The Threat of Alien Species

The Threat of Alien Species

Op-Ed: With climate change, we may witness sequoia forests convert to chaparral

by David Vetter / Grist

In the last few decades, the growth of a wide range of invasive species has led to the demise of more than 50% of native trees. Today, the majority of North America’s threatened species rely on one or another introduced species as a source of food, habitat, or reproduction. So as we face increasingly uncertain and unstable landscapes, an ever-increasing array of alien species is on the horizon.

This poses a threat for humans as well, because many alien species are vectors for diseases. A survey conducted in 2006 found that 19% of respondents were infected with one or more diseases. There are an estimated 60,000 known human diseases, which are spread through food, water, sex, air, insects, pets, agriculture, and, to a point, through other animals. Many of these diseases are caused by microorganisms, which have become especially diverse during the recent century, with the introduction of a number of species from warmer places.

Among the most common human microorganisms are bacteria, viruses, and prions. Prions are proteins that can mutate into abnormal, often dangerous forms. These include the prion diseases Alzheimer’s disease, Creutzfeldt-Jacob Disease and bovine spongiform encephalopathy. A second group, called protozoa, are microscopic organisms that must find a human host in order to reproduce. These include the parasitic worms, such as hookworms, schistosomes, and roundworms, that are transmitted by human contact with contaminated water from freshwater, soil, and sewage. All of these species are found in the intestinal tract of humans and are transmitted through contaminated food and water.

Most of these diseases are either not deadly or at least are very rare in developed countries. However, they can cause serious health problems in developing countries. Moreover, the pathogens, which can be infectious for years, are also often difficult or impossible to eliminate, leading to human infections that can be life-threatening or life-shortening.

There are also examples of microorganisms that are non-toxic or non-deadly. For example, the hepatitis C virus, transmitted by contact with infective blood, is non-toxic and non-deadly when it infects the liver.

Such pathogens range from bacteria to viruses to prions to protozoa that could potentially

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