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If you were to count all the species that live in your hometown, you would get one number. Then if you were to count all the species that live in your county, and then in your state, and then in the whole country, you would see a pattern. The larger the area, the more species present. If you plotted these numbers onto a graph, with area on the x-axis and number of species on the y-axis, it would look something like this:
Species area curve for amphibians and reptiles in the West Indies.
This is a general pattern that holds up among different types of species and regions. In bigger areas, not only is there more space, but there are potentially more habitats, too. If a species area curve rises very slowly, then not many new species are added as area gets bigger. But if a species area curve rises quickly, then there are probably many types of habitats and niches for the species to occupy. The number of species in an area is called species richness. But as we always say, mo' species, mo' problems.
There are differences in distributions of individual species, and some live in lots of places and others live in very few places. What happens when we look at all the distributions of species together? Then things start to get interesting. We start seeing patterns of where species occur—lots of species in the tropics, fewer at the poles. There are bands of latitude where species diversity is super high. In the Amazon rainforest, a patch of land less than one square mile is home to over 1,000 tree species, but there are only 50 different tree species in all of northern Europe. Similar patterns hold for birds and insects. So, what's the dealie-o?
Scientists are still actively researching this topic, but there are two main factors that people think are behind the high diversity in the tropics: climate and history.
In the first section, we talked about climate and how the tropics receive more intense sunlight than other places on Earth. Sunlight is key when it comes to driving biodiversity. Plants need sunlight to grow, and herbivores (plant-eating animals) need plants for food. With all this sunlight, there is more evaporation of water from plants and the soil, a process called evapotranspiration. This means there is more water available in the ecosystem, so more species can be supported there. Since the tropics receive more light than other parts of the Earth, the growing season is longer for plants in the tropics. That means species have more opportunities to reproduce and therefore, more opportunities for evolution to act.
The history of the Earth is important, too. During the Earth's ice ages, most of the planet was much colder and ice covered much of the land. Sounds…great. The tropics remained ice-free though, and were a refuge for warm-loving species. Now that's what we're talking about. Because they were free of ice at that time, the tropics have a more continuous history of having the same habitat. This means there was more time for species to diversify and evolve into different ecological roles.
Coral reefs are tropical ecosystems, and have a much higher diversity of species in the west Pacific Ocean than in the Atlantic Ocean. Scientists think that the current center of diversity is the place where corals first evolved, and corals have dispersed to the Atlantic from there.
History of the continents is also important for modern species distributions. All the continents have moved around through time, but we'll look at the Americas as an example.
North America and South America were not always connected by land. What is now Central America did not exist until 3-5 million years ago. Geologic activity created mountains that rose out of the ocean, connecting North and South America at what is now the country of Panama. This land bridge was completed about 2.8 million years ago, and revolutionized life in the Americas. Nature is a pretty good civil engineer, if you ask us. Two major things happened: terrestrial plants and animals could move freely between North and South America, and marine organisms could no longer move freely between the Atlantic and Pacific.
Panama connects South and North America.
This was a huge deal. Fish, corals, and other marine life on both sides of Central America were cut off from each other, so their evolution started taking different directions on each side of the continent. Where one species once lived, two sister species evolved. The land bridge also changed circulation patterns in the oceans, and the Caribbean Sea became warmer and saltier. Now there is a waterway connecting the two oceans, but it is the manmade Panama Canal.
It takes a ship 8 to 10 hours to pass through the Panama Canal, which is 47.9 miles long.
Meanwhile, on land, North American animals started to spread south, and South American animals started to spread north. Pumas, jaguars, deer, and mice expanded their ranges south. Ground sloths, giant anteaters, and armadillos went north. This is called the Great American Interchange. Even though many types of animals spread into new territories, the North American animals that expanded their ranges into South America did much better than the animals going the opposite direction. Why?
Pumas (Puma concolor) expanded their ranges south after Central America was formed.
One theory is that animals in North America had a history of expanding into new ranges, since many of them originally came from Asia, when it was connected to North America by a land bridge. South America had been relatively isolated for quite some time, so the animals living there may not have been very good at colonizing new territories.
We know about all this history, not from watching the History Channel, but from the fossil record. Animal fossils, especially teeth, have been found all over the Americas. By keeping track of the age of fossils are in each location, biogeographers can tell when animals expanded their ranges. We're pretty sure this is what Indian Jones does in his down time.
Natural range expansions show how important dispersal is. Organisms that expand their range naturally walk, fly, or somehow get themselves to a new area. They usually don't completely change the ecosystem that they expand into. This is different from non-native species expanding into an area because humans brought them. When humans get involved, we purposefully or accidentally move species around, resulting in a few really noxious invasive species that survive better in their new ranges than their old ones. These species cause millions of dollars of damage in the US every year.