By: Marisa M.
Photo from: Go-Lab
Equilibrium Theory of Island Biogeography
We all have heard about and maybe seen the Great Butterfly Migration, where monarch butterflies fly every winter to Mexico, or the V-shaped geese migrations, when geese fly south for warmer temperatures. But what we don’t hear often is the species variation within these groups of animals. We may not see it, but animals are constantly adapting to their new environments as the climate and environment is constantly changing .
Photo from: Truth in Science
As shown on the cover of this study, an experiment led by Charles Darwin on finches from the Galápagos Islands showed the evolution of finch beaks over many years due to the changes in habitat of the islands and the finches’ ability to adapt to those changes. Darwin focused on geological barriers, such as species being separated from each other, and what factors affected the formation of new species. He explained that organisms evolved into new species through the slow process of natural selection, and thus different species would form on different sides of a barrier. Only the “fittest” finches who survived with desirable and practical traits needed for the environment carried on these traits to their offspring, completely changing finch beak formation. This is what Charles Darwin called natural selection. Only the finches with the proper sized beaks to eat the food on the island survived while the finches with bigger or smaller beaks than the optimal size died.
Some animals may completely become extinct, though, due to the unavailability of resources such as.... This study focuses on the equilibrium theory of island biogeography, which explains how species dispersed themselves onto oceanic islands. This theory was first introduced in 1963 by Edward O. Wilson and Robert MacArthur.. The theory predicted the island’s biodiversity based on the island's size and distance from the mainland, and is as follows:
Species are more likely to colonize nearer islands than farther islands.
Larger islands have higher immigration rates than smaller islands.
Larger islands have lower extinction rates than smaller islands.
Figure 1.1 visually shows what the island biogeography theory states about the immigration, extinction, and evolution of species who travel to different sized and distanced islands.
Marsupials
Pangea, a single formation of land that existed 175 to 335 million years ago,connected all the continents that we now have. After Pangea broke apart due to natural causes, the continents slowly moved into their current positions. Due to these continents drifting so far away from each other, the animals and organisms on these continents were separated as well. These organisms had to adapt to the certain climates and habitats that these continents created, and over millions of years, turned into completely different species. An example of this concept are marsupials. Based on fossil evidence from North and South America, Antarctica, and Australia, marsupials all once looked the same. Due to their geographic separation from when Pangea broke apart, they evolved to the specific habitat that they now live in today. There are no marsupials in Antarctica, though, due to the cold climate there. These organisms are still evolving, but at a very slow rate that one human lifetime would not be able to see. Pangea could be considered an example of the island biogeography theory due to the fact that the continents are different distances away from each other and are different sizes. Some bigger continents, though, aren’t as rich in species due to their climate and urbanization. The island biogeography theory clearly states that larger islands (and habitats) have more species diversity than smaller islands with less area.
Figure 1.2 shows the different marsupial species that have evolved all over the world due to their geographic isolation from when Pangea broke apart into the continents that we now have today.
Equilibrium
The theory’s name emphasizes finding equilibrium, where immigration rates and extinction rates are able to find a neutral point where there isn’t a surplus or decline of species. The number of species supported on the island, or the equilibrium in which these species are able to coexist and survive, is determined by their immigration and extinction rates shown by the diagram below. The “Rate of Immigration” curve is falling because the likelihood that species is not already present on the island is very low. The reason why the “Rate of Extinction” curve is increasing is because the more species there are on an island, the more likely that a species will go extinct due to the competition of resources and habitat.
Figure 1.3 shows the relationship between the rate of immigration and extinction, and the amount of species that actually are able to survive and coexist on the island/reach equilibrium (the meeting point between the immigration and extinction rates).
General Dynamic Theory of Oceanic Islands
Robert J. Whittaker came up with the General Dynamic Theory of Oceanic Island Biogeography in 2008 to further add to the factors that affect species richness. Besides immigration, evolution, and extinction, Whittaker found that the island’s age and development also was a very important factor for island richness.
This theory can be applied to the level of endemism found on islands. An endemic species is a species that only lives in one geographic region of the world and is unique to that particular region. This occurs very often on islands due to the restricted amount of genes from the organisms that live on the island. Bigger islands have a greater amount of endemic species due to their greater range of different habitats. Older islands are also shown to have a greater amount of endemic species due to the amount of time organisms had to diverge and form new species from their ancestors.
Figure 1.4 shows the endemic penguin species on the Galapagos Islands called the “Galapagos penguins” due to their existence only on these islands.
Area Effect
Islands create a phenomenon called the area effect, which proves that larger islands tend to have more species due to their greater area in habitual land. These islands are surrounded by habitats that are unsuitable for species to live in e.g. oceans or large bodies of water. This means that the area effect can be applied to national parks! National Parks are areas of land set aside by the government to protect the animals and land that live and thrive in these parks. These are pieces of land and wildlife conserved for the future of the human population because of the fast urbanization of many cities and neighborhoods. National parks are surrounded and isolated by industrial development, which includes roads, buildings (cities and towns), and homes.
Testing the Theory
As stated by William Newmark from the University of Michigan, the island biogeography theory could be applied to National Parks. Newmark in 1983 studied the disappearance and the appearance of red foxes, river otters, white-tailed jackrabbits, and spotted skunks from national parks in the United States and Canada. This study collected data from 19 different national parks in the western United States, recording the number of species they have and their areas. It is predicted that this collected information from the 21st century will continue to prove the island biogeography theory.
Data
Table 1.1 shows the number of species and area in square miles of each of the given 19 National Parks found in the western United States. These are the same National Parks Newmark studied.
Figure 1.5 is a visual representation that shows the relationship between the number of species and the area in square miles of each of the 19 National Parks studied.
Conclusion
Though there were some outliers in the data set, it can be concluded that the island biogeography theory can be applied to the “habitat islands” of National Parks. As shown by Figure 1.5, the more area a National Park had, the more species richness it contained on average. This is due to the fact that more land area means there are more available resources and niches for organisms to find and adapt to. Unlike in smaller national parks, where there are fewer available resources and niches for these animals to adapt to.
National Parks are indeed considered islands due to their surrounding areas being unsuitable for wildlife. These National Parks have preserved, protected, and restored biological diversity that would’ve otherwise been lost if there were no National Parks protecting their existence.
Discussion Questions:
What else do you think could be applied to the biogeography theory?
In what ways have humans caused the evolution of animals and different species?