Biodiversity and Biogeography
Elephants, tigers, and zebras, oh my! Most children in the United States see these fantastic beasts only in zoos. Why? Because they do not live in the wild in the US. Today, these animals are only found on the African and Asian continents, but not North or South America. In contrast, there are many species found only in the Americas, but not the rest of the world.
Biodiversity describes the species richness of a given place at a given time. Are there very few species, or many? Are most of the species very similar (from the same genus or family) or is the ecosystem very diverse, with many, many types of plants and animals? There are several factors that influence biodiversity, including evolution, climate change, and anthropogenic impacts.
Biodiversity describes the species richness of a given place at a given time. Are there very few species, or many? Are most of the species very similar (from the same genus or family) or is the ecosystem very diverse, with many, many types of plants and animals? There are several factors that influence biodiversity, including evolution, climate change, and anthropogenic impacts.
Evolution and Wallace's Line
Most people associate the theory of evolution with the famous naturalist Charles Darwin. But at the same time Darwin was developing his theories based on trips to the Galapagos Islands, another young naturalist was discovering evidence of evolution halfway around the world, in southeast Asia. Alfred Russel Wallace was traveling through the Indonesian archipelago observing the fauna when he noticed something very peculiar. When he left the island of Bali and crossed the Lombok Strait to the Island of Lombok (a distance of only 20 miles), there were suddenly no more large animals. On Bali, there had been monkey, tigers, boars, lizards, and more. But on Lombok, there were only birds and insects. After visiting several more islands in the region and assessing the biodiversity of those islands, Wallace drew a line on a map and used it to support his theory about evolution. Can you guess what his theory was?
Wallace theorized that the islands to the left (west and north) of his line must have all been connected once, and that the animals found there were all related. But the islands to the right (south and east) of his line must have been separated somehow. He theorized that this separation caused completely different species to develop in isolation on these islands, even though they are climatically and vegetatively the same as the rest of Indonesia.
Today, we know the reason for this separation -- plate tectonics. There is a deep ocean trench along Wallace's line, but a shallow continental shelf underlying the rest of Indonesia and southeast Asia. During the ice ages, when the ocean was 300 feet lower in elevation, the islands west of the line were connected to the mainland, and animals were free to roam across and populate the islands. But the smaller islands to the east of the line remained disconnected and separated by a deep, relatively cold ocean channel that prevented mammals from crossing to them. Only species that could fly were able to make the crossing, and even then only rarely would they establish. This led to different evolutionary paths for the species found on each side of the line. The species west of the line are very similar, with divergence in their characteristics only occurring over the last couple of hundred thousand year. The species east of the line, however, have been separated and evolving for millions of years since the break-up of Pangea.
Today, we know the reason for this separation -- plate tectonics. There is a deep ocean trench along Wallace's line, but a shallow continental shelf underlying the rest of Indonesia and southeast Asia. During the ice ages, when the ocean was 300 feet lower in elevation, the islands west of the line were connected to the mainland, and animals were free to roam across and populate the islands. But the smaller islands to the east of the line remained disconnected and separated by a deep, relatively cold ocean channel that prevented mammals from crossing to them. Only species that could fly were able to make the crossing, and even then only rarely would they establish. This led to different evolutionary paths for the species found on each side of the line. The species west of the line are very similar, with divergence in their characteristics only occurring over the last couple of hundred thousand year. The species east of the line, however, have been separated and evolving for millions of years since the break-up of Pangea.
Factors determining biodiversity and biogeography
Scientists have been able to study evolution and biogeography in action thanks to some tectonic events that have produced remarkable natural laboratories. One of these types of events is the presence of highly isolated islands across the globe, where scientists have found some of the most unique species on Earth. Places like New Zealand (home of the famous, and extinct, Dodo bird) and Hawaii host numerous unique species and incredible biodiversity.
The other type of tectonic event is a volcanic eruption. In 1883, the volcanic island Krakatau erupted violently (as it is a composite volcano) and completely destroyed all flora and fauna on the island. In the aftermath of this event, naturalists have been able to study how the island repopulated, how the initial colonizing species have evolved, and how the biodiversity has fluctuated over the intervening decades. How quickly new species have moved from other islands and established on Krakatau is related to the process of diffusion, or species dispersal rates.
To learn more about biogeography and factors controlling biodiversity, log in to BBLearn and complete "Assignment 6.2: Diffusion and Island Biogeography."
The other type of tectonic event is a volcanic eruption. In 1883, the volcanic island Krakatau erupted violently (as it is a composite volcano) and completely destroyed all flora and fauna on the island. In the aftermath of this event, naturalists have been able to study how the island repopulated, how the initial colonizing species have evolved, and how the biodiversity has fluctuated over the intervening decades. How quickly new species have moved from other islands and established on Krakatau is related to the process of diffusion, or species dispersal rates.
To learn more about biogeography and factors controlling biodiversity, log in to BBLearn and complete "Assignment 6.2: Diffusion and Island Biogeography."