Humans and Biodiversity: Invasions and Extinctions
Beyond evolution and diffusion, several other factors contribute to biodiversity. Birth and death rates among species are also important, and contribute to evolutionary rates. Species with short lifespans and high birth rates evolve the most rapidly, while species with lower birth rates and longer lifespans evolve more slowly. As we have seen, climate is a considerable factor; the most biodiverse places in the world tend to be in the tropics, while there is much lower biodiversity in places likes deserts and the arctic and antarctic. Finally, humans place a significant role in determining biodiversity thanks to our unique ability to facilitate two types of species change: extinctions and invasions.
A species is said to go extinct when the last known specimen dies. While extinctions have occurred naturally over the history of the Earth (for example, the mass extinction of dinosaurs following the meteor impact 65 million years ago), humans have produced extinctions at a nearly unprecedented rate globally over the last few centuries. These extinctions have followed three types of human activities: over-hunting, introduction of competitors, and habitat destruction. Let's look at each of these more closely.
A species is said to go extinct when the last known specimen dies. While extinctions have occurred naturally over the history of the Earth (for example, the mass extinction of dinosaurs following the meteor impact 65 million years ago), humans have produced extinctions at a nearly unprecedented rate globally over the last few centuries. These extinctions have followed three types of human activities: over-hunting, introduction of competitors, and habitat destruction. Let's look at each of these more closely.
Over-hunting
One of the great debates in the anthropological sciences is whether early man could have been responsible for exterminating mammoths from the North American continent. If so, this would be one of the earliest examples of how effective humans can be at producing extinctions. Globally, humans have over-hunted many species to the point of extinction, and many more to nearly that point (which is part of what predicated the Endangered Species Act). While most of the over-hunted species have been large animals and birds consumed for food or killed for sport or as predatory competitors to humans in the food chain, others have been over-hunted for their commercial value. For example, the African elephant is in danger of becoming extinct thanks to the high value placed on its ivory tusks by some cultures, which leads to the slaughter of thousands of animals each year (and elephants have a very low birth rate). The American bison was nearly hunted to extinction in the 19th century before beginning a slow and lengthy revival in the 20th century.
Invasions
An invasive species is defined as one which did not evolve in a place and is found outside of its evolutionary range thanks to migration or transportation. Some species have migrated over long periods in the past when climates were changing, other species migrated during periods when plates collided (such as North and South America about 27 million years ago). But when people use the term 'invasive' today, they are generally referring to species which have been transported by humans from their native location to someplace new, where they thrive. These species can then outcompete the native species for resources, sometimes causing the native species to go extinct.
In Idaho, we have several examples of invasive species that are slowly exterminating the native species and destroying some of the ecosystems Idaho is known for. For example, boaters have brought in invasive Asian mussels (such as zebra and quagga mussels) that are now infesting the waters of Idaho Panhandle lakes, degrading the water quality and covering surfaces that were formerly occupied by native species. Any Idaho fisherman knows that one of the few fish you can catch as many as you want of is the Northern Pike, an invasive fish that eats many of the native species, including salmon and trout. On land, the invasive cheatgrass (also called downy brome grass) has replaced the native sagebrush across millions of acres of southern Idaho because its seeds spread and establish so easily. While there is little danger of sagebrush going extinct any time soon, the loss of sagebrush is endangering the sage grouse that depends on it for habitat (little known fact: cheatgrass, which is native to Asia, was introduced by Washington State University researchers in an experiment gone wrong back in the early 1900s. Oops.). Think about the following questions that governmental agencies wrestle with every day regarding these invasive species:
1) What is the best way to limit the introduction of new invasive species from outside of Idaho?
2) How do we restore lakes, rivers, and landscapes by getting rid of invasive species?
3) What species are the top priorities? Which ones are doing the most harm?
4) Is it even possible to get rid of invasive species once they are established?
5) How do we grow food that is susceptible to attack from invasive species like insect pests or weeds?
What other invasive species have you heard of or know of in Idaho and the Northwest? How do those species negatively impact Idaho (for example, do they threaten water quality, native fishes or terrestrial animals, are they weeds in your garden, or pests that destroy crops)?
In Idaho, we have several examples of invasive species that are slowly exterminating the native species and destroying some of the ecosystems Idaho is known for. For example, boaters have brought in invasive Asian mussels (such as zebra and quagga mussels) that are now infesting the waters of Idaho Panhandle lakes, degrading the water quality and covering surfaces that were formerly occupied by native species. Any Idaho fisherman knows that one of the few fish you can catch as many as you want of is the Northern Pike, an invasive fish that eats many of the native species, including salmon and trout. On land, the invasive cheatgrass (also called downy brome grass) has replaced the native sagebrush across millions of acres of southern Idaho because its seeds spread and establish so easily. While there is little danger of sagebrush going extinct any time soon, the loss of sagebrush is endangering the sage grouse that depends on it for habitat (little known fact: cheatgrass, which is native to Asia, was introduced by Washington State University researchers in an experiment gone wrong back in the early 1900s. Oops.). Think about the following questions that governmental agencies wrestle with every day regarding these invasive species:
1) What is the best way to limit the introduction of new invasive species from outside of Idaho?
2) How do we restore lakes, rivers, and landscapes by getting rid of invasive species?
3) What species are the top priorities? Which ones are doing the most harm?
4) Is it even possible to get rid of invasive species once they are established?
5) How do we grow food that is susceptible to attack from invasive species like insect pests or weeds?
What other invasive species have you heard of or know of in Idaho and the Northwest? How do those species negatively impact Idaho (for example, do they threaten water quality, native fishes or terrestrial animals, are they weeds in your garden, or pests that destroy crops)?
Habitat Loss
The most widespread cause of species extinction by humans is destruction of habitat. As humans have increased and multiplied across the surface of the Earth, we have shaped the landscape to our needs and extracted the natural resources to develop our civilizations and economies. This is particularly well-documented by scientists in the case of deforestation. Because wood has been a necessary to fuel cooking and heating fires, build homes and early ships, and make paper and a myriad of other products, forests have always been valuable to humans, who have cut them down as rapidly as possible to make money. Deforestation is also necessary to expand agriculture; today, much of the deforestation taking place in the tropics is to clear space for new fields.
To better understand how scientists monitor and understand the impacts of deforestation, log in to BBLearn and complete "Assignment 6.3: Deforestation."
To better understand how scientists monitor and understand the impacts of deforestation, log in to BBLearn and complete "Assignment 6.3: Deforestation."