whereas in a large one could be corrected by another species (Henderson and Whittaker). For example, a certain bush that is the major food source to many small rodents cannot live under dryer conditions. For a couple of seasons, there is a drought and many of the bushes die out. In a small environment such as a fragment, there may not be other places for the rodents to get their nutrients but in a large forest, where there is increased species diversity, it is more likely that the problem can be fixed by another bush. Therefore, larger forests are more impervious to change than smaller ones.
Another consequence of habitat fragmentation is increased forest edge.
In conservation, a common procedure to decrease the effect of fragments is corridors. Corridors are small strips of forest between patches that to a certain extent, allow increased motion of organisms between different environments (“Habitat”). These corridors can also be “built” to the ocean to allow for animals that leave on coastal setting to interact with those of the interior land. With limited resources, this is the most that many conservation biologists can do. Unfortunately, the habitat is becoming increasingly fragmented, and the effects will be seen in all forests.
In conclusion, the best only way to preserve the true natural environment is to set aside the largest amount of continuous land with the smallest amount of edge to be saved. A large connected piece of land will allow species interactions to continue and relationships to thrive. Much like a work of art, all the pieces must be present to inspire the same message. Hopefully, people will realize that much like the necessity of maintaining the unity of all the pieces of
“Edge Effects.” 31 October 2005. Wikipedia. 7 November 2005.
“Habitat Fragmentation.” 31 October 2005. Wikipedia. 7 November 2005.
Henderson, Scott J. and Whittaker, Robert J. “
Quammen, David. The Song of the Dodo: